CHE 120 - Introduction To Organic Chemistry - Textbook

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5. Chapter 1 - Organic Chemistry Review / Hydrocarbons

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• Chapter 1 - Organic Chemistry Review / Hydrocarbons
  • Introduction: Alkanes and Halogenated Hydrocarbons
  • 1.1 Organic Chemistry
  • 1.2 Structures and Names of Alkanes
  • 1.3 Branched-Chain Alkanes
  • 1.4 Condensed Structural and Line-Angle Formulas
  • 1.5 IUPAC Nomenclature
  • 1.6 Physical Properties of Alkanes
  • 1.7 Chemical Properties of Alkanes
  • 1.8 Halogenated Hydrocarbons
  • 1.9 Cycloalkanes
  • 1.10 End-of-Unit Material: Alkanes and Halogenated Hydrocarbons
  • Introduction: Unsaturated and Aromatic Hydrocarbons
  • 1.11 Alkenes: Structures and Names
  • 1.12 Cis-Trans isomers (Geometric Isomers)
  • 1.13 Physical Properties of Alkenes
  • 1.14 Chemical Properties of Alkenes
  • 1.15 Polymers
  • 1.16 Alkynes
  • 1.17 Aromatic Compounds: Benzene
  • 1.18 Structure and Nomenclature of Aromatic Compounds
  • 1.19 End-of-Unit Material: Unsaturated and Aromatic Hydrocarbons
• Chapter 2 - Alcohols, Phenols, Thiols, Ethers
• Chapter 3 - Aldehydes, Ketones
• Chapter 4 - Carboxylic Acids, Esters
• Chapter 5 - Amines and Amides
• Chapter 6 - Carbohydrates
• Chapter 7 - Lipids
• Chapter 8 - Amino Acids
• Chapter 9 - Proteins and Enzymes
• Chapter 10 - Nucleic Acids and Protein Synthesis
• Chapter 11 - Metabolic Pathways and Energy Production

CHE 120 Introduction to Organic Chemistry

Adapted by Nelson Nuñez-Rodriguez

Conditions of Use:

Unless otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Chapters derived from:

The Basics of General, Organic, and Biological Chemistry

By David W. Ball, John W. Hill, and Rhonda J. Scott

Attribution-NonCommercial-ShareAlike CC BY-NC-SA

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Opening Essay

Hydrocarbons are the simplest organic compounds, but they have interesting physiological effects. These effects depend on the size of the hydrocarbon molecules and where on or in the body they are applied. Alkanes of low molar mass—those with from 1 to approximately 10 or so carbon atoms—are gases or light liquids that act as anesthetics. Inhaling (“sniffing”) these hydrocarbons in gasoline or aerosol propellants for their intoxicating effect is a major health problem that can lead to liver, kidney, or brain damage or to immediate death by asphyxiation by excluding oxygen.

Swallowed, liquid alkanes do little harm while in the stomach. In the lungs, however, they cause “chemical” pneumonia by dissolving fatlike molecules from cell membranes in the tiny air sacs (alveoli). The lungs become unable to expel fluids, just as in pneumonia caused by bacteria or viruses. People who swallow gasoline or other liquid alkane mixtures should not be made to vomit, as this would increase the chance of getting alkanes into the lungs. (There is no home-treatment antidote for gasoline poisoning; call a poison control center.)

Liquid alkanes with approximately 5–16 carbon atoms per molecule wash away natural skin oils and cause drying and chapping of the skin, while heavier liquid alkanes (those with approximately 17 or more carbon atoms per molecule) act as emollients (skin softeners). Such alkane mixtures as mineral oil and petroleum jelly can be applied as a protective film. Water and aqueous solutions such as urine will not dissolve such a film, which explains why petroleum jelly protects a baby’s tender skin from diaper rash.

We begin our study of organic chemistry with the alkanes, compounds containing only two elements, carbon and hydrogen, and having only single bonds. There are several other kinds of hydrocarbons, distinguished by the types of bonding between carbon atoms and by the properties that result from that bonding. Later in this chapter we will examine hydrocarbons with double bonds, with triple bonds, and with a special kind of bonding called aromaticity.

1.1 Organic Chemistry

Learning Objective

1. Recognize the composition and properties typical of organic and inorganic compounds.

Scientists of the 18th and early 19th centuries studied compounds obtained from plants and animals and labeled them organic because they were isolated from “organized” (living) systems. Compounds isolated from nonliving systems, such as rocks and ores, the atmosphere, and the oceans, were labeled inorganic. For many years, scientists thought organic compounds could be made by only living organisms because they possessed a vital force found only in living systems. The vital force theory began to decline in 1828, when the German chemist Friedrich Wöhler synthesized urea from inorganic starting materials. He reacted silver cyanate (AgOCN) and ammonium chloride (NH4Cl), expecting to get ammonium cyanate (NH4OCN). What he expected is described by the following equation.

AgOCN + NH4Cl → AgCl + NH4OCN

Instead, he found the product to be urea (NH2CONH2), a well-known organic material readily isolated from urine. This result led to a series of experiments in which a wide variety of organic compounds were made from inorganic starting materials. The vital force theory gradually went away as chemists learned that they could make many organic compounds in the laboratory.

Today organic chemistry is the study of the chemistry of the carbon compounds, and inorganic chemistry is the study of the chemistry of all other elements. It may seem strange that we divide chemistry into two branches—one that considers compounds of only one element and one that covers the 100-plus remaining elements. However, this division seems more reasonable when we consider that of tens of millions of compounds that have been characterized, the overwhelming majority are carbon compounds.

Note

The word organic has different meanings. Organic fertilizer, such as cow manure, is organic in the original sense; it is derived from living organisms. Organic foods generally are foods grown without synthetic pesticides or fertilizers. Organic chemistry is the chemistry of compounds of carbon.

Carbon is unique among the other elements in that its atoms can form stable covalent bonds with each other and with atoms of other elements in a multitude of variations. The resulting molecules can contain from one to millions of carbon atoms. In Chapter 1 "Organic Chemistry Review/Hydrocarbons" through Chapter 5 "Amines and Amides", we survey organic chemistry by dividing its compounds into families based on functional groups. We begin with the simplest members of a family and then move on to molecules that are organic in the original sense—that is, they are made by and found in living organisms. These complex molecules (all containing carbon) determine the forms and functions of living systems and are the subject of biochemistry, a topic presented in Chapter 6 "Carbohydrates" through Chapter 11 "Metabolic Pathways and Energy Production".

Organic compounds, like inorganic compounds, obey all the natural laws. Often there is no clear distinction in the chemical or physical properties among organic and inorganic molecules. Nevertheless, it is useful to compare typical members of each class, as in Table 1.1 "General Contrasting Properties and Examples of Organic and Inorganic Compounds". (Keep in mind, however, that there are exceptions to every category in this table.) To further illustrate typical differences among organic and inorganic compounds, Table 1.1 also lists properties of the inorganic compound sodium chloride (common table salt, NaCl) and the organic compound hexane (C6H14), a solvent that is used to extract soybean oil from soybeans (among other uses). Many compounds can be classified as organic or inorganic by the presence or absence of certain typical properties, as illustrated in Table 1.1.

Table 1.1 General Contrasting Properties and Examples of Organic and Inorganic Compounds

Organic	Hexane	Inorganic	NaCl
low melting points	−95°C	high melting points	801°C
low boiling points	69°C	high boiling points	1,413°C
low solubility in water; high solubility in nonpolar solvents	insoluble in water; soluble in gasoline	greater solubility in water; low solubility in nonpolar solvents	soluble in water; insoluble in gasoline
flammable	highly flammable	nonflammable	nonflammable
aqueous solutions do not conduct electricity	nonconductive	aqueous solutions conduct electricity	conductive in aqueous solution
exhibit covalent bonding	covalent bonds	exhibit ionic bonding	ionic bonds

Concept Review Exercises

1. Classify each compound as organic or inorganic.

   1. C3H8O
   2. CaCl2
   3. Cr(NH3)3Cl3
   4. C30H48O3N

2. Which compound is likely organic and which is likely inorganic?

   1. a flammable compound that boils at 80°C and is insoluble in water
   2. a compound that does not burn, melts at 630°C, and is soluble in water

Answers

1. 1. organic
   2. inorganic
   3. inorganic
   4. organic
2. 1. organic
   2. inorganic

Key Takeaway

• Organic chemistry is the study of carbon compounds, nearly all of which also contain hydrogen atoms.

Exercises

1. Classify each compound as organic or inorganic.

   1. C6H10
   2. CoCl2
   3. C12H22O11

2. Classify each compound as organic or inorganic.

   1. CH3NH2
   2. NaNH2
   3. Cu(NH3)6Cl2

3. Which member of each pair has a higher melting point?

   1. CH3OH and NaOH
   2. CH3Cl and KCl

4. Which member of each pair has a higher melting point?

   1. C2H6 and CoCl2
   2. CH4 and LiH

Answers

1. a. organic b. inorganic c. organic

3. a. NaOH b. KCI

1.2 Structures and Names of Alkanes

Learning Objective

1. Identify and name simple (straight-chain) alkanes given formulas and write formulas for straight-chain alkanes given their names.

The simplest organic compounds are composed of carbon and hydrogen atoms only. As we know, there are several different kinds of hydrocarbons. They are distinguished by the types of bonding between carbon atoms and the properties that result from that bonding. Hydrocarbons with only carbon-to-carbon single bonds (C–C) and existing as a continuous chain of carbon atoms also bonded to hydrogen atoms are called alkanes (or saturated hydrocarbons). Saturated, in this case, means that each carbon atom is bonded to four other atoms (hydrogen or carbon)—the most possible; there are no double or triple bonds in the molecules.

Note

The word saturated has the same meaning for hydrocarbons as it does for the dietary fats and oils: the molecule has no carbon-to-carbon double bonds (C=C). (For more information about fats and oils, see Chapter 7 "Lipids", Section 7.1 "Fatty Acids" and Section 7.2 "Fats and Oils".)

The three simplest alkanes—methane (CH4), ethane (C2H6), and propane (C3H8)—are shown in Figure 1.1 "The Three Simplest Alkanes". The flat representations shown do not accurately portray bond angles or molecular geometry. Methane has a tetrahedral shape that chemists often portray with wedges indicating bonds coming out toward you and dashed lines indicating bonds that go back away from you. An ordinary solid line indicates a bond in the plane of the page.

Figure 1.1 The Three Simplest Alkanes

The VSEPR theory correctly predicts a tetrahedral shape for the methane molecule (Figure 1.2 "The Tetrahedral Methane Molecule").

Figure 1.2 The Tetrahedral Methane Molecule

Methane (CH4), ethane (C2H6), and propane (C3H8) are the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH2 unit. The first 10 members of this series are given in Table 1.2 "The First 10 Straight-Chain Alkanes".

Table 1.2 The First 10 Straight-Chain Alkanes

Name	Molecular Formula (CnH2n + 2)	Condensed Structural Formula	Number of Possible Isomers
methane	CH4	CH4	—
ethane	C2H6	CH3CH3	—
propane	C3H8	CH3CH2CH3	—
butane	C4H10	CH3CH2CH2CH3	2
pentane	C5H12	CH3CH2CH2CH2CH3	3
hexane	C6H14	CH3CH2CH2CH2CH2CH3	5
heptane	C7H16	CH3CH2CH2CH2CH2CH2CH3	9
octane	C8H18	CH3CH2CH2CH2CH2CH2CH2CH3	18
nonane	C9H20	CH3CH2CH2CH2CH2CH2CH2CH2CH3	35
decane	C10H22	CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3	75

Consider the series in Figure 1.3 "Members of a Homologous Series". The sequence starts with C3H8, and a CH2 unit is added in each step moving up the series. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH2 group) is called a homologous series. The members of such a series, called homologs, have properties that vary in a regular and predictable manner. The principle of homology gives organization to organic chemistry in much the same way that the periodic table gives organization to inorganic chemistry. Instead of a bewildering array of individual carbon compounds, we can study a few members of a homologous series and from them deduce some of the properties of other compounds in the series.

Figure 1.3 Members of a Homologous Series

Each succeeding formula incorporates one carbon atom and two hydrogen atoms more than the previous formula.

The principle of homology allows us to write a general formula for alkanes: CnH2n + 2. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. For example, an alkane with eight carbon atoms has the molecular formula C8H(2 × 8) + 2 = C8H18.

Concept Review Exercises

1. In the homologous series of alkanes, what is the molecular formula for the member just above C8H18?

2. Use the general formula for alkanes to write the molecular formula of the alkane with 12 carbon atoms.

Answers

1. C9H20

2. C12H26

Key Takeaway

• Simple alkanes exist as a homologous series, in which adjacent members differ by a CH2 unit.

Exercises

1. What compounds contain fewer carbon atoms than C3H8 and are its homologs?

2. What compounds contain five to eight carbon atoms and are homologs of C4H10?

Answer

1. CH4 and C2H6

v1.3 Branched-Chain Alkanes

Learning Objective

1. Learn how alkane molecules can have branched chains and recognize compounds that are isomers.

We can write the structure of butane (C4H10) by stringing four carbon atoms in a row,

–C–C–C–C–

and then adding enough hydrogen atoms to give each carbon atom four bonds:

The compound butane has this structure, but there is another way to put 4 carbon atoms and 10 hydrogen atoms together. Place 3 of the carbon atoms in a row and then branch the fourth one off the middle carbon atom:

Now we add enough hydrogen atoms to give each carbon four bonds.

There is a hydrocarbon that corresponds to this structure, which means that two different compounds have the same molecular formula: C4H10. The two compounds have different properties—for example, one boils at −0.5°C; the other at −11.7°C. Different compounds having the same molecular formula are called isomers. The compound with this branched chain is called isobutane (Figure 1.4 "Butane and Isobutane").

Figure 1.4 Butane and Isobutane

The ball-and-stick models of these two compounds show them to be isomers; both have the molecular formula C4H10.

Notice that C4H10 is depicted with a bent chain in Figure 1.4. The four-carbon chain may be bent in various ways because the groups can rotate freely about the C–C bonds. However, this rotation does not change the identity of the compound. It is important to realize that bending a chain does not change the identity of the compound; all of the following represent the same compound:

The formula of isobutane shows a continuous chain of three carbon atoms only, with the fourth attached as a branch off the middle carbon atom of the continuous chain.

Unlike C4H10, the compounds methane (CH4), ethane (C2H6), and propane (C3H8) do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds.

Next beyond C4H10 in the homologous series is pentane. Each compound has the same molecular formula: C5H12. (Table 1.2 "The First 10 Straight-Chain Alkanes" has a column identifying the number of possible isomers for the first 10 straight-chain alkanes.) The compound at the far left is pentane because it has all five carbon atoms in a continuous chain. The compound in the middle is isopentane; like isobutane, it has a one CH3 branch off the second carbon atom of the continuous chain. The compound at the far right, discovered after the other two, was named neopentane (from the Greek neos, meaning “new”). Although all three have the same molecular formula, they have different properties, including boiling points: pentane, 36.1°C; isopentane, 27.7°C; and neopentane, 9.5°C.

Note

A continuous (unbranched) chain of carbon atoms is often called a straight chain even though the tetrahedral arrangement about each carbon gives it a zigzag shape. Straight-chain alkanes are sometimes called normal alkanes, and their names are given the prefix n-. For example, butane is called n-butane. We will not use that prefix here because it is not a part of the system established by the International Union of Pure and Applied Chemistry.

Concept Review Exercises

1. In alkanes, can there be a two-carbon branch off the second carbon atom of a four-carbon chain? Explain.

2. A student is asked to write structural formulas for two different hydrocarbons having the molecular formula C5H12. She writes one formula with all five carbon atoms in a horizontal line and the other with four carbon atoms in a line, with a CH3 group extending down from the first attached to the third carbon atom. Do these structural formulas represent different molecular formulas? Explain why or why not.

Answers

1. No; the branch would make the longest continuous chain of five carbon atoms.

2. No; both are five-carbon continuous chains.

Key Takeaway

• Alkanes with four or more carbon atoms can exist in isomeric forms.

Exercises

1. Briefly identify the important distinctions between a straight-chain alkane and a branched-chain alkane.

2. How are butane and isobutane related? How do they differ?

3. Name each compound.

   a.

   b.

4. Write the structural formula for each compound.

   a. hexane

   b. octane

5. Indicate whether the structures in each set represent the same compound or isomers.

   a. CH3CH2CH2CH3 and

   b. CH3CH2CH2CH2CH3 and

Answers

1. Straight-chain alkanes and branched-chain alkanes have different properties as well as different structures.

3.

a. pentane b. heptane

5.

a. yes b. no

1.4 Condensed Structural and Line-Angle Formulas

Learning Objectives

1. Write condensed structural formulas for alkanes given complete structural formulas.
2. Draw line-angle formulas given structural formulas.

We use several kinds of formulas to describe organic compounds. A molecular formula shows only the kinds and numbers of atoms in a molecule. For example, the molecular formula C4H10 tells us there are 4 carbon atoms and 10 hydrogen atoms in a molecule, but it doesn’t distinguish between butane and isobutane. A structural formula shows all the carbon and hydrogen atoms and the bonds attaching them. Thus, structural formulas identify the specific isomers by showing the order of attachment of the various atoms.

Unfortunately, structural formulas are difficult to type/write and take up a lot of space. Chemists often use condensed structural formulas to alleviate these problems. The condensed formulas show hydrogen atoms right next to the carbon atoms to which they are attached, as illustrated for butane:

The ultimate condensed formula is a line-angle formula, in which carbon atoms are implied at the corners and ends of lines, and each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. For example, we can represent pentane (CH3CH2CH2CH2CH3) and isopentane [(CH3)2CHCH2CH3] as follows:

Note

Parentheses in condensed structural formulas indicate that the enclosed grouping of atoms is attached to the adjacent carbon atom.

Key Takeaways

• Condensed chemical formulas show the hydrogen atoms (or other atoms or groups) right next to the carbon atoms to which they are attached.
• Line-angle formulas imply a carbon atom at the corners and ends of lines. Each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds.

Exercises

1. Write the condensed structural formula for each structural formula.

   a.

   b.

   c.

2. A condensed structural formula for isohexane can be written as (CH3)2CHCH2CH2CH3. Draw the line-angle formula for isohexane.

3. Draw a line-angle formula for the compound CH3CH2CH(CH3)CH2CH2CH3.

4. Give the structural formula for the compound represented by this line-angle formula:

   

Answers

1.

a. CH3CH3

b. CH3CH2CH3

c. CH3CH2CH2CH2CH3

3.

1.5 IUPAC Nomenclature

Learning Objective

1. Name alkanes by the IUPAC system and write formulas for alkanes given IUPAC names.

As noted in Table 1.2 "The First 10 Straight-Chain Alkanes", the number of isomers increases rapidly as the number of carbon atoms increases. There are 3 pentanes, 5 hexanes, 9 heptanes, and 18 octanes. It would be difficult to assign unique individual names that we could remember. A systematic way of naming hydrocarbons and other organic compounds ha s been devised by the International Union of Pure and Applied Chemistry (IUPAC). These rules, used worldwide, are known as the IUPAC System of Nomenclature. (Some of the names we used earlier, such as isobutane, isopentane, and neopentane, do not follow these rules and are called common names.) A stem name (Table 1.3 "Stems That Indicate the Number of Carbon Atoms in Organic Molecules") indicates the number of carbon atoms in the longest continuous chain (LCC). Atoms or groups attached to this carbon chain, called substituents, are then named, with their positions indicated by numbers. For now, we will consider only those substituents called alkyl groups.

Table 1.3 Stems That Indicate the Number of Carbon Atoms in Organic Molecules

Stem	Number
meth-	1
eth-	2
prop-	3
but-	4
pent-	5
hex-	6
hept-	7
oct-	8
non-	9
dec-	10

An alkyl group is a group of atoms that results when one hydrogen atom is removed from an alkane. The group is named by replacing the -ane suffix of the parent hydrocarbon with -yl. For example, the CH3 group derived from methane (CH4) results from subtracting one hydrogen atom and is called a methyl group.

The alkyl groups we will use most frequently are listed in Table 1.4 "Common Alkyl Groups". Alkyl groups are not independent molecules; they are parts of molecules that we consider as a unit to name compounds systematically.

Table 1.4 Common Alkyl Groups

Parent Alkane	Alkyl Group	Condensed Structural Formula
methane		methyl		CH3–
ethane		ethyl		CH3CH2–
propane		propyl		CH3CH2CH2–
isopropyl		CH3–
butane		butyl*		CH3CH2CH2CH2–

*There are four butyl groups, two derived from butane and two from isobutane. We will introduce the other three where appropriate.

Simplified IUPAC rules for naming alkanes are as follows (demonstrated in Example 12.1).

1. Name alkanes according to the LCC of carbon atoms in the molecule (rather than the total number of carbon atoms). This LCC, considered the parent chain, determines the base name, to which we add the suffix -ane to indicate that the molecule is an alkane.

2. If the hydrocarbon is branched, number the carbon atoms of the LCC. Numbers are assigned in the direction that gives the lowest numbers to the carbon atoms with attached substituents. Hyphens are used to separate numbers from the names of substituents; commas separate numbers from each other. (The LCC need not be written in a straight line; for example, the LCC in the following has five carbon atoms.)

3. Place the names of the substituent groups in alphabetical order before the name of the parent compound. If the same alkyl group appears more than once, the numbers of all the carbon atoms to which it is attached are expressed. If the same group appears more than once on the same carbon atom, the number of that carbon atom is repeated as many times as the group appears. Moreover, the number of identical groups is indicated by the Greek prefixes di-, tri-, tetra-, and so on. These prefixes are not considered in determining the alphabetical order of the substituents. For example, ethyl is listed before dimethyl; the di- is simply ignored. The last alkyl group named is prefixed to the name of the parent alkane to form one word.

When these rules are followed, every unique compound receives its own exclusive name. The rules enable us to not only name a compound from a given structure but also draw a structure from a given name. The best way to learn how to use the IUPAC system is to put it to work, not just memorize the rules. It’s easier than it looks.

Example 1

Name each compound.

1. 
2. 
3. 

Solution

1. The LCC has five carbon atoms, and so the parent compound is pentane (rule 1). There is a methyl group (rule 2) attached to the second carbon atom of the pentane chain. The name is therefore 2-methylpentane.
2. The LCC has six carbon atoms, so the parent compound is hexane (rule 1). Methyl groups (rule 2) are attached to the second and fifth carbon atoms. The name is 2,5-dimethylhexane.
3. The LCC has eight carbon atoms, so the parent compound is octane (rule 1). There are methyl and ethyl groups (rule 2), both attached to the fourth carbon atom (counting from the right gives this carbon atom a lower number; rule 3). The correct name is thus 4-ethyl-4-methyloctane.

Skill-Building Exercise

Name each compound.

1. 
2. 
3. 

Example 2

Draw the structure for each compound.

1. 2,3-dimethylbutane
2. 4-ethyl-2-methylheptane

Solution

In drawing structures, always start with the parent chain.

1. The parent chain is butane, indicating four carbon atoms in the LCC.

   

   Then add the groups at their proper positions. You can number the parent chain from either direction as long as you are consistent; just don’t change directions before the structure is done. The name indicates two methyl (CH3) groups, one on the second carbon atom and one on the third.

   

   Finally, fill in all the hydrogen atoms, keeping in mind that each carbon atom must have four bonds.

   

2. The parent chain is heptane in this case, indicating seven carbon atoms in the LCC.

   –C–C–C–C–C–C–C–

   Adding the groups at their proper positions gives

   

   Filling in all the hydrogen atoms gives the following condensed structural formulas:

   

   Note that the bonds (dashes) can be shown or not; sometimes they are needed for spacing.

Skill-Building Exercise

Draw the structure for each compound.

1. 4-ethyloctane

2. 3-ethyl-2-methylpentane

3. 3,3,5-trimethylheptane

Concept Review Exercises

1. What is a CH3 group called when it is attached to a chain of carbon atoms—a substituent or a functional group?

2. Which type of name uses numbers to locate substituents—common names or IUPAC names?

Answers

1. substituent

2. IUPAC names

Key Takeaway

• Alkanes have both common names and systematic names, specified by IUPAC.

Exercises

1. Briefly identify the important distinctions between an alkane and an alkyl group.

2. How many carbon atoms are present in each molecule?

   a. 2-methylbutane

   b. 3-ethylpentane

3. How many carbon atoms are present in each molecule?

   a. 2,3-dimethylbutane

   b. 3-ethyl-2-methylheptane

4. Draw the structure for each compound.

   a. 3-methylpentane

   b. 2,2,5-trimethylhexane

   c. 4-ethyl-3-methyloctane

5. Draw the structure for each compound.

   a. 2-methylpentane

   b. 4-ethyl-2-methylhexane

   c. 2,2,3,3-tetramethylbutane

6. Name each compound according to the IUPAC system.

   a.

   b.

7. Name each compound according to the IUPAC system.

   a.

   b.

8. What is a substituent? How is the location of a substituent indicated in the IUPAC system?

9. Briefly identify the important distinctions between a common name and an IUPAC name.

Answers

1. An alkane is a molecule; an alkyl group is not an independent molecule but rather a part of a molecule that we consider as a unit.

3.

a. 6

b. 10

5.

a.

b.

c.

7.

a. 2,2,4,4-tetramethylpentane

b. 3-ethylhexane

9. Common names are widely used but not very systematic; IUPAC names identify a parent compound and name other groups as substituents.

1.6 Physical Properties of Alkanes

Learning Objective

1. Identify the physical properties of alkanes and describe trends in these properties.

Because alkanes have relatively predictable physical properties and undergo relatively few chemical reactions other than combustion, they serve as a basis of comparison for the properties of many other organic compound families. Let’s consider their physical properties first.

Table 1.5 "Physical Properties of Some Alkanes" describes some of the properties of some of the first 10 straight-chain alkanes. Because alkane molecules are nonpolar, they are insoluble in water, which is a polar solvent, but are soluble in nonpolar and slightly polar solvents. Consequently, alkanes themselves are commonly used as solvents for organic substances of low polarity, such as fats, oils, and waxes. Nearly all alkanes have densities less than 1.0 g/mL and are therefore less dense than water (the density of H2O is 1.00 g/mL at 20°C). These properties explain why oil and grease do not mix with water but rather float on its surface.

Table 1.5 Physical Properties of Some Alkanes

Molecular Name	Formula	Melting Point (°C)	Boiling Point (°C)	Density (20°C)*	Physical State (at 20°C)
methane	CH4	–182	–164	0.668 g/L	gas
ethane	C2H6	–183	–89	1.265 g/L	gas
propane	C3H8	–190	–42	1.867 g/L	gas
butane	C4H10	–138	–1	2.493 g/L	gas
pentane	C5H12	–130	36	0.626 g/mL	liquid
hexane	C6H14	–95	69	0.659 g/mL	liquid
octane	C8H18	–57	125	0.703 g/mL	liquid
decane	C10H22	–30	174	0.730 g/mL	liquid

*Note the change in units going from gases (grams per liter) to liquids (grams per milliliter). Gas densities are at 1 atm pressure.

Figure 1.5 Oil Spills

Crude oil coats the water’s surface in the Gulf of Mexico after the Deepwater Horizon oil rig sank following an explosion. The leak was a mile below the surface, making it difficult to estimate the size of the spill. One liter of oil can create a slick 2.5 hectares (6.3 acres) in size. This and similar spills provide a reminder that hydrocarbons and water don’t mix.

Source: Photo courtesy of NASA Goddard / MODIS Rapid Response Team, http://www.nasa.gov/topics/earth/features/oilspill/oil-20100519a.html.

Looking Closer: Gas Densities and Fire Hazards

Table 1.5 "Physical Properties of Some Alkanes" indicates that the first four members of the alkane series are gases at ordinary temperatures. Natural gas is composed chiefly of methane, which has a density of about 0.67 g/L. The density of air is about 1.29 g/L. Because natural gas is less dense than air, it rises. When a natural-gas leak is detected and shut off in a room, the gas can be removed by opening an upper window. On the other hand, bottled gas can be either propane (density 1.88 g/L) or butanes (a mixture of butane and isobutane; density about 2.5 g/L). Both are much heavier than air (density 1.2 g/L). If bottled gas escapes into a building, it collects near the floor. This presents a much more serious fire hazard than a natural-gas leak because it is more difficult to rid the room of the heavier gas.

As shown in Table 1.5 "Physical Properties of Some Alkanes", the boiling points of the straight-chain alkanes increase with increasing molar mass. This general rule holds true for the straight-chain homologs of all organic compound families. Larger molecules have greater surface areas and consequently interact more strongly; more energy is therefore required to separate them. For a given molar mass, the boiling points of alkanes are relatively low because these nonpolar molecules have only weak dispersion forces to hold them together in the liquid state.

Looking Closer: An Alkane Basis for Properties of Other Compounds

An understanding of the physical properties of the alkanes is important in that petroleum and natural gas and the many products derived from them—gasoline, bottled gas, solvents, plastics, and more—are composed primarily of alkanes. This understanding is also vital because it is the basis for describing the properties of other organic and biological compound families. For example, large portions of the structures of lipids consist of nonpolar alkyl groups. Lipids include the dietary fats and fatlike compounds called phospholipids and sphingolipids that serve as structural components of living tissues. (For more information about lipids, see Chapter 7 "Lipids".). These compounds have both polar and nonpolar groups, enabling them to bridge the gap between water-soluble and water-insoluble phases. This characteristic is essential for the selective permeability of cell membranes.

Tripalmitin (a), a typical fat molecule, has long hydrocarbon chains typical of most lipids. Compare these chains to hexadecane (b), an alkane with 16 carbon atoms.

Concept Review Exercises

1. Without referring to a table, predict which has a higher boiling point—hexane or octane. Explain.

2. If 25 mL of hexane were added to 100 mL of water in a beaker, which of the following would you expect to happen? Explain.

   a. Hexane would dissolve in water.

   b. Hexane would not dissolve in water and would float on top.

   c. Hexane would not dissolve in water and would sink to the bottom of the container.

Answers

1. octane because of its greater molar mass

2. b; hexane is insoluble in water and less dense than water.

Key Takeaway

• Alkanes are nonpolar compounds that are low boiling and insoluble in water.

Exercises

1. Without referring to a table or other reference, predict which member of each pair has the higher boiling point.

   a. pentane or butane

   b. heptane or nonane

2. For which member of each pair is hexane a good solvent?

   a. pentane or water

   b. sodium chloride or soybean oil

Answer

1. a. pentane

   b. nonane

1.7 Chemical Properties of Alkanes

Learning Objective

1. Identify the main chemical properties of alkanes.

Alkane molecules are nonpolar and therefore generally do not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents. Consider butane as an example:

Neither positive ions nor negative ions are attracted to a nonpolar molecule. In fact, the alkanes undergo so few reactions that they are sometimes called paraffins, from the Latin parum affinis, meaning “little affinity.”

Two important reactions that the alkanes do undergo are combustion and halogenation. Nothing happens when alkanes are merely mixed with oxygen (O2) at room temperature, but when a flame or spark provides the activation energy, a highly exothermic combustion reaction proceeds vigorously. For methane (CH4), the reaction is as follows:

CH4 + 2O2 → CO2 + 2H2O + heat

If the reactants are adequately mixed and there is sufficient oxygen, the only products are carbon dioxide (CO2), water (H2O), and heat—heat for cooking foods, heating homes, and drying clothes. Because conditions are rarely ideal, however, other products are frequently formed. When the oxygen supply is limited, carbon monoxide (CO) is a by-product:

2CH4 + 3O2 → 2CO + 4H2O

This reaction is responsible for dozens of deaths each year from unventilated or improperly adjusted gas heaters. (Similar reactions with similar results occur with kerosene heaters.)

Alkanes also react with the halogens chlorine (Cl2) and bromine (Br2) in the presence of ultraviolet light or at high temperatures to yield chlorinated and brominated alkanes. For example, chlorine reacts with excess methane (CH4) to give methyl chloride (CH3Cl).

CH4 + Cl2 → CH3Cl + HCl

With more chlorine, a mixture of products is obtained: CH3Cl, CH2Cl2, CHCl3, and CCl4.

Fluorine (F2), the lightest halogen, combines explosively with most hydrocarbons. Iodine (I2) is relatively unreactive. Fluorinated and iodinated alkanes are produced by indirect methods. We will discuss the names and uses of halogenated hydrocarbons in Section 1.8 "Halogenated Hydrocarbons".

Concept Review Exercises

1. Why are alkanes sometimes called paraffins?

2. Which halogen reacts most readily with alkanes? Which reacts least readily?

Answers

1. Alkanes do not react with many common chemicals. They are sometimes called paraffins, from the Latin parum affinis, meaning “little affinity.”

2. most readily: F2; least readily: I2

Key Takeaway

• Alkanes react with oxygen (combustion) and with halogens (halogenation).

Exercises

1. Why do alkanes usually not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents?

2. Write an equation for the complete combustion of methane (CH4, the main component of natural gas).

3. What is the most important reaction of alkanes?

4. Name some substances other than oxygen that react readily with alkanes.

Answers

1. Alkanes are nonpolar; they do not attract ions.

3. combustion

1.8 Halogenated Hydrocarbons

Learning Objective

1. Name halogenated hydrocarbons given formulas and write formulas for these compounds given names.

Many organic compounds are closely related to the alkanes. As we noted in Section 1.7 "Chemical Properties of Alkanes", alkanes react with halogens to produce halogenated hydrocarbons, the simplest of which have a single halogen atom substituted for a hydrogen atom of the alkane. Even more closely related are the cycloalkanes, compounds in which the carbon atoms are joined in a ring, or cyclic fashion.

The reactions of alkanes with halogens produce halogenated hydrocarbons, compounds in which one or more hydrogen atoms of a hydrocarbon have been replaced by halogen atoms:

The replacement of only one hydrogen atom gives an alkyl halide (or haloalkane). The common names of alkyl halides consist of two parts: the name of the alkyl group plus the stem of the name of the halogen, with the ending -ide. The IUPAC system uses the name of the parent alkane with a prefix indicating the halogen substituents, preceded by number indicating the substituent’s location. The prefixes are fluoro-, chloro-, bromo-, and iodo-. Thus CH3CH2Cl has the common name ethyl chloride and the IUPAC name chloroethane. Alkyl halides with simple alkyl groups (one to four carbon atoms) are often called by common names. Those with a larger number of carbon atoms are usually given IUPAC names.

Example 3

Give the common and IUPAC names for each compound.

1. CH3CH2CH2Br
2. (CH3)2CHCl

Solution

1. The alkyl group (CH3CH2CH2–) is a propyl group, and the halogen is bromine (Br). The common name is therefore propyl bromide. For the IUPAC name, the prefix for bromine (bromo) is combined with the name for a three-carbon chain (propane), preceded by a number identifying the carbon atom to which the Br atom is attached, so the IUPAC name is 1-bromopropane.
2. The alkyl group [(CH3)2CH–] has three carbon atoms, with a chlorine (Cl) atom attached to the middle carbon atom. The alkyl group is therefore isopropyl, and the common name of the compound is isopropyl chloride. For the IUPAC name, the Cl atom (prefix chloro-) attached to the middle (second) carbon atom of a propane chain results in 2-chloropropane.

Skill-Building Exercise

Give common and IUPAC names for each compound.

1. CH3CH2I

2. CH3CH2CH2CH2F

Example 4

Give the IUPAC name for each compound.

1. 
2. 

Solution

1. The parent alkane has five carbon atoms in the longest continuous chain; it is pentane. A bromo (Br) group is attached to the second carbon atom of the chain. The IUPAC name is 2-bromopentane.
2. The parent alkane is hexane. Methyl (CH3) and bromo (Br) groups are attached to the second and fourth carbon atoms, respectively. Listing the substituents in alphabetical order gives the name 4-bromo-2-methylhexane.

Skill-Building Exercise

Give the IUPAC name for each compound.

1. 
2. 

A wide variety of interesting and often useful compounds have one or more halogen atoms per molecule. For example, methane (CH4) can react with chlorine (Cl2), replacing one, two, three, or all four hydrogen atoms with Cl atoms. Several halogenated products derived from methane and ethane (CH3CH3) are listed in Table 1.6 "Some Halogenated Hydrocarbons", along with some of their uses.

Table 1.6.a Some Halogenated Hydrocarbons Derived from CH4

Formula	Common Name	IUPAC Name	Some Important Uses
CH3Cl	methyl chloride	chloromethane	refrigerant; the manufacture of silicones, methyl cellulose, and synthetic rubber
CH2Cl2	methylene chloride	dichloromethane	laboratory and industrial solvent
CHCl3	chloroform	trichloromethane	industrial solvent
CCl4	carbon tetrachloride	tetrachloromethane	dry-cleaning solvent and fire extinguishers (but no longer recommended for use)
CBrF3	halon-1301	bromotrifluoromethane	fire extinguisher systems
CCl3F	chlorofluorocarbon-11 (CFC-11)	trichlorofluoromethane	foaming plastics
CCl2F2	chlorofluorocarbon-12 (CFC-12)	dichlorodifluoromethane	refrigerant

Table 1.6.b Some Halogenated Hydrocarbons Derived from CH3CH3

Formula	Common Name	IUPAC Name	Some Important Uses
CH3CH2Cl	ethyl chloride	chloroethane	local anesthetic
ClCH2CH2Cl	ethylene dichloride	1,2-dichloroethane	solvent for rubber
CCl3CH3	methylchloroform	1,1,1-trichloroethane	solvent for cleaning computer chips and molds for shaping plastics

To Your Health: Halogenated Hydrocarbons

Once widely used in consumer products, many chlorinated hydrocarbons are suspected carcinogens (cancer-causing substances) and also are known to cause severe liver damage. An example is carbon tetrachloride (CCl4), once used as a dry-cleaning solvent and in fire extinguishers but no longer recommended for either use. Even in small amounts, its vapor can cause serious illness if exposure is prolonged. Moreover, it reacts with water at high temperatures to form deadly phosgene (COCl2) gas, which makes the use of CCl4 in fire extinguishers particularly dangerous.

Ethyl chloride, in contrast, is used as an external local anesthetic. When sprayed on the skin, it evaporates quickly, cooling the area enough to make it insensitive to pain. It can also be used as an emergency general anesthetic.

Bromine-containing compounds are widely used in fire extinguishers and as fire retardants on clothing and other materials. Because they too are toxic and have adverse effects on the environment, scientists are engaged in designing safer substitutes for them, as for many other halogenated compounds.

To Your Health: Chlorofluorocarbons and the Ozone Layer

Alkanes substituted with both fluorine (F) and chlorine (Cl) atoms have been used as the dispersing gases in aerosol cans, as foaming agents for plastics, and as refrigerants. Two of the best known of these chlorofluorocarbons (CFCs) are listed in Table 1.6 "Some Halogenated Hydrocarbons".

Chlorofluorocarbons contribute to the greenhouse effect in the lower atmosphere. They also diffuse into the stratosphere, where they are broken down by ultraviolet (UV) radiation to release Cl atoms. These in turn break down the ozone (O3) molecules that protect Earth from harmful UV radiation. Worldwide action has reduced the use of CFCs and related compounds. The CFCs and other Cl- or bromine (Br)-containing ozone-destroying compounds are being replaced with more benign substances. Hydrofluorocarbons (HFCs), such as CH2FCF3, which have no Cl or Br to form radicals, are one alternative. Another is hydrochlorofluorocarbons (HCFCs), such as CHCl2CF3. HCFC molecules break down more readily in the troposphere, and fewer ozone-destroying molecules reach the stratosphere.

Ozone in the upper atmosphere shields Earth’s surface from UV radiation from the sun, which can cause skin cancer in humans and is also harmful to other animals and to some plants. Ozone “holes” in the upper atmosphere (the gray, pink, and purple areas at the center) are large areas of substantial ozone depletion. They occur mainly over Antarctica from late August through early October and fill in about mid-November. Ozone depletion has also been noted over the Arctic regions. The largest ozone hole ever observed occurred on 24 September 2006.

Source: Image courtesy of NASA, http://ozonewatch.gsfc.nasa.gov/daily.php?date=2006-09-24.

Concept Review Exercises

1. What is the IUPAC name for the HFC that has the formula CH2FCF3? (Hint: you must use a number to indicate the location of each substituent F atom.)

2. What is the IUPAC name for the HCFC that has the formula CHCl2CF3?

Answers

1. 1,1,1,2-tetrafluoroethane

2. 1,1,1-trifluoro-2,2-dichloroethane

Key Takeaway

• The replacement of an hydrogen atom on an alkane by a halogen atom—F, Cl, Br, or I—forms a halogenated compound.

Exercises

1. Write the condensed structural formula for each compound.

   a. methyl chloride

   b. chloroform

2. Write the condensed structural formula for each compound.

   a. ethyl bromide

   b. carbon tetrachloride

3. Write the condensed structural formulas for the two isomers that have the molecular formula C3H7Br. Give the common name and the IUPAC name of each.

4. Write the condensed structural formulas for the four isomers that have the molecular formula C4H9Br. Give the IUPAC name of each.

5. What is a CFC? How are CFCs involved in the destruction of the ozone layer?

6. Explain why each compound is less destructive to the ozone layer than are CFCs.

   a. fluorocarbons

   b. HCFCs

Answers

1.

a. CH3Cl

b. CHCl3

3. CH3CH2CH2Br, propyl bromide, 1-bromopropane; CH3CHBrCH3, isopropyl bromide, 2-bromopropane

5. compounds containing Cl, F, and C; by releasing Cl atoms in the stratosphere

1.9 Cycloalkanes

Learning Objective

1. Name cycloalkanes given their formulas and write formulas for these compounds given their names.

The hydrocarbons we have encountered so far have been composed of molecules with open-ended chains of carbon atoms. When a chain contains three or more carbon atoms, the atoms can join to form ring or cyclic structures. The simplest of these cyclic hydrocarbons has the formula C3H6. Each carbon atom has two hydrogen atoms attached (Figure 1.6 "Ball-and-Spring Model of Cyclopropane") and is called cyclopropane.

Figure 1.6 Ball-and-Spring Model of Cyclopropane

The springs are bent to join the carbon atoms.

To Your Health: Cyclopropane as an Anesthetic

With its boiling point of −33°C, cyclopropane is a gas at room temperature. It is also a potent, quick-acting anesthetic with few undesirable side effects in the body. It is no longer used in surgery, however, because it forms explosive mixtures with air at nearly all concentrations.

The cycloalkanes—cyclic hydrocarbons with only single bonds—are named by adding the prefix cyclo- to the name of the open-chain compound having the same number of carbon atoms as there are in the ring. Thus the name for the cyclic compound C4H8 is cyclobutane. The carbon atoms in cyclic compounds can be represented by line-angle formulas that result in regular geometric figures. Keep in mind, however, that each corner of the geometric figure represents a carbon atom plus as many hydrogen atoms as needed to give each carbon atom four bonds.

Some cyclic compounds have substituent groups attached. Example 5 interprets the name of a cycloalkane with a single substituent group.

Example 5

Draw the structure for each compound.

1. cyclopentane
2. methylcyclobutane

Solution

1. The name cyclopentane indicates a cyclic (cyclo) alkane with five (pent-) carbon atoms. It can be represented as a pentagon.

   

2. The name methylcyclobutane indicates a cyclic alkane with four (but-) carbon atoms in the cyclic part. It can be represented as a square with a CH3 group attached.

   

Skill-Building Exercise

1. Draw the structure for each compound.

   a. cycloheptane

   b. ethylcyclohexane

The properties of cyclic hydrocarbons are generally quite similar to those of the corresponding open-chain compounds. So cycloalkanes (with the exception of cyclopropane, which has a highly strained ring) act very much like noncyclic alkanes. Cyclic structures containing five or six carbon atoms, such as cyclopentane and cyclohexane, are particularly stable. We will see in Chapter 6 "Carbohydrates" that some carbohydrates (sugars) form five- or six-membered rings in solution.

Note

The cyclopropane ring is strained because the C–C–C angles are 60°, and the preferred (tetrahedral) bond angle is 109.5°. (This strain is readily evident when you try to build a ball-and-stick model of cyclopropane; see Figure 1.6 "Ball-and-Spring Model of Cyclopropane".) Cyclopentane and cyclohexane rings have little strain because the C–C–C angles are near the preferred angles.

Concept Review Exercises

1. What is the molecular formula of cyclooctane?

2. What is the IUPAC name for this compound?

   

Answers

1. C8H16

2. ethylcyclopropane

Key Takeaway

• Many organic compounds have cyclic structures.

Exercises

1. Draw the structure for each compound.

   a. ethylcyclobutane

   b. propylcyclopropane

2. Draw the structure for each compound.

   a. methylcyclohexane

   b. butylcyclobutane

3. Cycloalkyl groups can be derived from cycloalkanes in the same way that alkyl groups are derived from alkanes. These groups are named as cyclopropyl, cyclobutyl, and so on. Name each cycloalkyl halide.

   a.

   b.

4. Halogenated cycloalkanes can be named by the IUPAC system. As with alkyl derivatives, monosubstituted derivatives need no number to indicate the position of the halogen. To name disubstituted derivatives, the carbon atoms are numbered starting at the position of one substituent (C1) and proceeding to the second substituted atom by the shortest route. Name each compound.

   a.

   b.

Answers

1.

a.

b.

3.

a. cyclopentyl bromide

b. cyclohexyl chloride

1.10 End-of-Unit Material

Unit Summary

To ensure that you understand the material in this unit, you should review the meanings of the following bold terms in the summary and ask yourself how they relate to the topics in the unit.

Organic chemistry is the chemistry of carbon compounds, and inorganic chemistry is the chemistry of all the other elements. Carbon atoms can form stable covalent bonds with other carbon atoms and with atoms of other elements, and this property allows the formation the tens of millions of organic compounds. Hydrocarbons contain only hydrogen and carbon atoms.

Hydrocarbons in which each carbon atom is bonded to four other atoms are called alkanes or saturated hydrocarbons. They have the general formula CnH2n + 2. Any given alkane differs from the next one in a series by a CH2 unit. Any family of compounds in which adjacent members differ from each other by a definite factor is called a homologous series.

Carbon atoms in alkanes can form straight chains or branched chains. Two or more compounds having the same molecular formula but different structural formulas are isomers of each other. There are no isomeric forms for the three smallest alkanes; beginning with C4H10, all other alkanes have isomeric forms.

A structural formula shows all the carbon and hydrogen atoms and how they are attached to one another. A condensed structural formula shows the hydrogen atoms right next to the carbon atoms to which they are attached. A line-angle formula is a formula in which carbon atoms are implied at the corners and ends of lines. Each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds.

The IUPAC System of Nomenclature provides rules for naming organic compounds. An alkyl group is a unit formed by removing one hydrogen atom from an alkane.

The physical properties of alkanes reflect the fact that alkane molecules are nonpolar. Alkanes are insoluble in water and less dense than water.

Alkanes are generally unreactive toward laboratory acids, bases, oxidizing agents, and reducing agents. They do burn (undergo combustion reactions).

Alkanes react with halogens by substituting one or more halogen atoms for hydrogen atoms to form halogenated hydrocarbons. An alkyl halide (haloalkane) is a compound resulting from the replacement of a hydrogen atom of an alkane with a halogen atom.

Cycloalkanes are hydrocarbons whose molecules are closed rings rather than straight or branched chains. A cyclic hydrocarbon is a hydrocarbon with a ring of carbon atoms.

Additional Exercises

1. You find an unlabeled jar containing a solid that melts at 48°C. It ignites readily and burns readily. The substance is insoluble in water and floats on the surface. Is the substance likely to be organic or inorganic?

2. Give the molecular formulas for methylcyclopentane, 2-methylpentane, and cyclohexane. Which are isomers?

3. What is wrong with each name? (Hint: first write the structure as if it were correct.) Give the correct name for each compound.

   a. 2-dimethylpropane

   b. 2,3,3-trimethylbutane

   c. 2,4-diethylpentane

   d. 3,4-dimethyl-5-propylhexane

4. What is the danger in swallowing a liquid alkane?

5. Distinguish between lighter and heavier liquid alkanes in terms of their effects on the skin.

6. Following is the line formula for an alkane. Draw its structure and give its name.

   

7. Write equations for the complete combustion of each compound.

   a. propane (a bottled gas fuel)

   b. octane (a typical hydrocarbon in gasoline).

8. The density of a gasoline sample is 0.690 g/mL. On the basis of the complete combustion of octane, calculate the amount in grams of carbon dioxide (CO2) and water (H2O) formed per gallon (3.78 L) of the gasoline when used in an automobile.

9. Draw the structures for the five isomeric hexanes (C6H14). Name each by the IUPAC system.

10. Indicate whether the structures in each set represent the same compound or isomers.

    a.

    b.

    c.

11. Consider the line-angle formulas shown here and answer the questions.

    a. Which pair of formulas represents isomers? Draw each structure. b. Which formula represents an alkyl halide? Name the compound and write its condensed structural formula. c. Which formula represents a cyclic alkane? Name the compound and draw its structure. d. What is the molecular formula of the compound represented by (i)?

Answers

1. organic

3.

a. Two numbers are needed to indicate two substituents; 2,2-dimethylpropane.

b. The lowest possible numbers were not used; 2,2,3-trimethylbutane.

c. An ethyl substituent is not possible on the second carbon atom; 3,5-dimethylheptane.

d. A propyl substituent is not possible on the fifth carbon atom; 3,4,5-trimethyloctane.

5. Lighter alkanes wash away protective skin oils; heavier alkanes form a protective layer.

7.

a. C3H8 + 5O2 → 3CO2 + 4H2O

b. 2C8H18 + 25O2 → 16CO2 + 18H2O

9. CH3CH2CH2CH2CH2CH3; hexane

11.

a. ii and iii; CH3CH2CH2CH2CH2CH2CH3 and

b. iv; 3-chloropentane; CH3CH2CHClCH2CH3 c. i; ethylcyclopentane; d. C7H14

Opening Essay

Our modern society is based to a large degree on the chemicals we discuss in this chapter. Most are made from petroleum. Earlier in this chapter we noted that alkanes—saturated hydrocarbons—have relatively few important chemical properties other than that they undergo combustion and react with halogens. Unsaturated hydrocarbons—hydrocarbons with double or triple bonds—on the other hand, are quite reactive. In fact, they serve as building blocks for many familiar plastics—polyethylene, vinyl plastics, acrylics—and other important synthetic materials (e.g., alcohols, antifreeze, and detergents). Aromatic hydrocarbons have formulas that can be drawn as cyclic alkenes, making them appear unsaturated, but their structure and properties are generally quite different, so they are not considered to be alkenes. Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics (e.g., polyesters and polystyrene).

The two simplest unsaturated compounds—ethylene (ethene) and acetylene (ethyne)—were once used as anesthetics and were introduced to the medical field in 1924. However, it was discovered that acetylene forms explosive mixtures with air, so its medical use was abandoned in 1925. Ethylene was thought to be safer, but it too was implicated in numerous lethal fires and explosions during anesthesia. Even so, it remained an important anesthetic into the 1960s, when it was replaced by nonflammable anesthetics such as halothane (CHBrClCF3).

1.11 Alkenes: Structures and Names

Learning Objective

1. Name alkenes given formulas and write formulas for alkenes given names.

As we know, alkenes are hydrocarbons with carbon-to-carbon double bonds (R2C=CR2) and alkynes are hydrocarbons with carbon-to-carbon triple bonds (R–C≡C–R). Collectively, they are called unsaturated hydrocarbons because they have fewer hydrogen atoms than does an alkane with the same number of carbon atoms, as is indicated in the following general formulas:

Some representative alkenes—their names, structures, and physical properties—are given in Table 1.7 "Physical Properties of Some Selected Alkenes".

Table 1.7 Physical Properties of Some Selected Alkenes

IUPAC Name	Molecular Formula	Condensed Structural Formula	Melting Point (°C)	Boiling Point (°C)
ethene	C2H4	CH2=CH2	-169	-104
propene	C3H6	CH2=CHCH3	-185	-47
1-butene	C4H8	CH2=CHCH2CH3	-185	-6
1-pentene	C5H10	CH2=CH(CH2)2CH3	-138	30
1-hexene	C6H12	CH2=CH(CH2)3CH3	-140	63
1-heptene	C7H14	CH2=CH(CH2)4CH3	-119	94
1-octene	C8H16	CH2=CH(CH2)5CH3	-102	121

We used only condensed structural formulas in Table 1.7 "Physical Properties of Some Selected Alkenes". Thus, CH2=CH2 stands for

The double bond is shared by the two carbon atoms and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious. Note that the molecular formula for ethene is C2H4, whereas that for ethane is C2H6.

The first two alkenes in Table 1.7 "Physical Properties of Some Selected Alkenes"—ethene and propene (Figure 1.7 "Ethene and Propene")—are most often called by their common names—ethylene and propylene, respectively. Ethylene is a major commercial chemical. The US chemical industry produces about 25 billion kilograms of ethylene annually, more than any other synthetic organic chemical. More than half of this ethylene goes into the manufacture of polyethylene, one of the most familiar plastics. (For more information about polymers and plastics, see Section 1.15 "Polymers".) Propylene is also an important industrial chemical. It is converted to plastics, isopropyl alcohol, and a variety of other products. (For more information about alcohols, see Chapter 2 "Organic Compounds of Oxygen", Section 2.2 "Alcohols: Nomenclature and Classification".)

Figure 1.7 Ethene and Propene

The ball-and-spring models of ethene/ethylene (a) and propene/propylene (b) show their respective shapes, especially bond angles.

Note

Although there is only one alkene with the formula C2H4 (ethene) and only one with the formula C3H6 (propene), there are several alkenes with the formula C4H8. Section 1.12 "Cis-Trans Isomers (Geometric Isomers)" begins a discussion of butenes.

Here are some basic rules for naming alkenes from the International Union of Pure and Applied Chemistry (IUPAC):

1. The longest chain of carbon atoms containing the double bond is considered the parent chain. It is named using the same stem as the alkane having the same number of carbon atoms but ends in -ene to identify it as an alkene. Thus the compound CH2=CHCH3 is propene.
2. If there are four or more carbon atoms in a chain, we must indicate the position of the double bond. The carbons atoms are numbered so that the first of the two that are doubly bonded is given the lower of the two possible numbers. The compound CH3CH=CHCH2CH3, for example, has the double bond between the second and third carbon atoms. Its name is 2-pentene (not 3-pentene).

3. Substituent groups are named as with alkanes, and their position is indicated by a number. Thus,

   

   is 5-methyl-2-hexene. Note that the numbering of the parent chain is always done in such a way as to give the double bond the lowest number, even if that causes a substituent to have a higher number. The double bond always has priority in numbering.

Example 1

Name each compound.

1. 
2. 

Solution

1. The longest chain containing the double bond has five carbon atoms, so the compound is a pentene (rule 1). To give the first carbon atom of the double bond the lowest number (rule 2), we number from the left, so the compound is a 2-pentene. There is a methyl group on the fourth carbon atom (rule 3), so the compound’s name is 4-methyl-2-pentene.
2. The longest chain containing the double bond has four carbon atoms, so the parent compound is a butene (rule 1). (The longest chain overall has five carbon atoms, but it does not contain the double bond, so the parent name is not pentene.) To give the first carbon atom of the double bond the lowest number (rule 2), we number from the left, so the compound is a 1-butene. There is an ethyl group on the second carbon atom (rule 3), so the compound’s name is 2-ethyl-1-butene.

Skill-Building Exercise

Name each compound.

1. CH3CH2CH2CH2CH2CH=CHCH3

2. 

Just as there are cycloalkanes, there are cycloalkenes. These compounds are named like alkenes, but with the prefix cyclo- attached to the beginning of the parent alkene name.

Example 2

Draw the structure for each compound.

1. 3-methyl-2-pentene
2. cyclohexene

Solution

1. First write the parent chain of five carbon atoms: C–C–C–C–C. Then add the double bond between the second and third carbon atoms:

   

   Now place the methyl group on the third carbon atom and add enough hydrogen atoms to give each carbon atom a total of four bonds.

   

2. First, consider what each of the three parts of the name means. Cyclo means a ring compound, hex means 6 carbon atoms, and -ene means a double bond.

   

Skill-Building Exercise

Draw the structure for each compound.

1. 2-ethyl-1-hexene

2. cyclopentene

Concept Review Exercises

1. Briefly identify the important distinctions between a saturated hydrocarbon and an unsaturated hydrocarbon.

2. Briefly identify the important distinctions between an alkene and an alkane.

3. Classify each compound as saturated or unsaturated. Identify each as an alkane, an alkene, or an alkyne.

   a.

   b. CH3CH2C≡CCH3

   c.

Answers

1. Unsaturated hydrocarbons have double or triple bonds and are quite reactive; saturated hydrocarbons have only single bonds and are rather unreactive.

2. An alkene has a double bond; an alkane has single bonds only.

3. a. saturated; alkane

   b. unsaturated; alkyne

   c. unsaturated; alkene

Key Takeaway

• Alkenes are hydrocarbons with a carbon-to-carbon double bond.

Exercises

1. Draw the structure for each compound.

   a. 2-methyl-2-pentene

   b. 2,3-dimethyl-1-butene

   c. cyclohexene

2. Draw the structure for each compound.

   a. 5-methyl-1-hexene

   b. 3-ethyl-2-pentene

   c. 4-methyl-2-hexene

3. Name each compound according to the IUPAC system.

   a.

   b.

   c.

4. Name each compound according to the IUPAC system.

   a.

   b.

   c.

Answers

1.

a.

b.

c.

3.

a. 2-methyl-2-pentene

b. 3-methyl-2-heptene

c. 2,5-dimethyl-2-hexene

1.12 Cis-Trans Isomers (Geometric Isomers)

Learning Objectives

1. Recognize that alkenes that can exist as cis-trans isomers.
2. Classify isomers as cis or trans.
3. Draw structures for cis-trans isomers given their names.

As noted in earlier in this chapter, there is free rotation about the carbon-to-carbon single bonds (C–C) in alkanes. In contrast, the structure of alkenes requires that the carbon atoms of a double bond and the two atoms bonded to each carbon atom all lie in a single plane, and that each doubly bonded carbon atom lies in the center of a triangle. This part of the molecule’s structure is rigid; rotation about doubly bonded carbon atoms is not possible without rupturing the bond. Look at the two chlorinated hydrocarbons in Figure 1.8 "Rotation about Bonds".

Figure 1.8 Rotation about Bonds

In 1,2-dichloroethane (a), free rotation about the C–C bond allows the two structures to be interconverted by a twist of one end relative to the other. In 1,2-dichloroethene (b), restricted rotation about the double bond means that the relative positions of substituent groups above or below the double bond are significant.

In 1,2-dichloroethane (part (a) of Figure 1.8 "Rotation about Bonds"), there is free rotation about the C–C bond. The two models shown represent exactly the same molecule; they are not isomers. You can draw structural formulas that look different, but if you bear in mind the possibility of this free rotation about single bonds, you should recognize that these two structures represent the same molecule:

In 1,2-dichloroethene (part (b) of Figure 1.8 "Rotation about Bonds"), however, restricted rotation about the double bond means that the relative positions of substituent groups above or below the double bond become significant. This leads to a special kind of isomerism. The isomer in which the two chlorine (Cl) atoms lie on the same side of the molecule is called the cis isomer (Latin cis, meaning “on this side”) and is named cis-1,2-dichloroethene. The isomer with the two Cl atoms on opposite sides of the molecule is the trans isomer (Latin trans, meaning “across”) and is named trans-1,2-dichloroethene. These two compounds are cis-trans isomers (or geometric isomers), compounds that have different configurations (groups permanently in different places in space) because of the presence of a rigid structure in their molecule.

Consider the alkene with the condensed structural formula CH3CH=CHCH3. We could name it 2-butene, but there are actually two such compounds; the double bond results in cis-trans isomerism (Figure 1.9 "Ball-and-Spring Models of (a) Cis-2-Butene and (b) Trans-2-Butene").

Figure 1.9 Ball-and-Spring Models of (a) Cis-2-Butene and (b) Trans-2-Butene

Cis-trans isomers have different physical, chemical, and physiological properties.

Cis-2-butene has both methyl groups on the same side of the molecule. Trans-2-butene has the methyl groups on opposite sides of the molecule. Their structural formulas are as follows:

Note, however, that the presence of a double bond does not necessarily lead to cis-trans isomerism. We can draw two seemingly different propenes:

However, these two structures are not really different from each other. If you could pick up either molecule from the page and flip it over top to bottom, you would see that the two formulas are identical.

Thus there are two requirements for cis-trans isomerism:

1. Rotation must be restricted in the molecule.
2. There must be two nonidentical groups on each doubly bonded carbon atom.

In these propene structures, the second requirement for cis-trans isomerism is not fulfilled. One of the doubly bonded carbon atoms does have two different groups attached, but the rules require that both carbon atoms have two different groups.

In general, the following statements hold true in cis-trans isomerism:

• Alkenes with a C=CH2 unit do not exist as cis-trans isomers.
• Alkenes with a C=CR2 unit, where the two R groups are the same, do not exist as cis-trans isomers.
• Alkenes of the type R–CH=CH–R can exist as cis and trans isomers; cis if the two R groups are on the same side of the carbon-to-carbon double bond, and trans if the two R groups are on opposite sides of the carbon-to-carbon double bond.

Cis-trans isomerism also occurs in cyclic compounds. In ring structures, groups are unable to rotate about any of the ring carbon–carbon bonds. Therefore, groups can be either on the same side of the ring (cis) or on opposite sides of the ring (trans). For our purposes here, we represent all cycloalkanes as planar structures, and we indicate the positions of the groups, either above or below the plane of the ring.

Example 3

Which compounds can exist as cis-trans (geometric) isomers? Draw them.

1. CHCl=CHBr
2. CH2=CBrCH3
3. (CH3)2C=CHCH2CH3
4. CH3CH=CHCH2CH3

Solution

All four structures have a double bond and thus meet rule 1 for cis-trans isomerism.

1. This compound meets rule 2; it has two nonidentical groups on each carbon atom (H and Cl on one and H and Br on the other). It exists as both cis and trans isomers:

   
2. This compound has two hydrogen atoms on one of its doubly bonded carbon atoms; it fails rule 2 and does not exist as cis and trans isomers.
3. This compound has two methyl (CH3) groups on one of its doubly bonded carbon atoms. It fails rule 2 and does not exist as cis and trans isomers.

4. This compound meets rule 2; it has two nonidentical groups on each carbon atom and exists as both cis and trans isomers:

   

Skill-Building Exercise

1. Which compounds can exist as cis-trans isomers? Draw them.

a. CH2=CHCH2CH2CH3

b. CH3CH=CHCH2CH3

c. CH3CH2CH=CHCH2CH3

d.

e.

Concept Review Exercises

1. What are cis-trans (geometric) isomers? What two types of compounds can exhibit cis-trans isomerism?

2. Classify each compound as a cis isomer, a trans isomer, or neither.

   a.

   b.

   c.

   d.

Answers

1. Cis-trans isomers are compounds that have different configurations (groups permanently in different places in space) because of the presence of a rigid structure in their molecule. Alkenes and cyclic compounds can exhibit cis-trans isomerism.

2. a. trans

   b. cis

   c. cis

   d. neither

Key Takeaway

• Cis-trans (geometric) isomerism exists when there is restricted rotation in a molecule and there are two nonidentical groups on each doubly bonded carbon atom.

Exercises

1. Draw the structures of the cis-trans isomers for each compound. Label them cis and trans. If no cis-trans isomers exist, write none.

   a. 2-bromo-2-pentene

   b. 3-heptene

   c. 4-methyl-2-pentene

   d. 1,1-dibromo-1-butene

   e. 2-butenoic acid (CH3CH=CHCOOH)

2. Draw the structures of the cis-trans isomers for each compound. Label them cis and trans. If no cis-trans isomers exist, write none.

   a. 2,3-dimethyl-2-pentene

   b. 1,1-dimethyl-2-ethylcyclopropane

   c. 1,2-dimethylcyclohexane

   d. 5-methyl-2-hexene

   e. 1,2,3-trimethylcyclopropane

Answer

1. a.

   b.

   c. none

   d. none

   e.

1.13 Physical Properties of Alkenes

Learning Objective

1. Identify the physical properties of alkenes and describe trends in these properties.

The physical properties of alkenes are similar to those of the alkanes. Table 1.7 "Physical Properties of Some Selected Alkenes" (in Section 1.11 "Alkenes: Structures and Names") shows that the boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. For molecules with the same number of carbon atoms and the same general shape, the boiling points usually differ only slightly, just as we would expect for substances whose molar mass differs by only 2 u (equivalent to two hydrogen atoms). Like other hydrocarbons, the alkenes are insoluble in water but soluble in organic solvents.

Looking Closer: Environmental Note

Alkenes occur widely in nature. Ripening fruits and vegetables give off ethylene, which triggers further ripening. Fruit processors artificially introduce ethylene to hasten the ripening process; exposure to as little as 0.1 mg of ethylene for 24 h can ripen 1 kg of tomatoes. Unfortunately, this process does not exactly duplicate the ripening process, and tomatoes picked green and treated this way don’t taste much like vine-ripened tomatoes fresh from the garden.

Other alkenes that occur in nature include 1-octene, a constituent of lemon oil, and octadecene (C18H36) found in fish liver. Dienes (two double bonds) and polyenes (three or more double bonds) are also common. Butadiene (CH2=CHCH=CH2) is found in coffee. Lycopene and the carotenes are isomeric polyenes (C40H56) that give the attractive red, orange, and yellow colors to watermelons, tomatoes, carrots, and other fruits and vegetables. Vitamin A, essential to good vision, is derived from a carotene. The world would be a much less colorful place without alkenes.

Concept Review Exercises

1. Briefly describe the physical properties of alkenes. How do these properties compare to those of the alkanes?

2. Without consulting tables, arrange the following alkenes in order of increasing boiling point: 1-butene, ethene, 1-hexene, and propene.

Answers

1. Alkenes have physical properties (low boiling points, insoluble in water) quite similar to those of their corresponding alkanes.

2. ethene < propene < 1-butene < 1-hexene

Key Takeaway

• The physical properties of alkenes are much like those of the alkanes: their boiling points increase with increasing molar mass, and they are insoluble in water.

Exercises

1. Without referring to a table or other reference, predict which member of each pair has the higher boiling point.

   a. 1-pentene or 1-butene

   b. 3-heptene or 3-nonene

2. Which is a good solvent for cyclohexene, pentane or water?

Answer

1. a. 1-pentene

   b. 3-nonene

1.14 Chemical Properties of Alkenes

Learning Objective

1. Write equations for the addition reactions of alkenes with hydrogen, halogens, and water.

Alkenes are valued mainly for addition reactions, in which one of the bonds in the double bond is broken. Each of the carbon atoms in the bond can then attach another atom or group while remaining joined to each other by a single bond.

Perhaps the simplest addition reaction is hydrogenation—a reaction with hydrogen (H2) in the presence of a catalyst such as nickel (Ni) or platinum (Pt).

The product is an alkane having the same carbon skeleton as the alkene. (The use of hydrogenation to convert unsaturated vegetable oils to saturated fats is discussed in Chapter 7 "Lipids", Section 7.2 "Fats and Oils".)

Alkenes also readily undergo halogenation—the addition of halogens. Indeed, the reaction with bromine (Br2) can be used to test for alkenes. Bromine solutions are brownish red. When we add a Br2 solution to an alkene, the color of the solution disappears because the alkene reacts with the bromine:

Another important addition reaction is that between an alkene and water to form an alcohol. This reaction, called hydration, requires a catalyst—usually a strong acid, such as sulfuric acid (H2SO4):

The hydration reaction is discussed further in Chapter 2 "Alcohols, Phenols, Thiols, Ethers"", Section 2.4 "Reactions That Form Alcohols", where we deal with this reaction in the synthesis of alcohols.

Example 4

Write the equation for the reaction between CH3CH=CHCH3 and each substance.

1. H2 (Ni catalyst)
2. Br2
3. H2O (H2SO4 catalyst)

Solution

In each reaction, the reagent adds across the double bond.

1.

2. 3.

Skill-Building Exercise

Write the equation for each reaction.

1. CH3CH2CH=CH2 with H2 (Ni catalyst)

2. CH3CH=CH2 with Cl2

3. CH3CH2CH=CHCH2CH3 with H2O (H2SO4 catalyst)

Concept Review Exercises

1. What is the principal difference in properties between alkenes and alkanes? How are they alike?

2. If C12H24 reacts with HBr in an addition reaction, what is the molecular formula of the product?

Answers

1. Alkenes undergo addition reactions; alkanes do not. Both burn.

2. C12H24Br2

Key Takeaway

• Alkenes undergo addition reactions, adding such substances as hydrogen, bromine, and water across the carbon-to-carbon double bond.

Exercises

1. Complete each equation.

   a. (CH3) 2C=CH2 + Br2 →

   b.

   c.

2. Complete each equation.

   a. CH2=CHCH=CH2 + 2H2→Ni

   b. (CH3)2C=C(CH3)2 + H2O →H2SO4

   c.

Answer

1. a. (CH3)2CBrCH2Br

   b. CH3CH(CH3)CH2CH3

   c.

1.15 Polymers

Learning Objective

1. Draw structures for monomers that can undergo addition polymerization and for four-monomer-unit sections of an addition polymer.

The most important commercial reactions of alkenes are polymerizations, reactions in which small molecules, referred to in general as monomers (from the Greek monos, meaning “one,” and meros, meaning “parts”), are assembled into giant molecules referred to as polymers (from the Greek poly, meaning “many,” and meros, meaning “parts”). A polymer is as different from its monomer as a long strand of spaghetti is from a tiny speck of flour. For example, polyethylene, the familiar waxy material used to make plastic bags, is made from the monomer ethylene—a gas.

The Production of Polyethylene

There are two general types of polymerization reactions: addition polymerization and condensation polymerization. (For more information about condensation polymerization, see Chapter 4 "Organic Acids and Bases and Some of Their Derivatives", Section 4.8 "Preparation of Esters".) In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers. Ethylene molecules are joined together in long chains. The polymerization can be represented by the reaction of a few monomer units:

The bond lines extending at the ends in the formula of the product indicate that the structure extends for many units in each direction. Notice that all the atoms—two carbon atoms and four hydrogen atoms—of each monomer molecule are incorporated into the polymer structure. Because displays such as the one above are cumbersome, the polymerization is often abbreviated as follows:

nCH2=CH2→ [ CH2CH2 ] n

Note

Many natural materials—such as proteins, cellulose and starch, and complex silicate minerals—are polymers. (For more information about proteins and cellulose/starch, see Chapter 9 "Proteins, and Enzymes", Section 9.1 "Proteins", and Chapter 6 "Carbohydrates", Section 6.7 "Polysaccharides", respectively.) Artificial fibers, films, plastics, semisolid resins, and rubbers are also polymers. More than half the compounds produced by the chemical industry are synthetic polymers.

Some common addition polymers are listed in Table 1.8 "Some Addition Polymers". Note that all the monomers have carbon-to-carbon double bonds. Many polymers are mundane (e.g., plastic bags, food wrap, toys, and tableware), but there are also polymers that conduct electricity, have amazing adhesive properties, or are stronger than steel but much lighter in weight.

Table 1.8 Some Addition Polymers

Monomer	Polymer	Polymer Name	Some Uses
CH2=CH2	~CH2CH2CH2CH2CH2CH2~	polyethylene	plastic bags, bottles, toys, electrical insulation
CH2=CHCH3		polypropylene	carpeting, bottles, luggage, exercise clothing
CH2=CHCl		polyvinyl chloride	bags for intravenous solutions, pipes, tubing, floor coverings
CF2=CF2	~CF2CF2CF2CF2CF2CF2~	polytetrafluoroethylene	nonstick coatings, electrical insulation

Medical Uses of Polymers

An interesting use of polymers is the replacement of diseased, worn out, or missing parts in the body. For example, about a 250,000 hip joints and 500,000 knees are replaced in US hospitals each year. The artificial ball-and-socket hip joints are made of a special steel (the ball) and plastic (the socket). People crippled by arthritis or injuries gain freedom of movement and relief from pain. Patients with heart and circulatory problems can be helped by replacing worn out heart valves with parts based on synthetic polymers. These are only a few of the many biomedical uses of polymers.

Figure 1.10 Hip Joint Replacement

Synthetic polymers are an important part of a hip joint replacement. The hip is much like a ball-and-socket joint, and total hip replacements mimic this with a metal ball that fits in a plastic cup.

Concept Review Exercises

1. What is a monomer? What is a polymer? How do polymer molecules differ from the molecules we have discussed in earlier sections of this chapter?

2. What is addition polymerization? What structural feature usually characterizes molecules used as monomers in addition polymerization?

3. What is the molecular formula of a polymer molecule formed by the addition polymerization of 175 molecules of vinyl chloride (CH2=CHCl)?

Answers

1. Monomers are small molecules that can be assembled into giant molecules referred to as polymers, which are much larger than the molecules we discussed earlier in this chapter.

2. In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers.

3. C350H525Cl175

Key Takeaway

• Molecules having carbon-to-carbon double bonds can undergo addition polymerization.

Exercises

1. Write the condensed structural formula of the monomer from which Saran is formed. A segment of the Saran molecule has the following structure: CH2CCl2CH2CCl2CH2CCl2CH2CCl2.

2. Write the condensed structural formula for the section of a molecule formed from four units of the monomer CH2=CHF.

Answer

1. H2C=CCl2

1.16 Alkynes

Learning Objectives

1. Describe the general physical and chemical properties of alkynes.
2. Name alkynes given formulas and write formulas for alkynes given names.

The simplest alkyne—a hydrocarbon with carbon-to-carbon triple bond—has the molecular formula C2H2 and is known by its common name—acetylene (Figure 1.11 "Ball-and-Spring Model of Acetylene"). Its structure is H–C≡C–H.

Figure 1.11 Ball-and-Spring Model of Acetylene

Acetylene (ethyne) is the simplest member of the alkyne family.

Note

Acetylene is used in oxyacetylene torches for cutting and welding metals. The flame from such a torch can be very hot. Most acetylene, however, is converted to chemical intermediates that are used to make vinyl and acrylic plastics, fibers, resins, and a variety of other products.

Alkynes are similar to alkenes in both physical and chemical properties. For example, alkynes undergo many of the typical addition reactions of alkenes. The International Union of Pure and Applied Chemistry (IUPAC) names for alkynes parallel those of alkenes, except that the family ending is -yne rather than -ene. The IUPAC name for acetylene is ethyne. The names of other alkynes are illustrated in the following exercises.

Concept Review Exercises

1. Briefly identify the important differences between an alkene and an alkyne. How are they similar?

2. The alkene (CH3)2CHCH2CH=CH2 is named 4-methyl-1-pentene. What is the name of (CH3)2CHCH2C≡CH?

3. Do alkynes show cis-trans isomerism? Explain.

Answers

1. Alkenes have double bonds; alkynes have triple bonds. Both undergo addition reactions.

2. 4-methyl-1-pentyne

3. No; a triply bonded carbon atom can form only one other bond. It would have to have two groups attached to show cis-trans isomerism.

Key Takeaway

• Alkynes are hydrocarbons with carbon-to-carbon triple bonds and properties much like those of alkenes.

Exercises

1. Draw the structure for each compound.

   a. acetylene

   b. 3-methyl-1-hexyne

2. Draw the structure for each compound.

   a. 4-methyl-2-hexyne

   b. 3-octyne

3. Name each alkyne.

   a. CH3CH2CH2C≡CH

   b. CH3CH2CH2C≡CCH3

Answers

1. a. H–C≡C–H

   b.

3.

a. 1-pentyne

b. 2-hexyne

1.17 Aromatic Compounds: Benzene

Learning Objective

1. Describe the bonding in benzene and the way typical reactions of benzene differ from those of the alkenes.

Next we consider a class of hydrocarbons with molecular formulas like those of unsaturated hydrocarbons, but which, unlike the alkenes, do not readily undergo addition reactions. These compounds comprise a distinct class, called aromatic hydrocarbons, with unique structures and properties. We start with the simplest of these compounds. Benzene (C6H6) is of great commercial importance, but it also has noteworthy health effects (see “To Your Health: Benzene and Us”).

The formula C6H6 seems to indicate that benzene has a high degree of unsaturation. (Hexane, the saturated hydrocarbon with six carbon atoms has the formula C6H14—eight more hydrogen atoms than benzene.) However, despite the seeming low level of saturation, benzene is rather unreactive. It does not, for example, react readily with bromine, which, as mentioned in Section 1.11 "Alkenes: Structures and Names", is a test for unsaturation.

Note

Benzene is a liquid that smells like gasoline, boils at 80°C, and freezes at 5.5°C. It is the aromatic hydrocarbon produced in the largest volume. It was formerly used to decaffeinate coffee and was a significant component of many consumer products, such as paint strippers, rubber cements, and home dry-cleaning spot removers. It was removed from many product formulations in the 1950s, but others continued to use benzene in products until the 1970s when it was associated with leukemia deaths. Benzene is still important in industry as a precursor in the production of plastics (such as Styrofoam and nylon), drugs, detergents, synthetic rubber, pesticides, and dyes. It is used as a solvent for such things as cleaning and maintaining printing equipment and for adhesives such as those used to attach soles to shoes. Benzene is a natural constituent of petroleum products, but because it is a known carcinogen, its use as an additive in gasoline is now limited.

To explain the surprising properties of benzene, chemists suppose the molecule has a cyclic, hexagonal, planar structure of six carbon atoms with one hydrogen atom bonded to each. We can write a structure with alternate single and double bonds, either as a full structural formula or as a line-angle formula:

However, these structures do not explain the unique properties of benzene. Furthermore, experimental evidence indicates that all the carbon-to-carbon bonds in benzene are equivalent, and the molecule is unusually stable.

Chemists often represent benzene as a hexagon with an inscribed circle:

The inner circle indicates that the valence electrons are shared equally by all six carbon atoms (that is, the electrons are delocalized, or spread out, over all the carbon atoms). It is understood that each corner of the hexagon is occupied by one carbon atom, and each carbon atom has one hydrogen atom attached to it. Any other atom or groups of atoms substituted for a hydrogen atom must be shown bonded to a particular corner of the hexagon. We use this modern symbolism, but many scientists still use the earlier structure with alternate double and single bonds.

To Your Health: Benzene and Us

Most of the benzene used commercially comes from petroleum. It is employed as a starting material for the production of detergents, drugs, dyes, insecticides, and plastics. Once widely used as an organic solvent, benzene is now known to have both short- and long-term toxic effects. The inhalation of large concentrations can cause nausea and even death due to respiratory or heart failure, while repeated exposure leads to a progressive disease in which the ability of the bone marrow to make new blood cells is eventually destroyed. This results in a condition called aplastic anemia, in which there is a decrease in the numbers of both the red and white blood cells.

Concept Review Exercises

1. How do the typical reactions of benzene differ from those of the alkenes?

2. Briefly describe the bonding in benzene.

3. What does the circle mean in the chemist’s representation of benzene?

Answers

1. Benzene is rather unreactive toward addition reactions compared to an alkene.

2. Valence electrons are shared equally by all six carbon atoms (that is, the electrons are delocalized).

3. The six electrons are shared equally by all six carbon atoms.

Key Takeaway

• Aromatic hydrocarbons appear to be unsaturated, but they have a special type of bonding and do not undergo addition reactions.

Exercises

1. Draw the structure of benzene as if it had alternate single and double bonds.

2. Draw the structure of benzene as chemists usually represent it today.

Answer

1. 

1.18 Structure and Nomenclature of Aromatic Compounds

Learning Objectives

1. Recognize aromatic compounds from structural formulas.
2. Name aromatic compounds given formulas.
3. Write formulas for aromatic compounds given their names.

Historically, benzene-like substances were called aromatic hydrocarbons because they had distinctive aromas. Today, an aromatic compound is any compound that contains a benzene ring or has certain benzene-like properties (but not necessarily a strong aroma). You can recognize the aromatic compounds in this text by the presence of one or more benzene rings in their structure. Some representative aromatic compounds and their uses are listed in Table 1.9 "Some Representative Aromatic Compounds", where the benzene ring is represented as C6H5.

Table 1.9 Some Representative Aromatic Compounds

Name	Structure	Typical Uses
aniline	C6H5–NH2	starting material for the synthesis of dyes, drugs, resins, varnishes, perfumes; solvent; vulcanizing rubber
benzoic acid	C6H5–COOH	food preservative; starting material for the synthesis of dyes and other organic compounds; curing of tobacco
bromobenzene	C6H5–Br	starting material for the synthesis of many other aromatic compounds; solvent; motor oil additive
nitrobenzene	C6H5–NO2	starting material for the synthesis of aniline; solvent for cellulose nitrate; in soaps and shoe polish
phenol	C6H5–OH	disinfectant; starting material for the synthesis of resins, drugs, and other organic compounds
toluene	C6H5–CH3	solvent; gasoline octane booster; starting material for the synthesis of benzoic acid, benzaldehyde, and many other organic compounds

Example 5

Which compounds are aromatic?

1. 
2. 
3. 
4. 

Solution

1. The compound has a benzene ring (with a chlorine atom substituted for one of the hydrogen atoms); it is aromatic.
2. The compound is cyclic, but it does not have a benzene ring; it is not aromatic.
3. The compound has a benzene ring (with a propyl group substituted for one of the hydrogen atoms); it is aromatic.
4. The compound is cyclic, but it does not have a benzene ring; it is not aromatic.

Skill-Building Exercise

Which compounds are aromatic?

1. 
2. 
3. 

In the International Union of Pure and Applied Chemistry (IUPAC) system, aromatic hydrocarbons are named as derivatives of benzene. Figure 1.12 "Some Benzene Derivatives" shows four examples. In these structures, it is immaterial whether the single substituent is written at the top, side, or bottom of the ring: a hexagon is symmetrical, and therefore all positions are equivalent.

Figure 1.12 Some Benzene Derivatives

These compounds are named in the usual way with the group that replaces a hydrogen atom named as a substituent group: Cl as chloro, Br as bromo, I as iodo, NO2 as nitro, and CH3CH2 as ethyl.

Although some compounds are referred to exclusively by IUPAC names, some are more frequently denoted by common names, as is indicated in Table 1.9 "Some Representative Aromatic Compounds".

When there is more than one substituent, the corners of the hexagon are no longer equivalent, so we must designate the relative positions. There are three possible disubstituted benzenes, and we can use numbers to distinguish them (Figure 1.13 "The Three Isomeric Dichlorobenzenes"). We start numbering at the carbon atom to which one of the groups is attached and count toward the carbon atom that bears the other substituent group by the shortest path.

Figure 1.13 The Three Isomeric Dichlorobenzenes

In Figure 1.13 "The Three Isomeric Dichlorobenzenes", common names are also used: the prefix ortho (o-) for 1,2-disubstitution, meta (m-) for 1,3-disubstitution, and para (p-) for 1,4-disubstitution.

The substituent names are listed in alphabetical order. The first substituent is given the lowest number. When a common name is used, the carbon atom that bears the group responsible for the name is given the number 1:

Example 6

Name each compound using both the common name and the IUPAC name.

1. 
2. 
3. 

Solution

1. The benzene ring has two chlorine atoms (dichloro) in the first and second positions. The compound is o-dichlorobenzene or 1,2-dichlorobenzene.
2. The benzene ring has a methyl (CH3) group. The compound is therefore named as a derivative of toluene. The bromine atom is on the fourth carbon atom, counting from the methyl group. The compound is p-bromotoluene or 4-bromotoluene.
3. The benzene ring has two nitro (NO2) groups in the first and third positions. It is m-dinitrobenzene or 1,3-dinitrobenzene.

Note

The nitro (NO2) group is a common substituent in aromatic compounds. Many nitro compounds are explosive, most notably 2,4,6-trinitrotoluene (TNT).

Skill-Building Exercise

Name each compound using both the common name and the IUPAC name.

1. 
2. 
3. 

Sometimes an aromatic group is found as a substituent bonded to a nonaromatic entity or to another aromatic ring. The group of atoms remaining when a hydrogen atom is removed from an aromatic compound is called an aryl group. The most common aryl group is derived from benzene (C6H6) by removing one hydrogen atom (C6H5) and is called a phenyl group, from pheno, an old name for benzene.

Polycyclic Aromatic Hydrocarbons

Some common aromatic hydrocarbons consist of fused benzene rings—rings that share a common side. These compounds are called polycyclic aromatic hydrocarbons (PAHs).

The three examples shown here are colorless, crystalline solids generally obtained from coal tar. Naphthalene has a pungent odor and is used in mothballs. Anthracene is used in the manufacture of certain dyes. Steroids, a large group of naturally occurring substances, contain the phenanthrene structure. (For more information about steroids, see Chapter 7 "Lipids", Section 7.4 "Steroids".)

To Your Health: Polycyclic Aromatic Hydrocarbons and Cancer

The intense heating required for distilling coal tar results in the formation of PAHs. For many years, it has been known that workers in coal-tar refineries are susceptible to a type of skin cancer known as tar cancer. Investigations have shown that a number of PAHs are carcinogens. One of the most active carcinogenic compounds, benzopyrene, occurs in coal tar and has also been isolated from cigarette smoke, automobile exhaust gases, and charcoal-broiled steaks. It is estimated that more than 1,000 t of benzopyrene are emitted into the air over the United States each year. Only a few milligrams of benzopyrene per kilogram of body weight are required to induce cancer in experimental animals.

Biologically Important Compounds with Benzene Rings

Substances containing the benzene ring are common in both animals and plants, although they are more abundant in the latter. Plants can synthesize the benzene ring from carbon dioxide, water, and inorganic materials. Animals cannot synthesize it, but they are dependent on certain aromatic compounds for survival and therefore must obtain them from food. Phenylalanine, tyrosine, and tryptophan (essential amino acids) and vitamins K, B2 (riboflavin), and B9 (folic acid) all contain the benzene ring. (For more information about vitamins, see Chapter 9 "Proteins, and Enzymes", Section 9.6 "Enzyme Cofactors and Vitamins".) Many important drugs, a few of which are shown in Table 1.10 "Some Drugs That Contain a Benzene Ring", also feature a benzene ring.

Note

So far we have studied only aromatic compounds with carbon-containing rings. However, many cyclic compounds have an element other than carbon atoms in the ring. These compounds, called heterocyclic compounds, are discussed in Chapter 5 "Amines and Amides", Section 5.3 "Amines as Bases". Some of these are heterocyclic aromatic compounds.

Table 1.10 Some Drugs That Contain a Benzene Ring

Name	Structure
aspirin
acetaminophen
ibuprofen
amphetamine
sulfanilamide

Concept Review Exercises

1. Briefly identify the important characteristics of an aromatic compound.

2. What is meant by the prefixes meta, ortho, or para? Give the name and draw the structure for a compound that illustrates each.

3. What is a phenyl group? Give the structure for 3-phenyloctane.

Answers

1. An aromatic compound is any compound that contains a benzene ring or has certain benzene-like properties.

2. meta = 1,3 disubstitution; (answers will vary)

   

   ortho = 1,2 disubstitution

   

   para = 1,4 disubstitution or 1-bromo-4-chlorobenzene

   

3. phenyl group: C6H5 or

   

   3-phenyloctane:

   

Key Takeaway

• Aromatic compounds contain a benzene ring or have certain benzene-like properties; for our purposes, you can recognize aromatic compounds by the presence of one or more benzene rings in their structure.

Exercises

1. Is each compound aromatic?

   a.

   b.

2. Is each compound aromatic?

   a.

   b.

3. Draw the structure for each compound.

   a. toluene

   b. m-diethylbenzene

   c. 3,5-dinitrotoluene

4. Draw the structure for each compound.

   a. p-dichlorobenzene

   b. naphthalene

   c. 1,2,4-trimethylbenzene

5. Name each compound with its IUPAC name.

   a.

   b.

   c.

   d.

6. Name each compound with its IUPAC name.

   a.

   b.

   c.

   d.

Answers

1.

a. yes

b. no

3.

a.

b.

c.

5.

a. ethylbenzene

b. isopropylbenzene

c. o-bromotoluene

d. 3,5-dichlorotoluene

1.19 End-of-Unit Material

Unit Summary

To ensure that you understand the material in this unit, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the unit.

Any hydrocarbon containing either a double or triple bond is an unsaturated hydrocarbon. Alkenes have a carbon-to-carbon double bond. The general formula for alkenes with one double bond is CnH2n. Alkenes can be straight chain, branched chain, or cyclic. Simple alkenes often have common names, but all alkenes can be named by the system of the International Union of Pure and Applied Chemistry.

Cis-trans isomers (or geometric isomers) are characterized by molecules that differ only in their configuration around a rigid part of the structure, such as a carbon–to-carbon double bond or a ring. The molecule having two identical (or closely related) atoms or groups on the same side is the cis isomer; the one having the two groups on opposite sides is the trans isomer.

The physical properties of alkenes are quite similar to those of alkanes. Like other hydrocarbons, alkenes are insoluble in water but soluble in organic solvents.

More reactive than alkanes, alkenes undergo addition reactions across the double bond:

• Addition of hydrogen (hydrogenation):

  CH2=CH2 + H2 → CH3CH3

• Addition of halogen (halogenation):

  CH2=CH2 + X2 → XCH2CH2X

  where X = F, Cl, Br, or I.

• Addition of water (hydration):

  CH2=CH2 + HOH → HCH2CH2OH

Alkenes also undergo addition polymerization, molecules joining together to form long-chain molecules.

…CH2=CH2 + CH2=CH2 + CH2=CH2 +…→…CH2CH2–CH2CH2–CH2CH2–…

The reactant units are monomers, and the product is a polymer.

Alkynes have a carbon-to-carbon triple bond. The general formula for alkynes is CnH2n − 2. The properties of alkynes are quite similar to those of alkenes. They are named much like alkenes but with the ending -yne.

The cyclic hydrocarbon benzene (C6H6) has a ring of carbon atoms. The molecule seems to be unsaturated, but it does not undergo the typical reactions expected of alkenes. The electrons that might be fixed in three double bonds are instead delocalized over all six carbon atoms.

A hydrocarbon containing one or more benzene rings (or other similarly stable electron arrangements) is an aromatic hydrocarbon, and any related substance is an aromatic compound. One or more of the hydrogen atoms on a benzene ring can be replaced by other atoms. When two hydrogen atoms are replaced, the product name is based on the relative position of the replacement atoms (or atom groups). A 1,2-disubstituted benzene is designated as an ortho (o-) isomer; 1,3-, a meta (m-) isomer; and 1,4-, a para (p-) isomer. An aromatic group as a substituent is called an aryl group.

A polycyclic aromatic hydrocarbon (PAH) has fused benzene rings sharing a common side.

Additional Exercises

1. Classify each compound as saturated or unsaturated.

a.

b. CH3C≡CCH3

2. Classify each compound as saturated or unsaturated.

a.

b.

3. Give the molecular formula for each compound.

a.

b.

4. When three isomeric pentenes—X, Y, and Z—are hydrogenated, all three form 2-methylbutane. The addition of Cl2 to Y gives 1,2-dichloro-3-methylbutane, and the addition of Cl2 to Z gives 1,2-dichloro-2-methylbutane. Draw the original structures for X, Y, and Z.

5. Pentane and 1-pentene are both colorless, low-boiling liquids. Describe a simple test that distinguishes the two compounds. Indicate what you would observe.

6. Draw and name all the alkene cis-trans isomers corresponding to the molecular formula C5H10. (Hint: there are only two.)

7. The complete combustion of benzene forms carbon dioxide and water: C6H6 + O2 → CO2 + H2O

Balance the equation. What mass, in grams, of carbon dioxide is formed by the complete combustion of 39.0 g of benzene?

8. Describe a physiological effect of some PAHs.

9. What are some of the hazards associated with the use of benzene?

10. What is wrong with each name? Draw the structure and give the correct name for each compound.

a. 2-methyl-4-heptene

b. 2-ethyl-2-hexene

c. 2,2-dimethyl-3-pentene

11. What is wrong with each name?

a. 2-bromobenzene

b. 3,3-dichlorotoluene

c. 1,4-dimethylnitrobenzene

12. Following are line-angle formulas for three compounds. Draw the structure and give the name for each.

a.

b.

c.

12. Following are ball-and-stick molecular models for three compounds (blue balls represent H atoms; red balls are C atoms). Write the condensed structural formula and give the name for each.

a.

b.

c.

Answers

1.

a. unsaturated

b. unsaturated

3.

a. C6H10

b. C4H8

5. Add bromine solution (reddish-brown) to each. Pentane will not react, and the reddish-brown color persists; 1-pentene will react, leaving a colorless solution.

7. 2C6H6 + 15O2 → 12CO2 + 6H2O; 132 g

9. carcinogenic, flammable

11.

a. number not needed

b. can’t have two groups on one carbon atom on a benzene ring

c. can’t have a substituent on the same carbon atom as the nitro group

13.

a. CH3CH=CHCH2CH2CH3; 2-hexene

b.

c.

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