Chapter 7 Notes - Alcohols, Ethers, Thiols - Portland State University

Portland State University - - Professor Carl C. Wamser

Chemistry 331 - Fall 1997

Chapter 7 Notes - Alcohols, Ethers, Thiols

Functional groups

alcohol: C-O-H ether: C-O-C thiol: C-S-H

Alcohol Classification

• based on the carbon the OH group is attached to: 1° , 2° , 3°
• methyl alcohol CH3OH
• ethyl alcohol CH3CH2OH (1°)
• isopropyl alcohol (CH3)2CHOH (2°)
• t-butyl alcohol (CH3)3COH (3°)

Alcohol Nomenclature

• OH group takes priority (even over -ene or -yne) - it must be in the parent chain - the direction of numbering gives it the lowest possible number
• -ol suffix with number designation
• name other substituents and multiple bonds as usual

Alcohol Examples

• common names for alcohols: alkyl alcohol cyclohexyl alcohol or cyclohexanol trans-4-methylcyclohexanol

Alcohol Example

(R)-3-methyl-5-hexen-3-ol

Ether Nomenclature

• alkyl alkyl ether (common) benzyl methyl ether
• alkoxy substituent (IUPAC) (S)-2-ethoxypentane

Sulfur Functional Groups

• thiols: C-S-H group (analogous to alcohols)
• sulfides: C-S-C group (analogous to ethers)
• disulfides: C-S-S-C group (analogous to peroxides)
• similar to oxygen analogs except: better nucleophiles easier to oxidize

Hydrogen Bonding

• sp3 O with two covalent bonds and two lone pairs
• O lone pairs attract polar H bonds
• covalent O-H bond strength ~ 100 kcal/mole
• O...H (H-bond) strength ~ 5 kcal/mole

Effects of H-Bonding

• alcohols have higher boiling points than alkanes (nonpolar) or alkyl halides (polar, but no H-bonds)
• ethers are polar but have no H-bonds (pentane and diethyl ether both boil at about 35°)
• H-bonds hold together the strands of DNA ("velcro" effect)

Acid-Base Reactions

• remember analogy with water
• reactions as bases: H2O + H+ <==> H3O+ ROH + H+ <==> ROH2+ (an oxonium ion)
• reactions as acids: H2O + B- <==> B-H + OH- ROH + B- <==> B-H + RO- (an alkoxide ion)

Acidity of Alcohols

• alcohols about as acidic as water MeOH more acidic, EtOH less acidic 3° alcohols much weaker acids
• pKa values: 3° > 2° > 1° > MeOH 18 , 17, 16, 15.5 (compare H2O: pKa = 15.7)
• tBuOH + NaOH ---> unfavorable

Alkoxide Anions

• deprotonation of alcohols gives alkoxide anions CH3OH + NaNH2 ---> NH3 + CH3O- Na+ (sodium methoxide)
• most commonly made by direct reaction with active metals CH3OH + Na ---> 1/2 H2 + CH3O- Na+ (CH3)3COH + K ---> 1/2 H2 + (CH3)3CO-K+

Oxygen Functional Groups

• alcohols are just the first of the many possible oxygen functional groups
• oxidation leads to increasing number of bonds to oxygen alkane --> alcohol --> carbonyl --> carboxyl --> CO2
• reduction leads to decreasing number of bonds to oxygen CO2 --> carboxyl --> carbonyl --> alcohol --> alkane

Synthesis of Alcohols

• hydration of alkenes follows Markovnikov's Rule 1-hexene --(H+, H2O)--> 2-hexanol
• reduction of carbonyl and carboxyl compounds reducing agents: sodium borohydride (NaBH4) lithium aluminum hydride (LiAlH4)

Alcohol Redox Reactions

• reductions to prepare alcohols:

aldehydes or ketones plus NaBH4 carboxylic acids or esters plus LiAlH4

• oxidations of alcohols:

1° alcohol to aldehyde with PCC 2° alcohol to ketone with CrO3 1° alcohol to carboxylic acid with CrO3

Alcohol Redox Examples

Reactions of Alcohols

• acid/base reactions
• oxidation reactions
• elimination (dehydration)
• substitution (C-O bond cleavage)

Substitution Reactions

• substitution by halogens using HX OH is a poor leaving group but initial protonation creates a good leaving group (H2O)
• halide substitution (SN1 mechanism) tBuOH + HBr --> tBuOH2+ --> tBu+ --> tBuBr favored by relatively stable carbocation (3°)
• halide substitution (SN2 mechanism) MeOH + HBr --> MeOH2+ + Br- --> MeBr + H2O concerted displacement of H2O by Br- (unstable carbocation)

Dehydration (Elimination) Reactions

• same start as for substitution initial protonation creates a good leaving group (H2O)
• carbocation may lose H+ to form an alkene (E1 mechanism) tBuOH + H2SO4 --> tBuOH2+ --> tBu+ --> (CH3)2C=CH2 + H+ favored by relatively stable carbocation (3°), absence of nucleophile, high temperature
• less-substituted alcohols (unstable carbocations) show concerted loss of H2O and H+ (E2 mechanism)

The Zaitsev Rule

• predicts regiochemistry of alkene formation
• the major product in an elimination reaction is the more substituted alkene (generally more stable)
• dehydration of an alcohol forms the more stable (more substituted) alkene

Ether Reactions

• ethers are generally unreactive (make good solvents)
• react with strong acid (protonated form can undergo SN1 or SN2 substitution)

Epoxides

• cyclic 3-membered ring ethers
• named as 1,2-epoxyalkane
• prepared from alkenes with peroxyacids
• unlike other ethers, these react easily to undergo ring opening

Epoxide Reactions

• acid-catalyzed hydration
• base-catalyzed hydration
• product is trans diol (backside attack as in the reaction of bromonium ions)