Chapter 8 - Alcohols, Ethers, And Phenols

Chemistry 331 - Fall 1996 Elements of Organic Chemistry I

Professor Carl C. Wamser

Chapter 8 - Alcohols, Ethers, and Phenols

Alcohols, Ethers, Phenols

• functional groups:
• alcohol: C-O-H
• ether: C-O-C
• phenol: Ar-O-H (Ar = aryl group)

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

Phenol Nomenclature

• OH group assumed number 1
• 4-chloro-3-methylphenol

Ether Nomenclature

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

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+

Acidity of Phenols

• phenoxide anions are stabilized by resonance with the aromatic ring
• pKa of phenol is 10 (deprotonated by NaOH at pH 10)
• electron withdrawing groups further stabilize the phenoxide anion (especially ortho or para)
• pKa of p-nitrophenol is 7

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 Synthesis of Ethers

• Williamson ether synthesis an SN2 reaction with an alkoxide as nucleophile the alkyl halide should be methyl or 1°

Reactions of Alcohols

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

Substitution Reactions

• substitution (and elimination) OH is a poor leaving group but initial protonation creates a good leaving group (H2O)
• dehydration (E1 mechanism)
• halide substitution (SN1) tBuOH + HBr --> tBuOH2+ --> tBu+ --> tBuBr

Ether Reactions

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

Phenol Reactions

• the phenol C-O bond is rarely cleaved (SN1, SN2, E1, and E2 don't work for aryl groups)
• phenoxide anions are good nucleophiles
• electrophiles add readily to the benzene ring OH is activating and an o,p-director

Quinones

• phenols are readily oxidized to quinones
• reduction of quinones gives a hydroquinone a common reversible redox pair

Epoxides

• cyclic 3-membered ring ethers
• named as 1,2-epoxyalkane
• prepared from alkenes
• 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)

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