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)