Chapter 15 Alcohols, Diols, and Thiols

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Chapter 15Alcohols, Diols, and Thiols
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Sources of alcohols
Reactions discussed in earlier chapters (Table
15.1)

• Hydration of alkenes
• Hydroboration-oxidation of alkenes
• Hydrolysis of alkyl halides
• Syntheses using Grignard reagents organolithium
  reagents

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Sources of alcohols
New methods in Chapter 15

• Reduction of aldehydes and ketones
• Reduction of carboxylic acids
• Reduction of esters
• Reaction of Grignard reagents with epoxides
• Diols by hydroxylation of alkenes

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15.2Preparation of AlcoholsbyReduction of
Aldehydes and Ketones
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Reduction of Aldehydes Gives Primary Alcohols
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Example Catalytic Hydrogenation
Pt, ethanol

(92)
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Reduction of Ketones Gives Secondary Alcohols
R
C
O
R'
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Example Catalytic Hydrogenation
H
OH
Pt

H2
ethanol
(93-95)

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Retrosynthetic Analysis
H
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Metal Hydride Reducing Agents
Sodiumborohydride

• act as hydride donors

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Examples Sodium Borohydride
Aldehyde
NaBH4
methanol
Ketone
NaBH4
ethanol
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Lithium aluminum hydride

• more reactive than sodium borohydride
• cannot use water, ethanol, methanol etc.as
  solvents
• diethyl ether is most commonly used solvent

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Examples Lithium Aluminum Hydride
Aldehyde
1. LiAlH4diethyl ether
2. H2O
Ketone
1. LiAlH4diethyl ether
(C6H5)2CHCCH3
2. H2O
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Selectivity

• neither NaBH4 or LiAlH4reduces
  carbon-carbondouble bonds

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15.3Preparation of Alcohols By Reductionof
Carboxylic Acids and Esters
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Reduction of Carboxylic AcidsGives Primary
Alcohols
R
C
O
HO

• lithium aluminum hydride is only effective
  reducing agent

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Example Reduction of a Carboxylic Acid
1. LiAlH4diethyl ether
2. H2O
(78)
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Reduction of EstersGives Primary Alcohols

• Lithium aluminum hydride preferred forlaboratory
  reductions
• Sodium borohydride reduction is too slowto be
  useful
• Catalytic hydrogenolysis used in industrybut
  conditions difficult or dangerous to duplicate
  in the laboratory (special catalyst,
  hightemperature, high pressure

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Example Reduction of an Ester
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15.4Preparation of Alcohols From Epoxides
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Reaction of Grignard Reagentswith Epoxides

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Example
CH2
H2C
CH3(CH2)4CH2MgBr


O
1. diethyl ether 2. H3O