Chapter 15Chapter 15Alcohols, Diols, and ThiolsAlcohols, Diols, and Thiols

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15.115.1Sources of AlcoholsSources of Alcohols

Methanol is an industrial chemical.

End uses: solvent, antifreeze, fuel.

Principal use: preparation of formaldehyde.

Prepared by hydrogenation of carbon monoxide.

CO + 2H2 CH3OH

Methanol

Ethanol is an industrial chemical.

Most ethanol comes from fermentation.

Synthetic ethanol is produced by hydrationof ethylene.

Synthetic ethanol is denatured (madeunfit for drinking) by adding methanol, benzene,pyridine, castor oil, gasoline, etc.

Ethanol

Isopropyl alcohol is prepared by hydration of propene.

All alcohols with four carbons or fewer are readily available.

Most alcohols with five or six carbons are readily available.

Other Alcohols

Hydration of alkenes

Hydroboration-oxidation of alkenes

Hydrolysis of alkyl halides

Syntheses using Grignard reagentsOrganolithium reagents

Sources of Alcohols

Reactions discussed in earlier chapters (Table 15.1)

Reduction of aldehydes and ketones

Reduction of carboxylic acids

Reaction of Grignard reagents with epoxides

Diols by hydroxylation of alkenes

Sources of Alcohols

New methods in Chapter 15

15.215.2Preparation of AlcoholsPreparation of Alcohols

bybyReduction of Aldehydes and KetonesReduction of Aldehydes and Ketones

C

R

H OH

H

C

R

H

O

Reduction of Aldehydes Gives Primary Alcohols

Pt, ethanol

(92%)

Example: Catalytic Hydrogenation CH3O CH2OH

O

CH3O CH + H2

C

R

H OH

R'

C

R

R'

O

Reduction of Ketones Gives Secondary Alcohols

(93-95%)

Example: Catalytic Hydrogenation

+ H2

O Pt

ethanol

H OH

H:–C

R

H OH

H

C

R

H

O

H:–C

R

H OH

R'

C

R

R'

O

Retrosynthetic Analysis

Sodiumborohydride

Na+ –

B

H

H

HH

Lithiumaluminum hydride

Li+ –

Al

H

H

HH

Metal Hydride Reducing Agents

act as hydride donors

Examples: Sodium Borohydride

O

CH

O2N O NaBH4

methanol

(82%)

CH2OH

O2N H OH

(84%)

NaBH4

ethanol

Aldehyde

Ketone

Lithium Aluminum Hydride

More reactive than sodium borohydride.

Cannot use water, ethanol, methanol etc.as solvents.

Diethyl ether is most commonly used solvent.

Examples: Lithium Aluminum Hydride

Aldehyde

Ketone

O

CH3(CH2)5CH

1. LiAlH4

diethyl ether2. H2O

O

(C6H5)2CHCCH3

1. LiAlH4

diethyl ether2. H2O

(84%)

CH3(CH2)5CH2OH

(86%)

OH

(C6H5)2CHCHCH3

Neither NaBH4 or LiAlH4reduces carbon-carbondouble bonds.

O

H OH

1. LiAlH4

diethyl ether2. H2O

(90%)

Selectivity

15.315.3Preparation of Alcohols By ReductionPreparation of Alcohols By Reduction

of Carboxylic Acidsof Carboxylic Acids

lithium aluminum hydride is only effective reducing agent

Reduction of Carboxylic AcidsGives Primary Alcohols

C

R

H OH

H

C

R

HO

O

Example: Reduction of a Carboxylic Acid

1. LiAlH4

diethyl ether2. H2O

COH

O CH2OH

(78%)

15.415.4Preparation of Alcohols From EpoxidesPreparation of Alcohols From Epoxides

Reaction of Grignard Reagentswith Epoxides

H2C CH2

O

R MgX

CH2 CH2 OMgX

R

H3O+

RCH2CH2OH

CH3(CH2)4CH2MgBr H2C CH2

O

+

1. diethyl ether2. H3O+

CH3(CH2)4CH2CH2CH2OH

(71%)

Example

15.515.5Preparation of DiolsPreparation of Diols

Diols are Prepared by...

Reactions used to prepare alcohols

Hydroxylation of alkenes

O O

HCCH2CHCH2CH

CH3

H2 (100 atm)

Ni, 125°C

HOCH2CH2CHCH2CH2OH

CH3

3-Methyl-1,5-pentanediol

(81-83%)

Example: Reduction of a Dialdehyde

Vicinal diols have hydroxyl groups on adjacent carbons.

Ethylene glycol (HOCH2CH2OH) is most familiar example.

Hydroxylation of AlkenesGives Vicinal Diols

Osmium Tetraoxide is Key Reagent

C CHO OH

C C

OsO4O O

Os

OO

CC

Cyclic osmate ester

(CH3)3COOHOsO4 (cat)

tert-Butyl alcoholHO–

Example

(73%)

CH2CH3(CH2)7CH

CH3(CH2)7CHCH2OH

OH

Example H

H

(CH3)3COOHOsO4 (cat)

tert-Butyl alcoholHO–

(62%)

H

H

OHHO

Stereospecificsyn addition of —OH groups to each carbonof double bond