Chapter 15 Alcohols, Diols, and Thiols Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15.1 Sources of Alcohols

Methanol is an industrial chemical. End uses: solvent, antifreeze, fuel. Principal use: preparation of formaldehyde. Prepared by hydrogenation of carbon monoxide. CO + 2H 2  CH 3 OH Methanol

Ethanol is an industrial chemical. Most ethanol comes from fermentation. Synthetic ethanol is produced by hydration of ethylene. Synthetic ethanol is denatured (made unfit 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 reagents Organolithium 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

15.2 Preparation of Alcohols by Reduction of Aldehydes and Ketones

C R HOHOH H C R H O Reduction of Aldehydes Gives Primary Alcohols

Pt, ethanol (92%) Example: Catalytic Hydrogenation CH 3 OCH 2 OH O CH 3 OCH + H2H2

C R HOHOH R' C R O Reduction of Ketones Gives Secondary Alcohols

(93-95%) Example: Catalytic Hydrogenation + H2H2 O Pt ethanol HOH

H:–H:– C R HOHOH H C R H O H:–H:– C R HOHOH R' C R O Retrosynthetic Analysis

Sodium borohydride Na + – B H H HH Lithium aluminum hydride Li + – Al H H HH Metal Hydride Reducing Agents act as hydride donors

Examples: Sodium Borohydride O CH O2NO2N O NaBH 4 methanol (82%) CH 2 OH O2NO2N HOH (84%) NaBH 4 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 CH 3 (CH 2 ) 5 CH 1. LiAlH 4 diethyl ether 2. H 2 O O (C 6 H 5 ) 2 CHCCH 3 1. LiAlH 4 diethyl ether 2. H 2 O (84%) CH 3 (CH 2 ) 5 CH 2 OH (86%) OH (C 6 H 5 ) 2 CHCHCH 3

Neither NaBH 4 or LiAlH 4 reduces carbon-carbon double bonds. O HOH 1. LiAlH 4 diethyl ether 2. H 2 O (90%) Selectivity

15.3 Preparation of Alcohols By Reduction of Carboxylic Acids

lithium aluminum hydride is only effective reducing agent Reduction of Carboxylic Acids Gives Primary Alcohols C R HOHOH H C R HO O

Example: Reduction of a Carboxylic Acid 1. LiAlH 4 diethyl ether 2. H 2 O COH O CH 2 OH (78%)

15.4 Preparation of Alcohols From Epoxides

Reaction of Grignard Reagents with Epoxides H2CH2C CH 2 O RMgX CH 2 OMgX R H3O+H3O+ RCH 2 CH 2 OH

CH 3 (CH 2 ) 4 CH 2 MgBr H2CH2C CH 2 O + 1. diethyl ether 2. H 3 O + CH 3 (CH 2 ) 4 CH 2 CH 2 CH 2 OH (71%) Example

15.5 Preparation of Diols

Diols are Prepared by... Reactions used to prepare alcohols Hydroxylation of alkenes

O O HCCH 2 CHCH 2 CH CH 3 H 2 (100 atm) Ni, 125°C HOCH 2 CH 2 CHCH 2 CH 2 OH CH 3 3-Methyl-1,5-pentanediol (81-83%) Example: Reduction of a Dialdehyde

Vicinal diols have hydroxyl groups on adjacent carbons. Ethylene glycol (HOCH 2 CH 2 OH) is most familiar example. Hydroxylation of Alkenes Gives Vicinal Diols

Osmium Tetraoxide is Key Reagent C C HOHO OHOH C C OsO 4 O O Os OO C C Cyclic osmate ester

(CH 3 ) 3 COOH OsO 4 (cat) tert-Butyl alcohol HO – Example (73%) CH 2 CH 3 (CH 2 ) 7 CH CH 3 (CH 2 ) 7 CHCH 2 OH OHOH

Example H H (CH 3 ) 3 COOH OsO 4 (cat) tert-Butyl alcohol HO – (62%) H H OHOH HOHO Stereospecific syn addition of —OH groups to each carbon of double bond