Alcohols Alcohols Methanol and ethanol are industrial chemicals

Alcohols

Alcohols Methanol and ethanol are industrial chemicals: solvents, antifreeze, fuel, antiseptic, social / recreational 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.

Sources of alcohols Reactions discussed in earlier chapters Hydration of alkenes Hydroboration-oxidation of alkenes Hydrolysis of alkyl halides Diols by hydroxylation of alkenes

Sources of alcohols New methods: Reduction of aldehydes and ketones Reduction of carboxylic acids Reduction of esters Reaction of Grignard reagents

Preparation of Alcohols by Reduction of Aldehydes and Ketones

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

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

Reduction of Ketones Gives Secondary Alcohols R R C R' O H C R' OH

Example: Catalytic Hydrogenation H O + H 2 OH Pt ethanol (93 -95%)

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

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

Examples: Sodium Borohydride Aldehyde O 2 N O O 2 N Na. BH 4 CH 2 OH CH methanol (82%) Ketone O Na. BH 4 ethanol H OH (84%)

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 O CH 3(CH 2)5 CH 1. Li. Al. H 4 diethyl ether 2. H 2 O CH 3(CH 2)5 CH 2 OH (86%) Ketone O (C 6 H 5)2 CHCCH 3 1. Li. Al. H 4 diethyl ether 2. H 2 O OH (C 6 H 5)2 CHCHCH 3 (84%)

Selectivity neither Na. BH 4 or Li. Al. H 4 reduces isolated double bonds O 1. Li. Al. H 4 diethyl ether 2. H 2 O (90%) H OH

Preparation of Alcohols By Reduction of Carboxylic Acids and Esters

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

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

Reduction of Esters Gives Primary Alcohols Lithium aluminum hydride preferred for laboratory reductions Sodium borohydride reduction is too slow to be useful Catalytic hydrogenolysis used in industry but conditions difficult or dangerous to duplicate in the laboratory (special catalyst, high temperature, high pressure

Example: Reduction of a Carboxylic Acid O COCH 2 CH 3 1. Li. Al. H 4 diethyl ether 2. H 2 O CH 2 OH + (90%) CH 3 CH 2 OH