Alcohols

Alcohols as Acids

resonance in phenols

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. Alcohols

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

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

Preparation of Alcohols by Reduction of Aldehydes and Ketones

CRHOH H CRH O Reduction of Aldehydes Gives Primary Alcohols

Pt, ethanol (92%) Example: Catalytic Hydrogenation CH 3 O CH 2 OH O CH 3 O CH + H2H2H2H2

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

(93-95%) Example: Catalytic Hydrogenation + H2H2H2H2OPt ethanol HOH

H:–H:–H:–H:– H:–H:–H:–H:– CRH OHOHOHOH H CRH O CRH OHOHOHOH R' CRR' O Retrosynthetic Analysis

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

NaBH 4 (82%) Examples: Sodium Borohydride CH 2 OH O CH O2NO2NO2NO2Nmethanol O2NO2NO2NO2N O 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 (84%) Aldehyde KetoneO CH 3 (CH 2 ) 5 CH CH 3 (CH 2 ) 5 CH 2 OH 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 (86%)OH (C 6 H 5 ) 2 CHCHCH 3

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

Preparation of Alcohols By Reduction of Carboxylic Acids and Esters

lithium aluminum hydride is only effective reducing agent Reduction of Carboxylic Acids Gives Primary Alcohols CRHOH 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%)

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 Reduction of Esters Gives Primary Alcohols

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

Summary

Synthesis of Alcohols Sodium borohydride / Lithium aluminum hydride Reductions