Lithium aluminium hydride reduces carboxylic acids to primary alcohols in excellent yield, but it is an expensive reagent. How are carboxylic acids reduced to alcohols commercially?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → By converting the acids to esters, followed by reduction with hydrogen in the presence of a catalyst
Because LiAlH4 is costly, it is used only for special chemicals. On a large scale the acid is first converted to an ester, and the ester is then reduced by catalytic hydrogenation:
\( \displaystyle \ce{RCOOH + R'OH} \xrightarrow{\ce{H+}} \ce{RCOOR' + H2O} \)
\( \displaystyle \ce{RCOOR' + 2H2} \xrightarrow{\text{catalyst}} \ce{RCH2OH + R'OH} \)
Why the other options are incorrect
Option (1) By heating the sodium salts of the acids with sodalime — Incorrect. This is decarboxylation: the acid loses carbon dioxide and a hydrocarbon is formed, not an alcohol.
Option (3) By reducing the acids with sodium borohydride in place of lithium aluminium hydride — Incorrect. NaBH4 reduces aldehydes and ketones, but it does not reduce the carboxyl group.
Option (4) By converting the acids to acid chlorides, followed by hydrogenation over palladium on barium sulphate — Incorrect. This is the Rosenmund reduction; it stops at the aldehyde and does not give an alcohol.
NCERT (2026-27 edition):
7.4.1 Preparation of Alcohols – “(ii) By reduction of carboxylic acids and esters: Carboxylic acids are reduced to primary alcohols in excellent yields by lithium aluminium hydride, a strong reducing agent.” … “However, LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only. Commercially, acids are reduced to alcohols by converting them to the esters …, followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation).”