Alcohols can be prepared in many ways. They are prepared from alkenes by acid-catalysed hydration and by hydroboration–oxidation. Alcohols are also prepared from aldehydes and ketones by reduction. The reduction of carboxylic acids and esters also gives alcohols. Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones. Phenol, also known as carbolic acid, was first isolated in the early nineteenth century from coal tar. Nowadays, phenol is commercially produced synthetically. In the laboratory, phenols are prepared from benzene derivatives in many ways.
Which of the following reducing agents reduces carboxylic acids to primary alcohols?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Lithium aluminium hydride
The carboxyl group is hard to reduce. A strong hydride reagent, lithium aluminium hydride (LiAlH4, in ether, followed by acid work-up), reduces –COOH to –CH2OH, giving a primary alcohol:
\( \displaystyle \ce{R-COOH} \xrightarrow[\text{(ii) } \ce{H3O+}]{\text{(i) } \ce{LiAlH4}/\text{ether}} \ce{R-CH2OH} \)
Why the other options are incorrect
Option (2) Sodium borohydride — Incorrect. NaBH4 reduces aldehydes and ketones to alcohols, but it does not reduce the carboxyl group.
Option (3) Zinc amalgam and concentrated hydrochloric acid — Incorrect. This is the Clemmensen reduction, which reduces the C=O group of aldehydes and ketones to a CH2 group, giving a hydrocarbon, not an alcohol.
Option (4) Sodium hydride — Incorrect. It is not a reagent for reducing the carboxyl group; for carboxylic acids the reducing agents used are LiAlH4 or, better, diborane.
NCERT (2026-27 edition):
8.9.3 Reactions Involving –COOH Group – “Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane.”