Which of the following is the best reducing agent to convert \( RCOOH \) into \( RCH_2OH \)?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(B_2H_6\)
The carboxyl group is one of the hardest carbonyl groups to reduce. Only strong or special reducing agents take it to a primary alcohol; lithium aluminium hydride does it, but diborane does it better, and diborane does not easily reduce groups such as ester, nitro or halo.
\( \displaystyle \ce{RCOOH} \xrightarrow[\text{(ii) }\ce{H3O^+}]{\text{(i) }\ce{B2H6}} \ce{RCH2OH} \)
So the best reagent among the options is diborane, \( \ce{B2H6} \).
Why the other options are incorrect
Option (1) \(LiBH_4\) — Incorrect. Lithium borohydride is a milder hydride than \( \ce{LiAlH4} \); it reduces aldehydes, ketones and esters, but it does not reduce the free –COOH group easily.
Option (3) \(NaBH_4\) — Incorrect. Sodium borohydride reduces aldehydes and ketones to alcohols, but it does not reduce the carboxyl group.
Option (4) Hydrogen in presence of Pd — Incorrect. Catalytic hydrogenation reduces C=C bonds, nitro groups and aldehydes, but it leaves a carboxylic acid group unchanged under ordinary conditions.
NCERT (2026-27 edition):
Reactions Involving –COOH Group
