The carbonyl (C=O) group in aldehydes and ketones reduces into alcohols in the presence of which of the following reagents.
(A) LiAlH4
(B) NaBH4
(C) CrO3-H2SO4
(D) Grignard reagents.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (D) only
The C=O group of aldehydes and ketones is polar, with a partial positive charge on carbon. Reagents that deliver a nucleophile to this carbon convert the carbonyl compound into an alcohol:
Hydride reagents (\( \ce{LiAlH4} \), \( \ce{NaBH4} \)) add \( \ce{H^{-}} \): aldehydes give primary alcohols and ketones give secondary alcohols.
Grignard reagents (RMgX) add an alkyl group \( \ce{R^{-}} \); hydrolysis of the adduct then gives an alcohol with a longer carbon chain.
Oxidising agents do the opposite and never give alcohols from carbonyl compounds.
(A) LiAlH4 — Correct. It is a strong reducing agent and reduces both aldehydes and ketones.
\( \displaystyle \ce{RCHO} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{LiAlH4}} \ce{RCH2OH} \)
\( \displaystyle \ce{R2C=O} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{LiAlH4}} \ce{R2CH-OH} \)
(B) NaBH4 — Correct. Sodium borohydride is a milder hydride reagent, but it also reduces aldehydes and ketones to alcohols.
\( \displaystyle \ce{R2C=O} \xrightarrow{\ce{NaBH4}} \ce{R2CH-OH} \)
(C) CrO3-H2SO4 — Incorrect. Chromic acid is an oxidising agent. It oxidises alcohols to carbonyl compounds and aldehydes to carboxylic acids, so it cannot give an alcohol.
(D) Grignard reagents. — Correct. Strictly this is nucleophilic addition, not reduction, but it also turns the C=O group into an alcohol. Methanal gives a primary alcohol, other aldehydes give secondary alcohols and ketones give tertiary alcohols.
\( \displaystyle \ce{R2C=O} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\mathrm{R^{\prime}}\ce{MgX}} \ce{R2C(OH)}\mathrm{R^{\prime}} \)
NCERT (2026-27 edition):
Reduction – “… (i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation (Unit 7, Class XII).”
From Grignard Reagents – “Alcohols are produced by the reaction of Grignard reagents (Unit 6, Class XII) with aldehydes and ketones. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields an alcohol.”