An organic compound \('X'\) on treatment with methyl magnesium bromide gives a compound \('Y'\) which on hydrolysis gives ethanol. The compound \('X'\) is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(HCHO\)
A Grignard reagent adds to the carbonyl group of an aldehyde or ketone (nucleophilic addition), and hydrolysis of the adduct gives an alcohol. Methanal gives a primary alcohol, other aldehydes give secondary alcohols and ketones give tertiary alcohols. The R group of RMgX ends up on the carbonyl carbon.
Ethanol (\( \ce{CH3CH2OH} \)) is a primary alcohol with one carbon more than the \( \ce{CH3} \) of the Grignard reagent, so X must be methanal:
HCHO + \( \ce{CH3MgBr} \) → \( \ce{CH3CH2OMgBr} \) (Y)
\( \ce{CH3CH2OMgBr} \) + \( \ce{H2O} \) → \( \ce{CH3CH2OH} \) + Mg(OH)Br
Why the other options are incorrect
Option (1) CH3CHO — Incorrect. Ethanal gives the secondary alcohol propan-2-ol, \( \ce{CH3CH(OH)CH3} \).
Option (2) CH3COCH3 — Incorrect. Propanone, a ketone, gives the tertiary alcohol 2-methylpropan-2-ol.
Option (4) CH3OH — Incorrect. An alcohol has no carbonyl group; its acidic O–H destroys the Grignard reagent, giving methane (\( \ce{CH3MgBr} \) + \( \ce{CH3OH} \) → \( \ce{CH4} \) + \( \ce{CH3OMgBr} \)).
NCERT (2026-27 edition):
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.”
From Grignard Reagents – “… You will notice that the reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones.”