Identify "Y" in the following reaction:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → RCOOH
In a Grignard reagent the carbon bonded to magnesium carries a partial negative charge, so the alkyl group acts as a nucleophile. It adds to the electron-poor carbon of carbon dioxide (dry ice) in dry ether, which keeps moisture away from the reagent. This gives the magnesium salt of a carboxylic acid (X). Acid hydrolysis then frees the carboxylic acid (Y):
\( \displaystyle \ce{R-MgX + O=C=O} \xrightarrow{\text{dry ether}} \ce{R-COOMgX}\ (\text{X}) \)
\( \displaystyle \ce{R-COOMgX} \xrightarrow{\ce{H3O+}} \ce{R-COOH}\ (\text{Y}) + \ce{Mg(OH)X} \)
So Y is the carboxylic acid, RCOOH, which has one carbon atom more than the alkyl halide used to make the Grignard reagent.
Why the other options are incorrect
Option (2) RCHO — Incorrect. An aldehyde would need a reduction; addition to \( \ce{CO2} \) gives the carboxyl group directly.
Option (3) RCOX — Incorrect. An acid halide is made from the acid with \( \ce{SOCl2} \) or \( \ce{PCl5} \); it would react with water and cannot survive the acid hydrolysis step.
Option (4) ROH — Incorrect. Grignard reagents give alcohols with aldehydes and ketones (for example HCHO gives \( \ce{RCH2OH} \)), not with carbon dioxide.
NCERT (2026-27 edition):
From Grignard Reagents
