Which of the following reagents can bring about the given conversion?…

The correct answer is Option (1) → \(\text{(a) Mg-dry ether (b) }CO_2 \text{ (c) }H^+/H_2O\)

A –COOH group can be put in place of a halogen through a Grignard reagent. The aryl bromide reacts with magnesium in dry ether to give the Grignard reagent; this adds to carbon dioxide (dry ice) to give the salt of a carboxylic acid, which on acidification gives the acid:

    3-\( \ce{O2NC6H4Br} \) + Mg → 3-\( \ce{O2NC6H4MgBr} \) (dry ether)

    3-\( \ce{O2NC6H4MgBr} \) + \( \ce{CO2} \) → 3-\( \ce{O2NC6H4COOMgBr} \)

    3-\( \ce{O2NC6H4COOMgBr} \) + \( \ce{H3O^{+}} \) → 3-\( \ce{O2NC6H4COOH} \) (3-nitrobenzoic acid) + Mg(OH)Br

The carbon of \( \ce{CO2} \) becomes the carbon of the new –COOH group, so the sequence is Mg/ether, CO₂, H₃O⁺.

Why the other options are incorrect

• Option (2) \(\text{(a) KCN (b) }H_3O^+\) — Incorrect. The C–Br bond of an aryl halide has partial double-bond character, so KCN does not replace the bromine; no nitrile is formed that acid could hydrolyse to –COOH.

• Option (3) \(\text{(a) }KMnO_4 / OH^- \text{ (b) }H^+ / H_2O\) — Incorrect. Alkaline \( \ce{KMnO4} \) oxidises an alkyl side chain to –COOH; 3-nitrobromobenzene has no carbon side chain, so there is nothing to oxidise.

• Option (4) \(\text{(a) }CrO_3/ H_2SO_4 \text{ followed by }H^+ /H_2O\) — Incorrect. Chromic acid also oxidises only a group that is already there (an alcohol or an alkyl side chain); it cannot put a new carbon atom in place of the bromine.

NCERT (2026-27 edition):
From Grignard Reagents

Note: The same/similar question has been asked in different years by NTA as per the details given below: