Answer the question on basis of passage given below:
Organic compounds containing carbon-oxygen double bonds (C=O) are called carbonyl compounds. These are highly polar molecules. Hence, their boiling points are higher than hydrocarbons and ethers. Aldehydes can be prepared from dehydrogenation or oxidation of alcohols and by reduction of acyl halides. Ketones are prepared by oxidation of secondary alcohols or by hydration of alkynes etc. Aldehydes and ketones undergo nucleophilic addition reaction with a number of nucleophiles such as HCN, NaHSO3, ammonia derivatives. Carboxylic acids are prepared by the oxidation of primary alcohols, aldehydes and alkenes, by hydrolysis of nitriles. Carboxylic acids are more acidic than alcohols and phenols.
The correct sequence of reagents for the conversion of 3-nitrobromobenzene to 3-nitrobenzoic acid is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(Mg/ether, CO_2, H_3O^+\)
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) \(KCN, LiAlH_4, NaOH\) — Incorrect. The C–Br bond of an aryl halide has partial double-bond character, so KCN does not replace the bromine; and even a nitrile would be reduced by \( \ce{LiAlH4} \) to an amine (–\( \ce{CH2NH2} \)), not converted into an acid.
Option (3) \(KMnO_4/OH^-, dil.H_2SO_4,CO_2/H^+\) — 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) \(AgNO_3\) (ammoniacal), \(CO_2, H_3O^+\) — Incorrect. Ammoniacal \( \ce{AgNO3} \) (Tollens’ reagent) oxidises aldehydes; it does not react with an aryl bromide and cannot introduce a carbon atom.
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:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 09-06 S3 07-06 S3 31-05 S2 | – |