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Which of the following compounds reacts with HNO2 (nitrous acid) to produce a carboxylic acid?

1. C6H5COCl
2. C6H5CONH2
3. CH3COOCOCH3
4. CH3COOC2H5

Correct answer: C6H5CONH2

Solution

Primary amides react with nitrous acid (HNO2) through a diazotization-like mechanism: the NH2 of the amide is converted to a diazonium-like intermediate that decomposes releasing N2, yielding the parent carboxylic acid. Acid chlorides, anhydrides, and esters do not react with HNO2 to give carboxylic acids under normal conditions.

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