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JEE Main levelSingle correct optionDifficulty 2 of 3co012

Semicarbazide, $NH X_{2} NHCONH X_{2}$ , has two $NH X_{2}$ groups but only one takes part in semicarbazone formation, because

Athe $NH X_{2}$ attached to $C = O$ is more basic than the other
Bthe terminal $NH X_{2}$ is sterically hindered by the carbonyl oxygen
Cthe lone pair of the $NH X_{2}$ attached to $C = O$ is delocalised by resonance and is not nucleophilic
Dthe $NH$ group in the middle is protonated first

Answer: C

The amide-type nitrogen donates its lone pair into

C = O

, which lowers its nucleophilicity. The

NH X_{2}

on the other nitrogen is a free hydrazine-type nucleophile and attacks the carbonyl carbon.

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