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JEE Main levelSingle correct optionDifficulty 2 of 3o1082

In semicarbazide, $H X_{2} N - NH - CO - NH X_{2}$ , only the $NH X_{2}$ attached to the NH of the hydrazine unit takes part in semicarbazone formation because

Athe other $NH X_{2}$ is bonded to a carbonyl carbon which makes it sp3
Bthe other $NH X_{2}$ is more basic and gets protonated first
Cthe other $NH X_{2}$ is more hindered than the hydrazine nitrogen
Dthe lone pair of the other $NH X_{2}$ is delocalised into the carbonyl group (amide resonance), making it non-nucleophilic

Answer: D

The amide-type

NH X_{2}

has its lone pair delocalised over the

N - C = O

unit and is therefore a very poor nucleophile. The terminal hydrazine-type nitrogen keeps its localised lone pair and attacks the carbonyl carbon.

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