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Question
Give plausible explanation for the following:
There are two −NH2 groups in semicarbazide. However, only one is involved in the formation of semicarbazones.
Solution
Semicarbazide only has one of the two −NH2 groups resonance and this group is directly attached to the carbonyl-carbon atom.
Therefore, the electron density on −NH2 group involved in the resonance also decreases. As a result, it cannot act as a nucleophile. Since the other −NH2 group is not involved in resonance, it can act as a nucleophile and attack carbonyl-carbon atoms of aldehydes and ketones to produce semicarbazones.
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