Question

There are two – NH₂ group in semicarbazide. However, only one is involved in the formation of semicarbazone. Why?

Answer 1

Although semicarbazide has two – NH₂ groups but only one of them is involved in resonance as shown below.

As a result, e density on one of the – NH₂ group is reduced and hence it does not act as nucleophile. One pair of the other – NH₂ group is not involved in resonance and hence it is available for nucleophilic attack.