Oximes, semicarbazones and 2, 4- dinitrophenyl- hydrazones:

The reaction of aldehydes and ketones with hydroxylamine (NH₂OH), semicarbazide (NH₂NHCONH₂) and 2,4-dinitrophenylhydrazine occurs by similar mechanism explained for primary amines to provide oximes, semi carbazones, and 2,4- dinitrophenylhydrazones, correspondingly. These compounds were often synthesized to recognize a liquid aldehyde or ketone. The products are crystalline and solid, and through measuring their melting points, the original aldehyde or ketone could be recognized by looking up melting point tables of these derivatives. These days, it is easier to recognize liquid aldehydes and ketones spectroscopically.

Figure: Synthesis of oximes, semicarbazones, and 2,4-dinitrophenylhydrazones.