Preparation of solid derivatives of Carbonyl compounds (aldehydes & ketones) :- 2, 4- Dintrophenylhydrazones, Semicarbazones

The systematic procedure that involves several steps and preparations is carried out to identify unknown compounds to some extent. The preparation of derivatives usually establishes the identification of the unknown with certainty. Here the term derivative is simply referred to a compound prepared from an unknown, in order to identify the unknown compound. An ideal derivative should be a crystalline, easily purified solid with a sharp melting point, which can be prepared readily from the unknown in one direct and unambiguous step.

Many carbonyl compounds can be synthesized from the esterification reaction. There are excellent and conveniently prepared derivatives which use to identify carbonyl compounds. Oximes, Phenyl hydrazones, 2, 4-dinitrophenylhydrazones and semicarbazones are some of the best derivatives of aldehydes and ketones. In the preparation of each of these types of derivatives, the elimination of a water molecule between a molecule of the carbonyl compound and a molecule of the reagent is involved.

Dinitrophenylhydrazine is relatively sensitive to shock and friction. It is a red to orange solid usually supplied wet to reduce its explosive hazard. This is often used as a qualitative test for carbonyl groups, associated with aldehydes and ketones. The hydrazone derivatives can be used as evidence toward the identity of the original compound. These are usually yellow-red colour crystals. Crystals of different hydrazones have characteristic melting and boiling points allowing the identification of the unknown substance.

Semicarbazone is one of the most commonly used derivatives to identify aldehydes and ketones. It is a derivative of an aldehyde or ketone formed by a condensation reaction between a ketone or aldehyde and the reagent; semicarbazide. As semicarbazides are not very stable in the free form, they are usually stored in the laboratory in the form of their hydrochloride salts. Many semicarbazones are off white, crystalline solids, useful for the identification of the parent carbonyl group by melting point analysis.