Condensation reactions of the carbonyl bonds

Based on ammonia or more specifically hydrazine H₂N-NH₂ adds across the carbonyl bond. The addition is immediately followed by loss of water. This addition-elimination is called a condensation reaction

The most important of these is the reaction of carbonyls with 2,4-dinitrophenylhydrazine (2,4-DNPH).

This reaction occurs as follows:

This substituted 2,4-dinitrophenylhydrazone produces a precipitate of orange crystals. These are essential in identifying unknown carbonyl.

• React unknown with 2,4-DNPH, orange ppte formed
• Filter and re-crystalise the orange crystals, dry thoroughly.
• Find melting point of crystals, this will be sharp is crystals are pure
• Look up in melting point tables to identify the unknown carbonyl.

This is important – the exam board expect you to be able to reproduce this. You do not have to learn the equation of this reaction – shame as it is not hard!