Schiff base

General structure of an azomethine

General structure of a Schiff base (narrow definition)

A Schiff base, named after Hugo Schiff, is a compound with a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group, not hydrogen. Schiff bases in a broad sense have the general formula R¹R²C=NR³, where R is an organic side chain. In this definition, Schiff base is synonymous with azomethine. Some restrict the term to the secondary aldimines (azomethines where the carbon is connected to a hydrogen atom), thus with the general formula RCH=NR'.

The chain on the nitrogen makes the Schiff base a stable imine. A Schiff base derived from an aniline, where R³ is a phenyl or a substituted phenyl, can be called an anil.

Synthesis

Schiff bases can be synthesized from an aromatic amine and a carbonyl compound by nucleophilic addition forming a hemiaminal, followed by a dehydration to generate an imine. In a typical reaction, 4,4'-diaminodiphenyl ether reacts with o-vanillin:

A mixture of 4,4'-diaminodiphenyl ether 1 (1.00 g, 5.00 mmol) and o-vanillin 2 (1.52 g, 10.00 mmol) in methanol (40.00 mL) is stirred at room temperature for one hour to give an orange precipitate and after filtration and washing with methanol to give the pure Schiff base 3 (2.27 g, 97.00%)

There is a Schiff base intermediate in the fructose 1,6-bisphosphate aldolase catalyzed reaction during glycolysis and in the metabolism of amino acids.