enamine
/ EN-uh-meen /
An enamine is the nitrogen cousin of an enol: it has a carbon-carbon double bond with a nitrogen attached to one end (C=C-N). The name fuses 'ene' (the alkene) and 'amine' (the nitrogen). Enamines form when a secondary amine reacts with an aldehyde or ketone, and they are a clever way to make a carbonyl behave as a carbon nucleophile.
Why does a secondary amine give an enamine while a primary amine gives an imine? Both start the same way: the amine adds to the carbonyl and loses water. A primary amine (R-NH2) still has an N-H, so it can finish by forming a stable C=N double bond, an imine. A secondary amine (R2N-H) has only one N-H to start with, and once it is on the carbon it has no N-H left to make C=N. Instead the molecule sheds a proton from the neighbouring carbon and forms a C=C double bond instead, parking the nitrogen on the carbon next door. Common secondary amines for this are pyrrolidine and morpholine.
Enamines matter because the carbon at the far end of the C=C is electron-rich and nucleophilic (nitrogen pushes electron density into the double bond), so it can attack electrophiles, like an alkyl halide or another carbonyl. This is the Stork enamine reaction, a milder alternative to harsh enolate chemistry for alkylating or acylating the alpha carbon of a ketone. After the new bond is made, simple hydrolysis removes the amine and gives back a carbonyl, now bearing the new group.
Cyclohexanone plus pyrrolidine (a secondary amine), with acid and removal of water, gives the enamine 1-pyrrolidinocyclohexene, whose alpha carbon can then be alkylated by an alkyl halide.
Secondary amine plus carbonyl gives an enamine (C=C-N) instead of an imine.
The fork is simply the number of N-H bonds: a primary amine (two N-H) can keep one to form C=N (imine), a secondary amine (one N-H) cannot, so it forms C=C (enamine) instead. Tertiary amines have no N-H and react with neither.