enamine
Summary
An enamine is a compound with a nitrogen bonded directly to a C=C double bond (N–C=C). It forms from an aldehyde or ketone and a secondary amine with loss of water. Its β carbon is nucleophilic, like an enol or enolate; primary amines usually give imines instead.
Detail
Meaning: An enamine has the arrangement N–C=C. It forms when an aldehyde or ketone that has an α hydrogen reacts with a secondary amine (R₂NH) under mild acid catalysis. The amine adds to the carbonyl carbon, a carbinolamine forms, and water is lost. The nitrogen has no second H to give up, so the double bond forms toward the α carbon instead of toward nitrogen. The nitrogen lone pair delocalizes into the C=C, so the β carbon (the original α carbon of the carbonyl) is electron-rich and nucleophilic. It acts like a neutral enolate.
On the MCAT: Expect a reaction scheme in which a ketone and a secondary amine give an enamine, which then attacks an alkyl halide or a conjugated carbonyl (the Stork enamine reaction). Aqueous acid then hydrolyzes the product back to a carbonyl. The net result is α-alkylation of the carbonyl compound. Questions may ask you to pick the product by amine class: a primary amine gives an imine (C=N), a secondary amine gives an enamine. Imine–enamine tautomerism parallels keto–enol tautomerism.
Common confusion: Students mix up imine and enamine. An imine has C=N, and an enamine has N–C=C. Also, the carbon is the nucleophilic site in the enamine, not the nitrogen, even though nitrogen's lone pair supplies the electrons.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (Clayden)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.