imines
Summary
An imine has a C=N double bond (R₂C=NR′) and is also called a Schiff base. It forms when an aldehyde or ketone condenses with ammonia or a primary amine, losing water. A secondary amine gives an enamine instead.
Detail
Meaning: An imine has a carbon double-bonded to nitrogen, R₂C=NR′, the nitrogen analogue of a carbonyl. Imines with an R group on N are also called Schiff bases. They form when an aldehyde or ketone reacts with ammonia or a primary amine: the nitrogen lone pair adds to the carbonyl carbon, giving a carbinolamine, and acid-catalyzed loss of water forms C=N. The reaction is reversible, and aqueous acid hydrolyzes the imine back to carbonyl and amine.
Equation: R₂C=O + R′NH₂ ⇌ R₂C=NR′ + H₂O, where R and R′ are alkyl, aryl or H groups. As a reaction, it has no SI unit.
On the MCAT: Expect to predict the product of an aldehyde or ketone with a primary amine, or to follow the mechanism (nucleophilic addition, then dehydration). Biochemistry uses Schiff bases too: a lysine side chain forms one with retinal in rhodopsin and with pyridoxal phosphate. Reducing an imine (reductive amination) gives an amine.
Common confusion: A primary amine gives an imine, but a secondary amine, which has no second N–H to lose, gives an enamine. Also, an imine is not an amine: an imine has a C=N double bond, while an amine has only single bonds to N.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.