Like acetals, imines are unstable with respect to their parent carbonyl compound and amine, and must be formed by a method that allows removal of water from the reaction mixture.

Imines are formed from aldehydes or ketones with most primary amines. In general, they are stable enough to be isolated only if either the C or N of the imine double bond bears an aro matic substituent. Imines formed from ammonia are unstable, but can be detected in solution. CH₂ NH, for example, decomposes at temperatures above –80 °C, but PhCH NH is detectable by UV spectroscopy in a mixture of benzaldehyde and ammonia in methanol.

Imines are readily hydrolysed to the carbonyl compound and amine by aqueous acid—in fact, except for the particularly stable special cases we discuss below, most can be hydrolysed by water without acid or base catalysis. You have, in fact, already met an imine hydrolysis: at the end of Chapter 10 we talked about the addition of Grignard reagents to nitriles. The product is an imine that hydrolyses in acid solution to ketone plus ammonia.

The mechanism of the hydrolysis is the reverse of imine formation, going through the same hemiaminal intermediate and the same iminium and oxonium ions. All these steps are reversible and this should remind you that the relative stability of the starting material and product is as important in imine formation and hydrolysis as it is in acetal formation and hydrolysis.

مواضيع ذات صلة

Cyanide will attack iminium ions: the Strecker synthesis of amino acids

Lithium aluminium hydride reduces amides to amines

Iminium ions can react as electrophilic intermediates

Secondary amines react with carbonyl compounds to form enamines

Iminium ions and oxonium ions

Some imines are stable

Imines are the nitrogen analogues of carbonyl compounds

Amines react with carbonyl compounds

Modifying reactivity using acetals

Cyclic acetals are more stable than acyclic acetals

Acetals hydrolyse only in the presence of acid

Amines from imines: reductive amination