In fact, the oxime formed from a ketone and hydroxylamine is just a special example of an imine. All imines have a C N double bond and are formed when any primary amine reacts with an aldehyde or a ketone under appropriate conditions, for example aniline and benzaldehyde.

You shouldn’t need us to tell you the mechanism of this reaction: even without looking at the mechanism we gave for the formation of the oxime it should come as no surprise to you by now. But as the reaction is very important in chemistry and biology, we’ll discuss it in some depth. First, the amine attacks the aldehyde and the intermediate known as a hemiacetal is formed. Amines are good nucleophiles for carbonyl groups, and aldehydes and ketones are electrophilic. There is no need for any catalysis in this step. Indeed, addition of acid would slow the reaction down as the nucleophilic amine would be removed as a salt.

First step in imine formation: the amine attacks the carbonyl group to form the hemiaminal intermediate:

Dehydration of the hemiaminal gives the imine. Now there is some need for catalysis: acid must be added so that the OH group can become a good leaving group. This step resembles the conversion of hemiacetals to acetals. The difference is that the iminium ion can lose a proton and become a neutral imine.

Second step in imine formation: acid-catalysed dehydration of the hemiaminal intermediate:

●Imine formation requires acid catalysis.

So acid is needed for the second step but hinders the first step. Clearly some compromise is needed. Without an acid catalyst, the reaction is very slow, although in some cases it may still take place. Imine formation is in fact fastest at about pH 4–6: at lower pH, too much amine is protonated and the rate of the fi rst step is slow; above this pH the proton concentration is too low to allow protonation of the OH leaving group in the dehydration step. Imine formation is like a biological reaction: it is fastest near neutrality.

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 usually unstable and are easily hydrolysed

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