Imines in which the nitrogen atom carries an electronegative group are usually stable: exam Ples include oximes, hydrazones, and semi carbazones.

These compounds are more stable than imines because the electronegative substituent can participate in delocalization of the imine double bond. Delocalization decreases the small positive charge on the carbon atom of the imine double bond and raises the energy of the LUMO, making it less susceptible to nucleophilic attack. Oximes, hydrazones, and semi carbazones require acid or base catalysis to be hydrolysed.

Historical note:

Because the hydrazone and semi carbazone derivatives of carbonyl compounds are often stable, crystalline solids, they used to be used to confirm the supposed identity of aldehydes and ketones. For example, the boiling points of these three isomeric five-carbon ketones are all similar and before the days of NMR spectroscopy it would have been hard to distinguish between them.

Their semi carbazones and 2,4-dinitrophenylhydrazones, on the other hand, all differ in their melting points. By making these derivatives of the ketones, identifi cation was made much easier. Of course, all of this has been totally super seded by NMR! However, these crystalline derivatives are still useful in the purification of volatile aldehydes and ketones, and in solving structures by X-ray crystallography.

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

Imines are usually unstable and are easily hydrolysed

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