The reaction of aldehydes or ketones with primary amines generates imines . The reaction is favored in a slightly acid medium (pH=4.5).
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Control of the pH is essential, since protonation of the carbonyl oxygen is required to promote nucleophilic attack.

Mechanism:

Stage 1. Protonation of the carbonyl group that increases the positive polarity on carbon and favors the nucleophilic attack.
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Stage 2. Nucleophilic attack of the primary amine on the carbonyl carbon.
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Stage 3. Protonation of the hydroxyl group to transform it into a good leaving group.
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Stage 4. Loss of water and formation of the protonated imine.

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Stage 5. Deprotonation of the cation.

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Video

imines