Imine formation: the mechanism, step by step
The amine adds to the carbonyl, the resulting alcohol-amine loses water, and a carbon–nitrogen double bond is left.
The pathway runs in 3 steps. What sits between them is a real structure: it is what the reaction passes through, and on a bad day it is what you isolate instead of the product.
Below is the sequence as it is taught. On the platform it is drawn on whatever structures you supply, with the curly arrows resolved to the actual atoms involved rather than to a general case with R groups on it.
The steps
The amine adds to the carbonyl
The nitrogen shares its pair of electrons with the carbon of the C=O group, and the double bond's electrons move up onto the oxygen.
Addition of the amine lone pair into the carbonyl π*, giving a zwitterion with a negative oxygen and a positive nitrogen.
The proton moves from nitrogen to oxygen
The spare hydrogen on the nitrogen hops across to the oxygen. Now there is a hydroxyl group and a neutral nitrogen.
Proton transfer giving the neutral hemiaminal. In the flask this happens through the solvent rather than directly.
Water leaves and the double bond forms
The nitrogen pushes its electrons into the bond to carbon, making a double bond, and that forces the water off.
The nitrogen lone pair forms the C=N π bond as the C–O bond breaks, the oxygen leaving as water.
What this drawing assumes
- Drawn without the acid catalysis that normally carries it: in practice the hydroxyl is protonated before it leaves, and the rate is fastest at mildly acidic pH.
- The proton transfers between nitrogen and oxygen are collected into one step rather than drawn individually.
Other pathways this class runs by
- Under strongly acidic conditions the carbonyl is protonated first, which makes the addition faster but ties up the amine as its salt.
Run it on your own structures
Enter the reactants and the product you expect. If the transformation is one the engine can perform and this pathway reaches that product from those structures, it is drawn on them — with the arrows on the right atoms and the intermediates you would actually pass through. If it does not reach the product, you are told that instead of being shown a drawing that does not apply.
Other mechanisms
Nucleophilic acyl substitution
The nucleophile adds to the carbonyl, the carbon holds four groups for a moment, and the leaving group is pushed out as the double bond comes back.
Bimolecular nucleophilic substitution
One step. The nucleophile comes in on the opposite side from the leaving group, and the carbon turns inside out as the exchange happens.
Diels-Alder cycloaddition
One step. Six electrons move round a ring at once, two new single bonds form at the ends and the double bond ends up in the middle.
Bimolecular elimination
One step. The base takes a hydrogen from one carbon while the leaving group departs from the next, and a double bond forms between them.
Unimolecular nucleophilic substitution
The leaving group goes first, on its own, leaving a flat carbon with a positive charge. Whatever is around then attacks it from either side.
Organometallic addition to a carbonyl
The carbon attached to the metal is nucleophilic. It adds to the carbonyl, and the alkoxide that results is protonated when the reaction is worked up.