Electrophilic aromatic substitution: the mechanism, step by step
The ring gives up its aromaticity for one step to attack the electrophile, then loses a proton to get it back.
The pathway runs in 2 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 ring attacks the electrophile
Two of the ring's electrons reach out to the electrophile and make a bond. That carbon now holds four groups, the ring is no longer aromatic, and a positive charge is left spread round the rest of it.
Addition of the arene π system to the electrophile, giving the arenium ion. This is the slow step: the ring pays its aromatic stabilisation here and gets it back in the next one.
A base takes the proton and the ring comes back
The carbon that was attacked still carries its old hydrogen. Something basic takes it, those electrons drop back into the ring, and the ring is aromatic again.
Loss of the proton from the sp3 carbon restores the aromatic sextet and neutralises the cation. A substitution rather than an addition, because this step is faster than anything adding to the cation.
What this drawing assumes
- The cation in the middle is drawn as one of its three forms. The charge is really spread over the three positions away from where the attack happened, and that is what decides where the next group goes.
- Losing the proton is drawn as fast. It is, which is why these reactions do not usually stop at the cation.
Other pathways this class runs by
- With a very reactive electrophile the first step can be reversible, and the product seen is then the one that is most stable rather than the one that forms fastest.
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.
Imine formation
The amine adds to the carbonyl, the resulting alcohol-amine loses water, and a carbon–nitrogen double bond is left.
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.