Question
Write the mechanism of electrophilic aromatic substitution (EAS) for the bromination of benzene using . Explain the role of the Lewis acid catalyst and why substitution occurs instead of addition.
(JEE Main 2023, similar pattern)
Solution — Step by Step
alone is not electrophilic enough to attack benzene’s stable aromatic ring. The Lewis acid polarises the Br-Br bond:
The bromonium ion (or the strongly polarised complex) is the actual electrophile.
The -electrons of benzene attack the electrophile , forming a C-Br bond. This creates a carbocation intermediate called the arenium ion (sigma complex):
The arenium ion has a positive charge delocalised over three carbon atoms (ortho, para positions relative to the point of attack). Aromaticity is temporarily lost — the ring now has only 4 -electrons.
A base () removes a proton from the carbon bearing the bromine:
The catalyst is regenerated — it is truly catalytic. Aromaticity is restored in the product.
Why This Works
Why substitution, not addition? In the arenium ion intermediate, the ring can either: (a) Lose H → substitution product (aromatic, stable) (b) Add Br → addition product (not aromatic, less stable)
Restoring aromaticity (option a) provides about 150 kJ/mol of stabilisation energy. This thermodynamic driving force overwhelmingly favours substitution. The aromatic ring “wants” to get its 6 -electron system back.
Why is a Lewis acid needed? Benzene’s -cloud is electron-rich, but is not electrophilic enough on its own. makes one bromine atom more electron-deficient, creating a strong enough electrophile to overcome the activation barrier.
Alternative Method
Other Lewis acid catalysts work too: for Friedel-Crafts reactions, or for nitration ( electrophile), and for sulphonation. The mechanism is the same three-step pattern: (1) generate electrophile, (2) electrophilic attack to form arenium ion, (3) loss of proton to restore aromaticity.
For JEE, the EAS mechanism is the foundation for understanding directing effects (ortho/para vs meta directors) and activation/deactivation of the ring. If you understand why the arenium ion intermediate is stabilised by electron-donating groups at ortho/para positions, you can predict the product of any EAS reaction.
Common Mistake
Students sometimes write the mechanism as a one-step concerted process: . This skips the crucial arenium ion intermediate. EAS is a two-step mechanism: electrophilic addition (slow, rate-determining) followed by elimination of H (fast). The arenium ion is real and detectable — skipping it loses marks in both boards and JEE.