Electrophilic Substitution

Benzene's delocalised ring is a region of high electron density, so it attracts electrophiles. An electrophile is an electron pair acceptor, usually a positive ion such as NO₂⁺ or RCO⁺.Alkenes react with electrophiles by addition, but benzene reacts by substitution: a hydrogen atom on the ring is replaced by the electrophile. Substitution restores the stable delocalised ring, while addition would destroy it and lose the delocalisation energy.The general mechanism, for an electrophile E⁺, has two steps.Step 1. A pair of electrons from the delocalised ring is attracted to E⁺ and forms a bond to it. One carbon now has both H and E attached and is no longer part of the delocalised system. The result is a positively charged intermediate, drawn as a horseshoe (a partial ring) with a + inside it.Step 2. The C-H bond on that carbon breaks. Its electrons return to the ring, restoring full delocalisation, and a hydrogen ion (H⁺) is lost.C₆H₆ + E⁺ → C₆H₅E + H⁺Because benzene is stable, the electrophile has to be strong, so it is usually generated in the reaction mixture with the help of a catalyst, which is regenerated when H⁺ is released. AQA covers two examples: nitration, where the electrophile is the nitronium ion NO₂⁺, and Friedel-Crafts acylation, where it is an acylium ion RCO⁺.