Electrophilic Substitution

Benzene has a high electron density in its ring of delocalised π electrons, so it is attacked by electrophiles: electron-pair acceptors, usually positive ions such as NO₂⁺ or Br⁺. Unlike alkenes, however, benzene reacts by substitution, not addition: the electrophile replaces one hydrogen atom on the ring. Substitution keeps the stable delocalised system, whereas addition would destroy it.Because benzene is so stable, a strong electrophile is needed, and it usually has to be generated using a catalyst.General mechanism (electrophile E⁺):Step 1: a pair of electrons from the delocalised ring is attracted to the electrophile and forms a new bond to E. The curly arrow starts at the ring and goes to E⁺. This partly breaks the delocalised system: the intermediate is a positive ion, drawn with a horseshoe of delocalised electrons over five carbon atoms and a + sign, while the sixth carbon is bonded to both E and H. The intermediate is less stable than benzene.Step 2: the C-H bond on that carbon breaks and its electron pair moves back into the ring, restoring the full delocalised ring. H⁺ is lost, and it usually regenerates the catalyst.Overall:
C₆H₆ + E⁺ → C₆H₅E + H⁺Worked example: in nitration the electrophile is NO₂⁺, so the product is C₆H₅NO₂, nitrobenzene. The product still has six delocalised electrons, so it keeps the stability of the ring. The same two-step mechanism applies to bromination, nitration and Friedel-Crafts reactions; only the electrophile changes.