Electrophilic aromatic substitution

Benzene's ring of six delocalised pi electrons is, in one sense, exactly the kind of electron-rich target that attracts electrophiles, just as an ordinary alkene's pi bond does. Yet where an alkene reacts with an electrophile by addition — the pi bond breaking and both carbons gaining new substituents — benzene instead reacts by substitution, in which one hydrogen on the ring is replaced by the electrophile while the aromatic ring itself survives intact. Understanding why requires following the mechanism through both of its steps and comparing the energetic cost of each possible outcome.

The first step, common to both reaction types, is the same: an electrophile is attracted to the ring's electron density and forms a new bond to one ring carbon, using two of the six delocalised pi electrons. This breaks the full aromatic delocalisation and produces a positively charged intermediate (sometimes called an arenium ion or Wheland intermediate) in which the remaining four pi electrons are delocalised over the other five carbons, and the carbon bonded to the electrophile is now sp3-hybridised, bearing both the electrophile and its original hydrogen.

At this point the two reaction types diverge. If the ring behaved like an ordinary alkene, a nucleophile would simply attack this cationic intermediate, permanently converting one ring carbon to sp3 and destroying aromaticity for good. Instead, the intermediate has an available path back to full aromaticity: losing the hydrogen still attached to the electrophile-bearing carbon, as a proton, regenerates a carbon-carbon pi bond there and restores the complete six-electron delocalised ring — the same enormous stabilisation energy (aromatic stabilisation) that made benzene so unreactive toward addition in the first place is recovered by taking this path, and essentially never recovered by the addition path, so substitution dominates overwhelmingly under normal conditions. The net transformation replaces one hydrogen with the electrophile while the aromatic ring itself — the six-membered ring with its full complement of delocalised electrons — is regenerated exactly as it started, just with one new substituent in place of one hydrogen.