Mechanism of Electrophilic Aromatic Substitution

Benzene's delocalised pi electron cloud makes the ring, like an alkene's pi bond, attractive to

electrophiles -- but because addition across any one carbon-carbon bond would destroy the

resonance-stabilised aromatic system (a costly loss of the $\approx 150\ \text{kJ mol}^{-1}$

resonance energy), benzene reacts instead by substitution, replacing a ring hydrogen with the

electrophile while regenerating the fully aromatic ring at the end of the reaction. Every

electrophilic aromatic substitution (EAS) -- nitration, sulphonation, halogenation, and the

Friedel-Crafts reactions taken up next -- follows the same three-stage mechanistic pattern.

Stage 1: generation of the electrophile. Benzene's ring is not reactive enough to attack a

neutral reagent directly, so the actual attacking electrophile is first generated by a separate

acid-base or catalysed step. For nitration, concentrated nitric acid is protonated by

concentrated sulphuric acid (acting as a stronger acid), which then loses a molecule of water to

generate the nitronium ion, the true electrophile:

For sulphonation, fuming sulphuric acid (oleum) supplies free sulphur trioxide, $\text{SO}_3$,

itself a powerful electrophile at its sulphur atom (which is short two electrons in one resonance

form and readily accepts a pair from the ring).

Stage 2: electrophilic attack forms the arenium ion. The ring's pi electrons attack the

electrophile, forming a new sigma bond from one ring carbon to the electrophile. Doing so converts

that carbon from $sp^2$ to $sp^3$ (breaking it out of the ring's plane and the ring's conjugation

at that one position) and leaves a positive charge delocalised over the remaining five, still

conjugated, ring carbons. This resonance-stabilised carbocation intermediate is called the

arenium ion (or sigma complex, or Wheland intermediate); it can be drawn as three

contributing resonance structures with the positive charge on the carbons ortho and para to the

newly attached electrophile, which is why it is meaningfully more stable -- and this substitution

pathway meaningfully more favourable -- than a comparable non-aromatic carbocation would be, even

though the arenium ion is itself no longer fully aromatic and is, overall, higher in energy than

the starting benzene.

Stage 3: loss of a proton restores aromaticity. A base (often the conjugate base generated in

stage 1, e.g. $\text{HSO}_4^-$) removes the hydrogen still attached to the $sp^3$ carbon that was

just attacked; the electron pair from that $\text{C--H}$ bond flows back into the ring, the carbon

reverts to $sp^2$, and the ring regains its full, six-pi-electron aromatic delocalisation --

this restoration of the large resonance-stabilisation energy is the strong thermodynamic driving …