SEAr Mechanism

An electron-deficient reagent attacks (or is attacked by) the electron-rich π cloud of benzene, giving a new benzene derivative (aromaticity is preserved) in which a hydrogen has been replaced by a functional group. This kind of reaction is called electrophilic aromatic substitution (SEAr).

Outline of SEAr: benzene, electron rich (nucleophile), attacks the electron-poor electrophile E+ and, with the help of a catalyst, an H is replaced by E

The reaction takes place when the HOMO (highest occupied molecular orbital) of benzene transfers electrons to the LUMO (lowest unoccupied molecular orbital) of the electrophile, leading to a positively-charged non-aromatic intermediate (benzenonium ion). The reacting carbon of benzene changes from its initial trigonal sp² hybridization to tetrahedral sp³ in the benzenonium ion intermediate.

The intermediate benzenonium ion, although higher in energy because of the loss of aromaticity, has a certain stability because the positive charge can be somewhat delocalized over the ortho and para positions relative to the attack site. The loss of a proton regenerates the aromaticity. The lost proton is captured by E+'s counterion.

SEAr mechanism: the ring attacks E+ and forms the benzenonium ion, stabilized by three resonance forms, which loses H⁺ and recovers its aromaticity

Have a look at the attack in motion. The electrophile approaches the aromatic ring in a perpendicular trajectory to the ring plane, just in the very direction of the p orbital of the reacting carbon.

As the new bond forms, the reacting carbon gradually changes from a trigonal to a tetrahedral geometry.

Animation of the attack of an electrophile E+ on the π cloud of benzene, with the electrostatic potential map of the ring

The energy diagram of the SEAr is as follows:

Energy diagram of SEAr: a first transition state with activation energy Ea, the benzenonium ion intermediate and a lower second transition state

What is the overall energy balance of the SEAr with chlorine, for instance?

Bond enthalpies (kcal/mol): Cl–Cl, 58; Cl–C₆H₅, 96; H–C₆H₅, 111; H–Cl, 103

ΔH° = ΔH°(Cl–Cl) + ΔH°(H–C₆H₅) − [ΔH°(H–Cl) + ΔH°(Cl–C₆H₅)] = 58 + 111 − (103 + 96) = −30 kcal/mol

SEAr reactions are usually thermodynamically favorable. Catalysts are generally used to lower the activation energy so that the reaction proceeds at a reasonable rate.