Aromatic Halogenation

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The price of an SEAr is the temporary loss of aromaticity. This is an energy cost that has to be compensated by increasing the electrophilicity of the attacking species.

Benzene does not react with Br₂ alone, but with Br₂/FeBr₃ it gives bromobenzene

In bromination, the Br₂ molecule has to be activated with FeBr₃:

FeBr₃ coordinates to Br₂ and activates it: activated bromine is a stronger electrophile

The thermodynamic balance of the reaction depends on the halogen used:

Enthalpy of benzene halogenation: F₂, −114 kcal/mol (explosive, no catalyst); Cl₂, −30 and Br₂, −12 (with FeCl₃ or FeBr₃); I₂, +11 (needs an oxidant)

For instance, the energy diagram of chlorination is the following:

Energy diagram of the chlorination of benzene with Cl₂ and AlCl₃: two transition states and the benzenonium ion intermediate; the reaction releases 30 kcal/mol

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 used to make the activation energy surmountable, so that the reaction proceeds at a reasonable rate.