Electrophilic Aromatic Substitution with Halogen
Electrophilic aromatic substitution of bromine is assisted by a ferric bromide catalyst, ferric chloride for chlorination. Unlike electrophilic addition to alkenes, a catalyst is necessary to enhance the electrophilicity of the halogen to react with an aromatic ring because aromatic π electrons are more stable than vinylic π electrons, Electrophilic aromatic substitution begins with the addition of the electrophile into the aromatic π system of the ring. A conjugated, carbocation intermediate is formed, a resonance combination of three forms, concentrating positive charge at three locations, the two ortho positions and one para relative to the halide substituent. After this addition, a proton departs, completing the overall substitution with aromaticity restored.
Consider the case where electrophilic aromatic substitution is attempted on an aromatic ring that already contains an original halogen substituent. If electrophilic aromatic substitution is attempted upon rings that already contain substituents, the location of the next substitution depends on the characteristics of the original substituent. Whether the new substitution occurs ortho, para or meta to the original substituent depends on whether the original substituent either stabilizes or destabilizes a concentration of positive charge upon its carbon at the carbocation intermediate stage. If the substituent already present is electron donating, it will stabilize the carbocation by donating negative charge. New substitutions will occur ortho or para to such electron donating substituents already present on the ring. Electron withdrawing substituents destabilize a carbocation, so the new substitution will most likely be meta. Being electronegative, halide substituents are ring deactivating by induction. However, nonbonded pairs of electrons are present on the halide substituent which can donate by resonance. Because of these combined effects, a halide substituent, already present on the ring, is a ring deactivating, ortho-para director for further electrophilic aromatic substitution.