Scope
If the ring contains a strongly activating substituent such as -OH, -OR or amines, a catalyst is not necessary, for example in the bromination of p-cresol:
However, if a catalyst is used with excess bromine, then a tribromide will be formed.
Halogenation of phenols is faster in polar solvents due to the dissociation of phenol, with phenoxide ions being more susceptible to electrophilic attack as they are more electron-rich.
Chlorination of toluene with chlorine without catalyst requires a polar solvent as well such as acetic acid. The ortho to para selectivity is low:
No reaction takes place when the solvent is replaced by tetrachloromethane. In contrast, when the reactant is 2-phenyl-ethylamine, it is possible to employ relatively apolar solvents with exclusive ortho- regioselectivity due to the intermediate formation of a chloramine making the subsequent reaction step intramolecular.
The food dye erythrosine can be synthesized by iodination of another dye called fluorescein:
This reaction is driven by sodium bicarbonate.
Read more about this topic: Electrophilic Halogenation
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