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 in a basic environment 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-phenylethylamine, it is possible to employ relatively apolar solvents with exclusive ortho- regioselectivity due to the intermediate formation of a chloramine, enabling the Intramolecular reaction. The food dye erythrosine can be synthesized by iodination of another dye called fluorescein: This reaction is driven by sodium bicarbonate. See also Electrophilic aromatic substitution Electrophilic fluorination Halogenation reactions References Halogenation reactions