Aromatic halogen displacement by cyclohexyl radicals

Abstract

Abstract Cyclohexyl radicals displace either halogen or hydrogen from dihalobenzenes to give mono‐ and di‐halogenated cyclohexylbenzenes. The unusual reactivity of fluorine in the sequence, F > I > Br > Cl, shows that breaking of the carbon‐halogen bond is not involved in the rate‐determining step. Halogens ortho to each other are consistently more reactive toward displacement than hydrogen in the same molecule. This higher reactivity is explained by a combination of electronic and steric effects plus the ability of halogen ortho to the reaction site to stabilize a cyclohexadienyl radical, while the preference for fluorine displacement is attributed to the nucleophilic nature of the cyclohexyl radical which reacts more readily at a position of low electron density.