CATION RADICALS AS INTERMEDIATES IN AROMATIC HALOGENATION WITH IODINE MONOCHLORIDE - SOLVENT AND SALT EFFECTS OIL THE COMPETITION BETWEEN CHLORINATION AND IODINATION

Three distinct classes of substitutional reactivity can be discerned in the halogenation of a series of methyl-substituted methoxybenzenes (ArH) with iodine monochloride (ICl), namely, exclusive iodination, exclusive chlorination, and mixed iodination/chlorination. Spectral studies establish the prior formation of the charge-transfer complex [ArH,ICl] which suffers electron transfer to afford the reactive triad [ArH.+, I-., Cl-] according to Scheme 1. Separate reactivity studies show that chlorination and iodination can result from the quenching of the aromatic cation radical by chloride and iodine (atom), respectively. Iodination versus chlorination thus represents the competition between radical-pair and ion-pair collapse from the reactive triad, and it is predictably modulated by solvent polarity and added salt.