Electron spin resonance studies. Part 65. The selectivity of radical attack on halogen-containing compounds

Abstract

E.s.r. experiments with some carboxylic acid derivatives containing iodine (e.g. CH₂ICO₂^–) show that Ph·,·OH, and CO₂^–˙ all react via iodine-rather than hydrogen-abstraction. Examples of bromine abstraction from related compounds (e.g. MeCHBrCO₂^–) by both Ph˙ and CO₂^–˙ are reported; these reactions are evidently facilitated by the presence of the electron-withdrawing carboxylate substituent adjacent to the halogen. With ·OH the exothermic hydrogen-atom abstraction appears to be preferred to the endothermic abstraction of bromine, though some examples of the latter are noted. For β-halogen-substituted radicals of the type ·CHXCH₂Y (X = CO₂^–, CN, SO₃^–, Y = Br, and X = CO₂^–, Y = Cl) the results indicate the occurrence of a ready homolytic cleavage to give Y· and CH₂CHX, which undergoes further reaction. Examples of oxidative decarboxylation by SO₄^–˙ are also described.