Mechanism of the thermal decomposition of [di(benzoyloxy)iodo]benzene in bromobenzene

Abstract

The decomposition of [di(benzoyloxy)iodo]benzene in bromobenzene at 145.5 °C is first-order with k_obs(8.48 ± 0.40)× 10^–5 s^–1. Product studies indicate that concurrent heterolytic and homolytic decomposition of starting material occurs. Heterolysis of the iodo-compound gives phenyl benzoate and benzoyl hypoiodite. The hypoiodite reacts under the experimental conditions to give, as determined from the results of control experiments, 1-bromo-2-iodo-and-4-iodo-benzene by electrophilic iodination of the solvent, and iodobenzene by a Hunsdiecker type of process. The homolytic pathway gives carbon dioxide, benzoic acid, iodobenzene, 2-, 3-, and 4-bromobiphenyl, an involatile residue, and free iodine as major products. Direct evidence for a radical-chain decomposition of [di(benzoyloxy)iodo]benzene was not obtained. Control experiments indicate that such a reaction is possible, whereas the kinetic and product studies indicate that the chain decomposition is likely to be no more than of minor importance. It is proposed that free benzoyloxyl radicals are rapidly decarboxylated at 145.5 °C, and that most of the benzoic acid is derived from the reactions of the benzoyloxy(phenyl)iodo-radical, which behaves essentially as a stabilized benzoyloxyl radical. Control experiments provide circumstantial evidence for the existence of the radical PhİO₂CPh. The major reaction pathway is considered to be adequately represented by not less than twelve individual steps. The reactions are highly susceptible to the presence of traces of water, and consistent results could only be obtained under essentially anhydrous conditions.