Reactions of diphenyl ether with chlorine and bromine atoms around 750 K -Relevance for gas-phase "dioxin" formation

The title reactions have been studied to scrutinize rate data recently infe rred for the two reverse steps - reaction of phenoxy radicals with chlorobe nzene and bromobenzene - which were at variance with commonly accepted mode l values. Both with chlorine and bromine atoms, splitting to halobenzene an d phenoxy radical was found to occur in competition with abstraction of o-, m-, p-hydrogen atoms. On this basis, the displacements of Cl and Br from t he benzene ring by phenoxy radicals must have activation energies above 20 kcal/mol, and are therefore slow. As a consequence, formation of "dioxins" from halogenated phenols, in (slow) combustion, should proceed by combinati on of two (halo)phenoxy radicals rather than by displacement of (ortho-)hal ogen in a halophenol molecule.