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 inferred for the two reverse steps – reaction of phenoxy radicals with chlorobenzene and bromobenzene – which were at variance with commonly accepted model values. Both with chlorine and bromine atoms, splitting to halobenzene and 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 the 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 combination of two (halo)phenoxy radicals rather than by displacement of (ortho-)halogen in a halophenol molecule.