Monosubstitution versus Disubstitution in the SRN1 Reaction of Dihalobenzenes with Sulfanions. The Role of the Monosubstitution Product and of Its Anion Radical

Article abstract of DOI:10.1021/jo00285a015

The competition between mono- and disubstitution of dihalobenzenes by a series of aromatic sulfanions, via the S_RN1 reaction, is shown to involve two radical chains.The first one, recognized in earliest works, involves one branching point at the level of the monosubstituted product anion radical.Reoxidation of the latter via electron transfer to the parent dihalide affords the monosubstituted product.Conversely, the route to the disubstituted product is opened when cleavage of the carbon-halogen bond in the monosubstituted product anion radical occurs before the electron transfer takes place; the disubstitution product is then obtained in its reduced (anion radical) form.Reoxidation of the latter, to afford the neutral disubstituted product, may involve competitively the parent dihalide or the neutral monosubstituted product, depending on the electron affinity of the arylthio moiety.In the first case the electron transfer propagates the first chain; in the second a new chain leading to the disubstitution is activated.The role of the two imbricated chains, under photochemical conditions, and thus that of the monosubstitution product is discussed quantitatively on the basis of the pertinent rate constants determined by cyclic voltammetry.