THEORETICAL-STUDY OF THE ELECTROREDUCTION OF HALOGENATED AROMATIC-COMPOUNDS .3. O-DIBROMOBENZENES, M-DIBROMOBENZENES AND P-DIBROMOBENZENES STUDIES BY AM1 AND PM3 METHODS

The electroreductive potential values of the o-, m- and p-dibromobenze nes (DBBs) follow an unusual pattern in that, unlike the structurally related dichlorobenzene (DCB) derivatives, these three isomers exhibit a strong 'ortho effect' and are accompanied by a large difference in the E1/2 values. Any interference with the mechanism of reduction on t he part of the chemical environment can be safely ruled out, given the consistency of the E1/2 values obtained in four different solvents. T he use of theoretical indices calculated by the PM3 method enables the experimental behaviour of the DBBs to be rationalised on the basis of the electronic structure of the neutral isolated molecule. Remarkably , PM3 indicates the formation of a sigma-type radical anion in the cas e of the reduction of the bromobenzene derivatives.