Electronic effect of substituents on the hydrodesulfurization of ring substituted benzenethiols

Article abstract of DOI:10.1016/0304-5102(93)85104-2

To clarify the electronic factors of the reactants affecting hydrodesulfurization (HDS) reactivity, the o-, m- and p-isomers of aminobenzenethiol (ABT), methoxybenzenethiol (MBT) and toluenethiol (TT) were hydrogenolyzed by a batch method over CoS₂, NiS₂ and a presulfided commercial HDS catalyst. In the hydrogenolysis of all the isomers of ABT, MBT and TT, HDS by cleavage of the C-S bond occurred selectively and was promoted by the presence of electron-releasing substituents in the ortho- and para-positions. Among the quantities obtained from the MINDO/3 calculation for a reactant, the next three are especially interesting: these are the coefficients of the ipso-carbon and the sulfur atoms in the highest occupied π-orbital (π-HOMO), C_C^HOMO and C_S^HOMO, and the energy level of π-HOMO. The differences in reactivity among the isomers of a substituted benzenethiol can be interpreted by use of the frontier π-electron densities (FED), 2(C_C^HOMO)² and 2(C_S^HOMO)². On the other hand, the differences in reactivity among the molecules, i.e., ABT, MBT, TT, and benzenethiol, shows a close correlation with the ratio of the two FEDs, (C_C^HOMO/C_S^HOMO)², and also with the energy level of π-HOMO. It is suggested that the energy level and the FED assume an important role in the HDS reactivities and that the magnitudes of the FEDs on the positions of both the sulfur and the ipso carbon atoms affect the reactivities not independently but concertedly.