Investigation of the electronic effect of the diphenylphosphino group

We have synthesized meta-substituted derivatives of benzoic acid, determined their ionization constants in water-alcohol solutions of concentration 11 and 14 by volume, and calculated the Hammett constant _m of the diphenylphosphino (0.12), diphenylamino (–0.04), diphenylphosphinyl (0.40) and diphenylthiophosphinyl (0.32) groups. Unlike the diphenylamine group, the diphenylphosphino group is metaorienting, which is probably to be explained by the absence or the weak nature of p- conjugation of incomplete electron pairs on the phosphorus and the presence of an electron-acceptor effect (d-p conjugation).