Arylphosphonic acids. I. Substituent effects on their first and second dissociations

Abstract

Thermodynamic pK1 and pK2 values of 36 arylphosphonic acids have been determined. The pK values of the meta and para compounds are correlated well by the σn parameter, suggesting that there is little conjugation between the phosphonic group and the aromatic ring; the ρ values of the first and second dissociations are 0.923 and 1.140, respectively. The —PO3H− group itself appears to be electron donating, contrary to earlier literature reports. The effect of ortho groups has been examined using multivariate analysis; the steric component of the proximity effect is acid weakening and has a larger effect on the second dissociation than on the first, presumably because a greater change in degree of solvation accompanies the second dissociation than the first.