Substituent effects on the acidities and rates of ionisation of protonated meso-tetra-arylporphyrins

Abstract

Equilibrium constants and rates of equilibration between the basic (B) and diprotonated (BH₂²⁺) forms of meso-tetraarylporphyrins with various para-substituents in the phenyl rings have been determined in 90% Me₂SO–H₂O (v/v) for comparison with previous results for meso-tetraphenylporphyrin. Spectrophotometric measurements were unable to detect the presence of significant concentrations of the monoprotonated porphyrins and an overall equilibrium constant K=[B][H₃O⁺]²/[BH₂²⁺] was measured. Values of the rate coefficients for ionisation of the di- to the mono-protonated porphyrins were obtained from the kinetic results. These rate coefficients and the acidities of the diprotonated porphyrins, as measured by the values of K, increase as the substituent is made more strongly electron withdrawing.