Protonation Constants for Triarylphosphines in Aqueous Acetonitrile Solutions

Two independent methods were used to determine protonation constants K H for triarylphosphines in mixed acetonitrile/water media. One method is based on 31 P chemical shifts, and the other on the kinetics of debromination of a vicinal dibromide. The protonation constants obtained by the two methods agree well with each other, but they are several orders of magnitude smaller than the values previously reported in the literature for purely aqueous solutions. Moreover, K H decreases with increasing water content. For PPh 3 at 1.0 M ionic strength (CF 3 SO 3 H + CF 3 SO 3 Li), the values of K H are 24.0 ± 0.8 M - 1 (CH 3 CN/H 2 O = 90:10, v/v), 7.9 ± 0.7 (85:15), 4.2 ± 0.3 (80:20), 2.2 ± 0.1 (70:30), and 1.5 ± 0.3 (50:50). The binding of the proton is weaker at lower electrolyte concentrations, such that at μ = 0.40 M in 90:10 CH 3 CN/H 2 O K H for PPh 3 is only 3.9 ± 0.3 M - 1 .