NONLINEAR BRONSTED RELATIONSHIPS IN THE REACTIONS OF PHENOXIDE IONS WITH BIS(4-NITROPHENYL) PHENYLPHOSPHONATE IN H2O-ME(2)SO MIXTURES - SOLVATION EFFECTS

The second-order rate constants (k(ArO)) for reaction of substituted phenoxide ions with bis(4-nitrophenyl) phenylphosphonate in aqueous as well as various aqueous dimethyl sulfoxide (Me(2)-SO) mixtures are reported. Traditional Bronsted plots of log k(ArO) vs pK(a)(ArOH) are linear at low pK(a)(ArOH) but show very strong downward curvature at high pK(a)(ArOH). In each solvent, the break point in the Bronsted plot occurs at a pK(a)(ArOH) value which is much higher than that corresponding to the basicity of the leaving p-nitrophenoxide ion, suggesting that the observed levelling off does not reflect a change in the identity of the rate-determining step in a two-step process involving formation of a pentacoordinate intermediate. An analysis of the data on the basis of a novel strategy recently designed by Buncel and co-workers for the construction of Bronsted plots in solvent mixtures confirms this conclusion. It is proposed that the curvature in the traditional Bronsted plots is the reflection of strong imbalances in the transition states of the reactions, and it is suggested that these imbalances are essentially of solvational nature.