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 p henoxide ions with bis(4-nitrophenyl) phenylphosphonate in aqueous as well as various aqueous dimethyl sulfoxide (Me(2)-SO) mixtures are rep orted. Traditional Bronsted plots of log k(ArO) vs pK(a)(ArOH) are lin ear 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 oc curs at a pK(a)(ArOH) value which is much higher than that correspondi ng to the basicity of the leaving p-nitrophenoxide ion, suggesting tha t the observed levelling off does not reflect a change in the identity of the rate-determining step in a two-step process involving formatio n of a pentacoordinate intermediate. An analysis of the data on the ba sis 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 Brons ted plots is the reflection of strong imbalances in the transition sta tes of the reactions, and it is suggested that these imbalances are es sentially of solvational nature.