Nonlinear Broensted Relationships in the Reaction of Phenoxide Ions with Bis(4-nitrophenyl) Phenylphosphonate in H2O-Me2SO Mixtures. Solvation Effects

Article abstract of DOI:10.1021/jo00111a037

The second-order rate constants (k^ArO) for reaction of substituted phenoxide ions with bis(4-nitrophenyl) phenylphosphonate in aqueous as well as vatious aqueous dimethyl sulfoxide (Me2SO) mixtures are reported.Traditional Broensted plots of log k^ArO vs pK^ArOH_a are linear at low pK^ArOH_a but show very strong downward curvature at high pK^ArOH_a.In each solvent, the break point in the Broensted plot occurs at a pK^ArOH_a 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 Broensted plots in solvent mixtures confirms this conclusion.It is proposed that the curvature in the traditional Broensted 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.