Solvolytic Reactivity of 2,4-Dinitrophenolates

A series of X,Y-substituted benzhydryl 2,4-dinitrophenolates (DNP, 1-5) were subjected to solvolysis in various methanol/water, ethanol/water, and acetone/water mixtures at 25 degrees C. The linear free energy relationship (LFER) equation, log k = s(f)(E-f + N-f), was used to derive the nucleofuge-specific parameters (N-f and S-f) for an S(N)1-type reaction. The magnitudes of nucleofugalities (N-f) are around zero, indicating that DNP falls in the middle of the established nucleofugality scale. The slope parameters (s(f)) and the Grunwald-Winstein m(OT)s parameters obtained demonstrate that benzhydryl DNPs solvolyze through a late transition state (TS) in which the negative charge delocalization causes considerably diminished solvation. Because of the late TS, the nucleofuge-specific slope parameters, s(f), are relatively high, i.e., the log k vs. E-f plots are steeper than for most of the previously investigated leaving groups. This may lead to intersection of the log k vs E-f plots that correspond to DNP and to some other leaving groups of similar reactivity, i.e., inversion of the relative reactivities may occur. Such inversion is shown here for DNPs and phenyl carbonates.