Brønsted βNuValues and Leaving Group Effects in SN2 Reactions. Tests of the Reactivity—Selectivity Postulate

The rate constants for reactions of a family of 19 carbanions, ArCHSO2Ph−(derived from benzyl phenyl sulfones) withn‐butyl chloride have been measured in Me2SO solution. A plot of logkvs. the pKaof the conjugate acids for 12 of these carbanions give a linear plot (R2= 0.999) with a Brønsted coefficient of βNu= 0.402. Points for para electron‐withdrawing substituents, SPh, SOPh, SO2Ph. COPh, CN and NO2, deviate substantially from the plot. The deviations are attributed to the enhanced solvation of these remote substituents in the anion which leads to rate retardation. A curved Brønsted plot can be drawn through all the points, which would be consistent with the predictions of the Hammond–Leffler postulate (HLP) and the reactivity–selectivity postulate (RSP), but this interpretation is rejected. Instead, it is suggested that the apparent curvature in Brønsted plots for acid–base reactions – upon which HLP and RSP are based – is also caused by deviations due to solvent effects, donor atom effects in the bases, mechanistic changes and/or the failure to keep electronic and steric effects constant. A reactivity–selectivity plot for reactions of 9 ArCHSO2Ph−ions withn‐butyl bromide andn‐butyl chloride indicated constant selectivity. A similar plot for 4 carbanions derived from α‐methylbenzyl phenyl sulfones reacting withn‐butyl bromide andn‐butyl chloride also showed constant selectivity. Based on these results and an examination of the literature, it is concluded that there is no firm experimental basis for HLP and RSP.