Pitfalls in Assessing the α-Effect: Reactions of Substituted Phenyl Methanesulfonates with HOO−, OH−, and Substituted Phenoxides in H2O

Abstract

Toward resolving the current controversy regarding the validity of the α-effect, we have examined the reactions of Y-substituted phenyl methanesulfonates 1a-1l with HOO(-), OH(-), and Z-substituted phenoxides in the gas phase versus solution (H(2)O). Criteria examined in this work are the following: (1) Brønsted-type and Hammett plots for reactions with HOO(-)and OH(-), (2) comparison of β(lg) values reported previously for the reactions of Y-substituted phenyl benzenesulfonates 2a-2k with HOO(-) (β(lg) = -0.73) and OH(-) (β(lg) = -0.55), and for those of 1a-1l with HOO(-) (β(lg) = -0.69) and OH(-) (β(lg) = -1.35), and (3) Brønsted-type plot showing extreme deviation of OH(-) for reactions of 2,4-dintrophenyl methanesulfonate 1a with aryloxides, HOO(-), and OH(-), signifying extreme solvation vs different mechanisms. The results reveal significant pitfalls in assessing the validity of current interpretations of the α-effect. The extreme negative deviation by OH(-) must be due, in part, to the difference in their reaction mechanisms. Thus, the apparent dependence of the α-effect on leaving-group basicity found in this study has no significant meaning due to the difference in operating mechanisms. The current results argue in favor of a further criterion, i.e., a consistency in mechanism for the α-nucleophiles and normal nucleophiles.