SEPARATION OF POLAR AND RESONANCE SUBSTITUENT EFFECTS IN THE REACTIONS OF ACETOPHENONES WITH BISULFITE AND OF BENZYL HALIDES WITH NUCLEOPHILES

Application of the modified Yukawa-Tsuno equation log (k/k0) pσn+pr(σ+-pσn+pr) to the reversible reaction of bisulfite with substituted acetophenones gives p = 0.6 and pr = 0.45 for the hydroxide ion catalyzed cleavage reaction, p = 1.8 and pr = 0.45 for attack of sulfite dianion, and p = 1.2 and pr = 0.95 for Keq for bisulfite addition. Rate constants for the cleavage reaction exhibit a U-shaped Hammett plot and require a negative value of the r+ parameter, although the reaction is facilitated by electron donation by resonance. Furthermore, the values of p and pr for the equilibrium and rate constants in the two directions are additive, but the r+ values are not. The same treatment accounts for most of the curvature in Hammett plots for the SN2 reactions of nucleophiles with benzyl halides without invoking changes in mechanism or transition-state structure. The changes in transition-state structure that do occur in some of these reactions are discussed and it is suggested that the “reactivity-selectivity principle” represents an oversimplification that should be abandoned for all but the simplest reactions. © 1979, American Chemical Society. All rights reserved.