NONLINEAR STRUCTURE-REACTIVITY CORRELATIONS . REACTIVITY OF NUCLEOPHILIC REAGENTS TOWARD ESTERS

Rate constants are reported for the reactions of water and of nucleophiles of pK = 3.1-15.8 with a series of acetate esters with leaving groups of pK = 10-2. The reactions with oxygen anions exhibit a small sensitivity to the basicity of both the attacking group and the leaving group when the former is more basic than the latter and a large sensitivity to the basicity of both groups in the converse case. Logarithmic plots of rate constants against the pK of the attacking and leaving groups are nonlinear with limiting slopes, β, approaching 0.3 and 1.0 in both cases. The behavior of nucleophiles with “abnormal” reactivity in such plots is governed by their basicity rather than their absolute nucleophilic reactivity, so that inversions of relative reactivities may occur as the leaving group is varied. The reactions of “normal” nitrogen nucleophiles show β values for the nucleophile close to 0.8 over a range of 108 in reaction rate and then level off to a much smaller sensitivity to basicity in the reactions of strongly basic amines with the most reactive esters. Values of β for varying leaving groups are close to 1.0 for most reactions, indicating a large change in the charge on the leaving group in the transition state, and decrease to 0.4 for the rapid reactions. It is argued that no tetrahedral addition intermediate is formed before the transition state is reached in any of the aminolysis reactions. The similarity in the behavior of primary, secondary, and tertiary amines means that proton transfer is not required in these reactions and that amine attack and leaving-group expulsion can occur through transition state(s) of zero net charge. The results do not discriminate unequivocally between a concerted mechanism and one involving a metastable tetrahedral intermediate for these reactions. © 1968, American Chemical Society. All rights reserved.