Enthalpy–Entropy Correlations in Reactions of Aryl Benzoates with Potassium Aryloxides in Dimethylformamide

Temperature dependences of the relative reactivity of potassium aryloxides XC 6 H 4 O − K + toward 4‐nitrophenyl ( 1 ), 3‐nitrophenyl ( 2 ), 4‐chlorophenyl ( 3 ), and phenyl ( 4 ) benzoates in dimethylformamide (DMF) were studied using the competitive reactions technique. The rate constants k X for the reactions of 1 with potassium 4‐cyanophenoxide, 2 with potassium 3‐bromophenoxide, 3 with potassium 3‐bromo‐, 4‐bromo‐, and unsubstituted phenoxides, 4 with potassium 4‐methoxy‐ and 3‐methylphenoxides were measured at 25°C. Correlation analysis of the relative rate constants k X / k H(3‐Me) and differences in the activation parameters (∆∆ Н ≠ and ∆∆ S ≠ ) of competitive reactions revealed the existence of six isokinetic series. We investigated the substituent effect of X on the activation parameters for each isokinetic series and concluded that the reactions of aryl benzoates PhCO 2 C 6 H 4 Y with potassium aryloxides in DMF proceed via a four‐step mechanism. The large ρ 0 (Y) and ρ XY values at 25°C obtained for the reactions of 1–3 with potassium aryloxides with an electron‐donating substituent refer to the rate‐determining formation of the spiro ‐σ‐complex. The Hammett plots for the reactions of 1 and 2 exhibit a downward curvature, causing the motion of the transition state for the rate‐determining step according to a Hammond effect as the substituent in aryloxide changes from electron‐donating to electron‐withdrawing. Analysis of data in the terms of two‐dimensional reaction coordinate diagrams leads to the conclusion that significant anti‐Hammond effects arise in the cases of ortho‐substituted and unsubstituted substrates. It was shown that the isokinetic and compensation effects observed for the reactions of aryl benzoates with potassium aryloxides in DMF can be interpreted in the terms of the electrostatic bonding between the reaction centers.