Kinetic Study on Nucleophilic Substitution Reactions of 4-Chloro-2-nitrophenyl X-Substituted-benzoates with Cyclic Secondary Amines: Effect of Substituent X on Reactivity and Reaction Mechanism

Second-order rate constants (kN) have been measured spectrophotometrically for the reactions of 4-chloro-2-nitrophenyl X-substituted-benzoates (1a-1h) with a series of cyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. The Hammett plot for the reactions of 1a-1h with piperidine consists of two intersecting straight lines, while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρX = 1.25 and r = 0.58, indicating that the nonlinear Hammett plot is not due to a change in the rate-determining step (RDS) but is caused by ground-state stabilization through resonance interactions for substrates possessing an electron-withdrawing group in the benzoyl moiety. The Brønsted-type plot for the reactions of 4-chloro-2-nitrophenyl benzoate (1d) with a series of cyclic secondary amines curves downward with β2 = 0.85, β1 = 0.24, and pKa o = 10.5, implying that a change in RDS occurs from the k2 step to the k1 process as the pKa of the conjugate acid of the amine exceeds 10.5. Dissection of kN into the microscopic rate constants k1 and k2/k−1 ratio associated with the reaction of 1d reveals that k2 is dependent on the amine basicity, which is contrary to generally held views