Electronic nature of substituent X governs reaction mechanism in aminolysis of 4-pyridyl X-substituted-benzoates in acetonitrile.
A kinetic study is reported for aminolysis of 4-pyridyl X-substituted-benzoates 5a-i. Plots of pseudo-first-order rate constants (k(obsd)) vs [amine] curve upward for the reactions of substrates possessing a strong electron-withdrawing group in the benzoyl moiety (5a-d) but are linear for the reactions of those bearing an electron-donating group (5e-i), indicating that the electronic nature of substituent X governs the reaction mechanism. The k(1)k(2)/k(-1) and k(1)k(3)/k(-1) values were calculated from the intercept and slope of the linear plots of k(obsd)/[amine] vs [amine], respectively. The Hammett plot for k(1)k(2)/k(-1) consists of two intersecting straight lines, while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρ(X) = 0.41 and r = 1.58, implying that the nonlinear Hammett plot is not due to a change in rate-determining step but is caused by stabilization of substrates possessing an electron-donating group through resonance interactions. The small ρ(X) suggests that the k(2)/k(-1) ratio is little influenced by the nature of substituent X. The Brønsted-type plots for aminolysis of 4-pyridyl 3,5-dinitrobenzoate 5a are linear with β(nuc) = 0.98 and 0.79 for k(1)k(2)/k(-1) and k(1)k(3)/k(-1), respectively. The effect of amine basicity on the microscopic rate constants is also discussed.
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea. ihum@ewha.ac.kr
Ik-Hwan
IH
Um
Ae Ri
AR
Bae
D000097:Acetonitriles / Q000737:chemistry*; D000588:Amines / Q000737:chemistry*; D001565:Benzoates / Q000737:chemistry*; D004583:Electrons; D007700:Kinetics; D015394:Molecular Structure
D000097:Acetonitriles; D000588:Amines; D001565:Benzoates; C032159:acetonitrile