Aminolysis of 2,4-dinitrophenyl X-substituted benzoates and Y-substituted phenyl benzoates in MeCN: effect of the reaction medium on rate and mechanism.
Second-order rate constants (k(N)) have been determined spectrophotometrically for the reactions of 2,4-dinitrophenyl X-substituted benzoates (1 a-f) and Y-substituted phenyl benzoates (2 a-h) with a series of alicyclic secondary amines in MeCN at 25.0 +/- 0.1 degrees C. The k(N) values are only slightly larger in MeCN than in H2O, although the amines studied are approximately 8 pK(a) units more basic in the aprotic solvent than in H2O. The Yukawa-Tsuno plot for the aminolysis of 1 a-f is linear, indicating that the electronic nature of the substituent X in the nonleaving group does not affect the rate-determining step (RDS) or reaction mechanism. The Hammett correlation with sigma- constants also exhibits good linearity with a large slope (rho(Y) = 3.54) for the reactions of 2 a-h with piperidine, implying that the leaving-group departure occurs at the rate-determining step. Aminolysis of 2,4-dinitrophenyl benzoate (1 c) results in a linear Brønsted-type plot with a beta(nuc) value of 0.40, suggesting that bond formation between the attacking amine and the carbonyl carbon atom of 1 c is little advanced in the transition state (TS). A concerted mechanism is proposed for the aminolysis of 1 a-f in MeCN. The medium change from H2O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T+/-) in aprotic solvent.
Um
Ik-Hwan
IH
Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea. ihum@mm.ewha.ac.kr
Jeon
Sang-Eun
SE
Seok
Jin-Ah
JA
D000097:Acetonitriles / Q000737:chemistry; D000588:Amines / Q000737:chemistry*; D001565:Benzoates / Q000737:chemistry*; D007700:Kinetics; D009025:Morpholines / Q000737:chemistry; D010880:Piperidines / Q000737:chemistry
D000097:Acetonitriles; D000588:Amines; D001565:Benzoates; D009025:Morpholines; D010880:Piperidines; C032727:piperidine; C037574:morpholine; C032159:acetonitrile