Kinetics and mechanism of the base-catalysed reaction of 4-nitrophenyl (4-methylphenoxy) formate in benzene

Abstract

The kinetics of n-butylaminolysis of 4-nitrophenyl (4-methylphenoxy) formate (NPMPF) in benzene has been investigated at 27oC and in the presence of functionally similar, but structurally different addenda, namely; imidazole, pyridine and triethylamine. The reaction is catalysed by the nucleophile and imidazole in a linear fashion across the entire range of concentrations employed, giving values of k3/k2 ratio of 398 and 267.5 respectively. In the presence of pyridine and triethylamine, which respectively gave k3/k2 values of 260 and 780, the modes of catalyses differed very significantly. While the observed second order rate constant, kA, showed a quadratic dependence on pyridine concentration, it manifested a curvilinear (downward) dependence on the concentration of triethylamine. Analyses of the kinetic data have revealed that all additives, regardless of their differing modes of activity, manifested genuine base catalyses of the reaction in accord with a Bunnett-type (Specific Base-General Acid, SB-GA) mechanism in which electrophilic catalyzing models assist the detachment of the leaving group.


(Global Journal of Pure and Applied Sciences: 2003 9(4): 481-490)