A Kinetic Study for the Reaction of 2,4-Dinitrophenyl Benzoate with Secondary Cyclic Amines

Abstract

Apparent second-order rate constants (kapp) have been measured spectrophotometrically for the reaction of 2,4- dinitrophenyl benzoate (DNPB) with 6 secondary cyclic amines in H2O containing 20 mole% DMSO at 25.0± 0.1°C. The Bronsted-type plot (log kapp vs. pKap) shows a break at pKa near 9.1, e.g. two straight lines with βapp values of 0.67 and 0.44 for the low basic (pKa<9.1) and the highly basic (pKa>9.1) amines, respectively. Using an estimated k2 value of 3 ×l 109 sec-1, all the other microconstants (k1, k-1 and K) involved in the present aminolysis have been calculated. The k-1 value decreases with increasing the basicity of amines while k1 and K values increase with increasing the amine basicity, as expected. Good linear Brönsted-type plots have been obtained for these microconstants of the present aminolysis of DNPB. The magnitudes of the slope of the Brönsted-type plots, k1, and k-1, have been calculated to be 0.43 and - 0.24, respectively, indicating the k-1 step is about two folds less sensitive than the k1 step to the amine basicity. The K value has been calculated to be 0.66, which appears to be much smaller than the one for other aminolyses showing general base catalysis. The small K value has been attributed to the absence of general base catalysis in the present aminolysis of DNPB.