Effects of mixed H2O?CH3CN solvents on the Br�nsted coefficient for the intramolecular general base-catalyzed cleavage of ionized phenyl salicylate in the presence of primary and secondary amines

Abstract

Nucleophilic second-order rate constants, k n ms , for the reactions of several primary and secondary amines with ionized phenyl salicylate (PS Ϫ ) show a nonlinear decrease with the increase in the content of CH 3 CN from 2 to Յ50% v/v in mixed aqueous solvent. The values of k n ms remain almost unchanged with the change in the content of CH 3 CN at Ͼ50% v/v. The nucleophilic reactivity of primary and secondary amines toward PS Ϫ reveal Brønsted plots of different Brønsted coefficients, ␤ nuc , at a constant content of CH 3 CN in mixed aqueous solvents. The values of ␤ nuc decrease from 0.4 to nearly 0 for primary amines and from 0.22 to 0.12 for secondary amines with the increase in CH 3 CN content from 2 to 70% v/v. The values of k n ms /k MeOH ms (where k MeOH ms represents the nucleophilic second-order rate constant for the reaction of MeOH with PS Ϫ in H 2 O-CH 3 CN solvents), obtained within 2-50%v/v CH 3 CN, fit to an empirical equation: log (k n ms /k MeOH ms ) ϭ ϩ X where X is the % v/v content of CH 3 CN, and and are empirical constants. It has been shown empirically that both and are the function of Brønsted coefficient ␤ lg . The values of are used to calculate ␤ lg and these ␤ lg values for all amines except Tris lie within Ϫ0.32 to Ϫ0.46. The effects of mixed water-acetonitrile solvents on pK a of leaving the group, phenol, and conjugate acid of amine nucleophile