International Journal of Chemical Kinetics p. 153 - 164 (2000)
Update date:2022-08-04
Topics:
Khan, M. Niyaz
Arifin
George
Wahab
Nucleophilic second-order rate constants, knms, for the reactions of several primary and secondary amines with ionized phenyl salicylate (PS-) show a nonlinear decrease with the increase in the content of CH3CN from 2 to ≤50% v/v in mixed aqueous solvent. The values of knms remain almost unchanged with the change in the content of CH3CN at >50% v/v. The nucleophilic reactivity of primary and secondary amines toward PS- reveal Bronsted plots of different Bronsted coefficients, βnuc, at a constant content of CH3CN in mixed aqueous solvents. The values of βnuc decrease from 0.4 to nearly 0 for primary amines and from 0.22 to 0.12 for secondary amines with the increase in CH3CN content from 2 to 70% v/v. The values of knms/kMeOHms (where kMeOHms represents the nucleophilic second-order rate constant for the reaction of MeOH with PS- in H2O-CH3CN solvents), obtained within 2-50% v/v CH3CN, fit to an empirical equation: log (knms/kMeOHms) = θ+λX where X is the % v/v content of CH3CN, and θ and λ are empirical constants. It has been shown empirically that both θ and λ are the function of Bronsted coefficient βlg. The values of λ are used to calculate βlg and these βlg values for all amines except Tris lie within -0.32 to -0.46. The effects of mixed water-acetonitrile solvents on pKa of leaving the group, phenol, and conjugate acid of amine nucleophile have been concluded to be the major source for the observed solvent effects on knms. The βlg for Tris is unusually very close to zero.
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