Nucleophilicities of para‐substituted aniline radical cations in acetonitrile: Kinetic investigation and structure–reactivity relationships

Abstract

AbstractThe kinetics of the coupling of 2‐bromo‐3,5‐dinitrothiophene 1 with various anilines 2a–f were studied in CH3CN at 20°C. The derived second‐order rate constants of anilines possessing an electron‐withdrawing group (e.g., 2d–f) were employed to determine the electrophilicity parameter E of the thiophene 1 according to the correlation equation log k20°C = sN (E + N), where N and sN are nucleophile‐specific parameters. Nonlinear Brønsted and Hammett relationships are discussed, and it is shown that the reactions for donor anilines 2a–d proceed through a single‐electron transfer mechanism. Further support for this assumption was given by the observation that the nucleophilicity parameters N and E° values of oxidation potential constants are linearly related in a direct relationship with a correlation parameter of R2 = 0.9914. The effect of aniline nucleophilicity on reactivity was examined quantitatively on the basis of kinetic measurements, leading to a nonlinear relationship of log (k20°C) with nucleophilicity parameters (N). It is suggested that the observed nonlinear Mayr correlation can be effectively explored to evaluate the nucleophilicity parameters N of four‐substituted aniline radical cations XC6H4NH2+• (X = OH, OMe, Me) and studying their reactivity patterns. On the other hand, it is confirmed that the N values of HOC6H4NH2+• (15.60), MeOC6H4NH2+• (15.23) and MeC6H4NH2+• (14.19) thus obtained can be used to predict the second‐order rate constants for their reactions with N1‐methyl‐4‐nitro‐2,1,3 benzothiadiazolium tetrafluoroborate of known electrophilicity E. This study was one of the first that employed the Mayr approach for determining nucleophilicity parameters of radical cations.