Equilibrium addition of nucleophiles to carbon–nitrogen double bonds. Kinetics of the addition of propanethiol to benzylideneanilines in non-aqueous solutions

Abstract

Rate constants for the addition of propanethiol to benzylideneanilines in methanol–acetonitrile (90 : 10 v/v) have been determined by following the reaction spectrophotometrically and by estimation of equilibrium constants. Electron-releasing groups (R¹ or R²) in strongly basic Schiff's bases such as R¹C₆H₄CHNPh and PhCHNC₆H₄R² and electron-withdrawing groups (R²) in weakly basic Schiff's bases such as p-NO₂C₆H₄CHNC₆H₄R² increase the second-order rate constants. These facts together with the occurrence of acid and base catalysis suggest a change in mechanism from acid catalysed addition of the thiol to the more basic Schiff's bases to attack of the thiolate ion on the unprotonated less basic Schiff's basic Schiff's bases. The kinetic solvent isotope effects, k_f^MeOH/k_f^MeOD, and the equilibrium solvent isotope effects, K^MeOH/K^MeOD, are 1·5 and 0·50 for N-benzylidene-m-chloroaniline and 1·4 and 0·52 for N-p-nitrobenzylidene-m-chloroaniline, respectively. The nucleophilic reactivity of the thiolate ion toward azomethine is compared with that of methoxide.