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

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, kfMeOH/kfMeOD, and the equilibrium solvent isotope effects, KMeOH/KMeOD, 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.