Kinetic and Mechanism of Benzylation of Anilines

Second-order rate constants for the reactions of benzyl halides (Cl, Br, I) and some para-substituted benzyl chlorides (OCH3, CH3, C1, NOz) with substituted anilines (p-OCH3, p-CH3, H, p-C1, m-C1) were measured in ethanol solution. The reactivity order found for benzyl halides (I > Br > C1) is in accord with the leaving group polarizability. The reaction rate is increased by electron-donating substituents and decreased by electron-withdrawing ones, both in the nucleophile and in the substrate. Hammett plots are linear on varying the substituents in the aniline, but show remarkable curvature on varying those in the benzyl chloride. The reaction can be adequately described as an sN2 bimolecular process. The reaction rate depends on the electronic availability on the nitrogen atom (bond making) and on the mesomeric interaction between the substrate substituent and the reaction center, which favors the halogen displacement (bond breaking). Electron-donating (electron-withdrawing) groups make the transition state looser (tighter). The C-halogen bond breaking is more advanced with respect to the C-N bond formation.