DFT Study of the SNAr Reactions of 1-Chloro-2-nitro- and 1-Phenoxy-2-nitro-benzenes with Aniline in Acetonitrile and Toluene: Concerted or Multistep Mechanism?

Computational studies to determine the rate constants and thermodynamic properties of the nucleophilic aromatic substitution reactions of 4-R-1-chloro-2,6-dinitrobenzenes 1, 6-R-1-chloro-2,4-dinitrobenzenes 2 and some of the corresponding 1-phenoxy derivatives 3 and 4 with aniline in the gas phase, acetonitrile and toluene is reported at the M06-2x/6-31g+(d,p) level of theory; R=H, NO2, CF3, CN and ring N. Formation of the Meisenheimer complex intermediate was observed in all cases and its stability was dependent on the number of electron-withdrawing groups present on the ring and their relative position. The overall reaction was found to be exothermic and exergonic in all cases. From kinetics calculations, the initial attack of the aniline nucleophile on the dinitrobenzene to form the intermediate is rate limiting with the chloride ion as the leaving group in acetonitrile and decomposition of the MC was rate limiting in vacuum and toluene. With the phenoxide ion as the leaving group, the decomposition of the MC intermediate is rate limiting in all cases. The observation of MC and thermodynamic data obtained are suggestive of the prevalence of a multistep SNAr mechanism and ring activation by electron withdrawing group increased the rate of reaction.

Automated summary

This study investigated the reaction mechanism and thermodynamic properties of nucleophilic aromatic substitution reactions of different functionalized dinitrobenzenes and corresponding phenoxy derivatives with aniline under different conditions using computational methods. The formation of Meisenheimer complex intermediates was observed, and their stability depended on the number and position of electron-withdrawing groups on the ring. The overall reactions were exothermic and exergonic. From kinetics calculations, it was found that the initial attack of aniline on dinitrobenzene was the rate-limiting step in acetonitrile, while the decomposition of the intermediate MC was the rate-limiting step in vacuum and toluene. The results suggest the prevalence of a multistep SNAr mechanism and indicate that electron-withdrawing groups can enhance the reaction rate.