Квантово-химическое исследование механизмов реакций бензамида и бензолсульфонамида с хлорангидридом 3-нитробензолсульфокислоты в газовой фазе

Abstract

Quantum-chemical simulation of mechanisms of 3-nitrobenzenesulfonic acid chloride interactions with benzoic and benzenesulfonic acids amides in the gas phase was carried out by calculating the three-dimensional potential energy surfaces of these reactions (DFT//B3LYP/6-311G(d,p) level). It was found that in both of the processes considered, a single route can be realized containing a single saddle point and starting as an axial attack of the nucleophile. Further approach of the reagent molecules proceeds with a decrease in the angle of nucleophilic attack to ≈ 130o in the reaction transition state and ≈ 100o – in the reaction product – sulfonamide. It was shown that the studied reactions proceed according to the bimolecular concerted mechanism of nucleophilic substitution SN2, which implies the formation of a single transition state along the reaction pathway. It was found that the geometric structure of the reaction centers in the transition states of the processes is intermediate between the trigonal bipyramid and the tetragonal pyramid, which is explained by the change in the angle of nucleophilic attack when the reagent molecules approach each other. It was found that in benzamide sulfonylation reaction, a cyclic transition state is formed, in which the forming and loosening bonds lie in the same plane, and the H-Cl distance corresponds to the length of the hydrogen bond. In benzenesulfonamide reaction with 3-nitrobenzenesulfonyl chloride, the transition state is not cyclic. The activation energies of the reactions are calculated; they were 155 kJ/mol in the benzamide sulfonylation reaction and 150 kJ/mol in the process with the participation of benzenesulfonic acid amide. The closeness of the obtained values is associated with the similar structure of the amide and sulfamide groups containing electrophilic centers near the amino groups. A significant difference in the rate constants of the studied reactions, which was found earlier, when they occur in aqueous dioxane, is explained by the features of –CONH2 and –SO2NH2 groups specific solvation and the contribution of the entropy factor to the reaction rate: the cyclic transition state of the benzamide reaction with 3-nitrobenzenesulfonyl chloride is more ordered in comparison with a non-cyclic transition state of the reaction with benzenesulfonamide participation, which can promote faster occurence of the first process.