Nucleophilic Substitution Reactions of 1-Halogeno-4-COR-2-nitrobenzenes and 1-Halogeno-6-COR-2-nitrobenzenes with Sodium Benzenethiolate and Piperidine. Can an "Inverted Built-in Solvation" Be Responsible for the Peculiar Activation by an o-Carboxamido Group in SNAr Reactions with an Anionic Nucleophile?
A kinetic study of the title reactions has allowed an interpretation of the higher efficiency of an o-carboxamido group with respect to an o-carbomethoxy group in activating the benzenethiolate-dehalogenation reactions in methanol (kCONH2/kCO2Me 2.2-3.0) as due to an interaction between the anionic nucleophile and the hydrogen atoms of the carboxamido group. An inversion of the activating power of the two groups (kCONH2/kCO2Me 0.14) in the reactions with the same nucleophile has been observed when they are in a para-position. Moreover, for piperidino-dehalogenation reactions in methanol kCONH2/kCO2Me ratios less than unity (0.2-0.6) have been observed independently of the position (ortho or para) of the carboxamido and carbomethoxy groups with respect to the reaction center.