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 S(N)Ar 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 (k(CONH)()2/k(CO)()2(Me) 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 (k(CONH)()2/k(CO)()2(Me) 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 k(CONH)()2/k(CO)()2(Me) 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.