Catalysis of trinitrobenzene–anilide σ-complex formation and decomposition in dimethyl sulphoxide in the presence of tetraethylammonium chloride and perchlorate

Abstract

The effect of tetraethylammonium chloride and perchlorate on the reaction TNB + PhNH₂+ DABCO ⇆ TNB,NHPh^–+ DABCO,H⁺, in dimethyl sulphoxide at 25°, has been investigated. The equilibrium constant for σ-complex formation increases by 340-fold on increasing [Et₄NCl] from 0 to 1.2M. This increase in K results from a 2.5-fold increase in the forward rate constant and a 140-fold decrease in the reverse rate constant. The corresponding changes in these parameters obtained with Et₄NClO₄ are modest though in the same direction. The effect observed with Et₄NCl is larger than in past studies of electrolyte effects on σ-complex formation and decomposition, or in S_NAr processes. The catalytic effect of Et₄NCl in the present system is ascribed to association of Cl^– with the conjugate acid of DABCO to yield the DABCO,H⁺⋯ Cl^– heteroconjugate complex with consequent decrease in the rate of the reverse reaction.