Kinetics of the reactions of fluoro- or chloro-2,4-dinitrobenzene with n-butyl-, s-butyl- or t-butyl-amine in benzene. Further evidence for an addition intermediate in nucleophilic aromatic substitution

Abstract

The second-order rate coefficients for the reactions of fluoro-2,4-dinitrobenzene with n-butyl-, s-butyl-, or t-butyl-amine in benzene increase with increasing amine concentration. The trend is only slightly less than linear in the case of the slowest amine, t-butylamine, while profoundly less than linear in the other two cases. These reactions are also accelerated by pyridine which exhibits a much more marked effect at low than at high concentration of primary amine. In contrast, the reactions of chloro-2,4-dinitrobenzene with n-butyl-, s-butyl-, or t-butyl-amine in benzene are of the first order with respect to the amine. These results are taken as evidence that in the reactions of the fluoro-compound there is an addition intermediate whose decomposition to products rather than its formation from reagents is amine-catalysed. Also discussed is the finding that the relative reactivity between fluoro- and chloro-compounds, as measured by the ratio of their second-order rate coefficients extrapolated to zero amine concentration, is not greatly dependent on the steric bulk of the amine.