Catalysis in nucleophilic aromatic substitution reactions. The presence of molecular complexes on the pathway of reactions between 1-fluoro- and 1-chloro-2,4-dinitrobenzene and aliphatic amines

Abstract

The kinetics of reactions between 1-halogeno-2,4-dinitrobenzene (halogen = F, Cl) and butylamine and piperidine were re-investigated in cyclohexane. The k_obs, values (s^–1 mol^–1 dm³) increased with increases in the initial concentration of the reacting amine. Investigation of the reaction mixtures at zero reaction time, indicated the presence of an interaction between the substrate and the amine which occurred in a rapidly established equilibrium and which preceded the substitution process. The nature of the molecular complexes observed is discussed. Present and previous data indicate that the k_obs increase was difficult to explain with the traditional hypothesis of proton and leaving group departure as a rate determining step. A better rationalisation of the experimental data may be obtained by involving the molecular complexes on the substitution reaction pathway.