The reaction of imidazole with some 1-halogeno-2,4-dinitrobenzenes in aprotic solvents

Abstract

The reactions of imidazole with 1-chloro- and 1-fluoro-2,4-dinitrobenzenes in dimethyl sulfoxide are not catalysed by imidazole. DABCO or imidazolium perchlorate. In acetonitrile these reactions are not catalysed by the nucleophile or DABCO but the second-order rate constant k_A has a linear dependence on the concentration of tetrabutylammonium chloride and is depressed by the addition of imidazolium perchlorate. In this solvent the reaction of imidazole with 1-bromo-2,4-dinitro-benzene is not catalysed by the nucleophile and the k_A^Br/k_A^Cl ratio is 0.90. These results are rationalised in terms of the conventional mechanism of aromatic nucleophilic substitution reactions in aprotic solvents of high relative permittivity, with chloride ion catalysing the formation of the intermediate in acetonitrile solution.

In benzene the reaction of imidazole with 1-chloro-2,4-dinitrobenzene is strongly catalysed by the nucleophile and by tetrabutylammonium chloride and weakly by pyridine. The corresponding reaction with 1-bromo-2,4-dinitrobenzene is also strongly catalysed by imidazole and there is little difference in the reactivity of the two substrates. The catalysis of the reactions by amines observed here, and that previously reported in the literature, is explained as being due to reaction taking place via complexes formed from the reactants and catalysts.