The use of radioisotopes in studies of reaction mechanism. Part II. Theoretical calculation and experimental measurement of halogen exchange of 1-halogeno-2,4-dinitrobenzenes in methanol and NN-dimethylformamide

Abstract

The symmetrical halogen (nucleophilic) exchange reactions of 1-chloro-, 1-bromo-, and 1-iodo-2,4-dinitro-benzene in methanol and NN-dimethylformamide have been studied, and compared with results of calculations by Miller's method based on the aromatic addition–elimination S_N2 mechanism. Agreement for reactions in dimethylformamide is excellent, and satisfactory also for reactions in methanol, especially when allowance is made for the previously reported heavy nucleophile interaction (values of ΔS^‡ more negative than those normally found for reactions with anionic nucleophilies, accompanied by low values of ΔE^‡, so that values of ΔG^‡ are little affected). The importance of this interaction in methanol but not in dimethylformamide is confirmed, extended, and discussed.

The major difference between the two solvents is the greater nucleophilicity of the halogenide ions in dimethylformamide, the magnitude of the increase in reactivity being in the order Cl^– > Br^– > I^–. As a result, the symmetrical exchange reactivity order in methanol, I > Br > Cl (rate ratio at 100 °C = 1 : 0·038₅ : 0·009₂ becomes I < Br < Cl in dimethylformamide (rate ratio at 100 °C = 1 : 3·82 : 17·7).

The solvent rate ratios for exchange are Cl, 7·2₄× 10⁴; Br, 1·1₉× 10⁴; I, 1·4₀× 10². Corresponding Δ(ΔG)^‡ values are 7·5₆, 6·5₂, and 3·1₇ kcal mol^–1.