Reactions of 1-X-2,4,6-trinitrobenzenes and 1-X-2,4-dinitrobenzenes with hydroxide ions. Comparison of the relative rates of nucleophilic attack at substituted and unsubstituted ring-positions

Abstract

Kinetic and equilibrium measurements are reported for the reactions with hydroxide ions of 1-X-2,4,6-trinitrobenzenes (X = H, F, Cl, Br, I) in water and of 1-X-2,4-dinitrobenzenes in dimethyl sulphoxide–water (80:20, v/v). Attack at unsubstituted ring-positions results in the formation of σ-adducts while attack at halogen-substituted positions leads to 2,4,6-trinitro- or 2,4-dinitro-phenolate ions by nucleophilic substitution; the rate constants for these processes are compared. The results provide evidence for two types of steric effects: (i) increasing the size of the halogen atom results in disruption of the planarity of the nitro-groups, giving a general decrease in reactivity at both unsubstituted and halogen-substituted positions; (ii) unfavourable steric and electrostatic repulsion between entering and leaving groups slows the attack of hydroxide at carbon atoms carrying Cl, Br, or I. In water there is evidence for ionization of added hydroxy groups [Scheme 1; (2)⇌(3)] and for nucleophilic displacement of nitrite by hydroxide within the 3-hydroxy adducts (2) and (3). Contrary to a recent report the reaction of hydroxide ions with nitro compounds to form hydroxy adducts is found to occur without the observation of spectrophotometrically detectable intermediates.