Highly electrophilic heteroaromatics: the hydrolysis of 7-chloro-4,6-dinitrobenzofurazan and 7-chloro-4,6-dinitrobenzofuroxan in aqueous solution

Abstract

The hydrolysis of 7-chloro-4,6-dinitrobenzofurazan (4) and 7-chloro-4,6-dinitrobenzofuroxan (5) to give the related 7-hydroxy derivatives has been kinetically investigated in the pH range 1–6 in aqueous solution. The reactions occur 3500 and 10000 times more rapidly, respectively, than the, hydrolysis of picryl chloride, emphasizing the much higher electrophilic character of the two heterocycles. Strong catalysis of the hydrolysis of 4 and 5 by carboxylate ions is observed, with Brønsted β values of 0.38 and 0.42, respectively, which support a concerted mechanism. Although the corresponding points (k^H₂O/55.55) show strong negative deviations from the Bronsted plots, the finding of appreciable solvent isotope effects (k^H₂O/k^D₂Oca.1.75) favours a similar mechanism for the water reactions, i.e.water attack is assisted by a second water molecule which acts as a base catalyst. Differences in the electrostatic effects contributing to the stabilization of the transition states for the water-catalysed and carboxylate ion-catalysed reactions would account for the deviations observed for the k^H₂O points.