The stabilities of Meisenheimer complexes. Part IX. The general acid catalysed ring opening of spiro-complexes

Abstract

The ring opening of two spiro-Meisenheimer complexes to give respectively 1-(2-hydroxyethoxy)-2,4,6-trinitrobenzene and 1-(2-hydroxyethoxy)-2,4-dinitronaphthalene is found to be subject to general acid catalysis in water. The α value of the Brønsted equation is 0·5. The isotope effect of the hydronium ion catalysed reaction K_D3O+/K_H3O+= 1·5. A mechanism is proposed involving concerted oxygen protonation and C–O bond breaking. The uncatalysed reaction is shown to proceed by a unimolecular mechanism rather than a bimolecular reaction with water. The results are compared with those for non-cyclic analogues.