Meisenheimer complexes: reaction of methoxide ions with 2-chloro-4,6-dinitroanisole in methanol and in methanol–dimethyl sulphoxide

The reaction of methoxide ions with 2-chloro-4,6-dinitroanisole (VI) has been studied kinetically in methanol and in various MeOH–dimethyl sulphoxide (DMSO) mixtures by the stopped-flow method. When the DMSO concentration reaches 70%(w/w), the appearance of the classical Meisenheimer 1,1-complex (VII) is preceded by the fast formation of the thermodynamically less stable 1,3-complex (VIII) which is produced by the attack of MeO– on the unsubstituted C-3 of (VI), and undergoes a rapid conversion into (VII). The interaction becomes still more complicated in the mixtures whose DMSO concentration is greater than 85%, where the initial formation of the 1,5-complex (IX) can be observed. The rate and equilibrium constants for the formation of the three sorts of complexes (VII)(k3, K3), (VIII)(k2, K2), and (IX)(k1, K1), and those for their decomposition (k–3, k–2, and k–1) have been determined. These data shed some light on the mechanism of the interaction and the factors affecting the stabilities of the complexes. Thus, it can be seen that the greater thermodynamic stability of the 1,3-complex (VIII) with respect to that of the 1,5-complex (IX) is essentially because it benefits from the strong delocalizing effect of a para-nitro-group.