The stabilities of Meisenheimer complexes. Part V. Association with barium and calcium ions in methanol

Abstract

The stoicheiometric equilibrium constants for methoxide ion addition to 2-methoxycarbonyl-4,6-dinitroanisole, 4-methoxycarbonyl-2,6-dinitroanisole, and 2-chloro-4,6-dinitroanisole are greatly increased by the presence of small concentrations of barium and calcium ions. This is attributed to the stabilisation of the Meisenheimer complexes by association with these cations. In the case of the methoxide adduct from 2,4,6-trinitroanisole ion association is demonstrated by changes in the visible spectrum in the presence of various cations. However the methoxide adduct from 1,3,5-trinitrobenzene shows little tendency to associate with cations. It is suggested that when ion pairs are formed the cation is held by a cage effect by the two oxygen atoms of the methoxy-groups at the position of addition and by the electronegative substituent at the ortho-position.