Reactions of aromatic nitro-compounds in alkaline media. Part X. The relationship between hydrogen exchange and colour formation in m-dinitrobenzene and related compounds

Abstract

The rate of tritium exchange between the hydrogen atom in the 2-position of 1,3-dinitrobenzene and basic media (sodium methoxide in methanol or in methanol–dimethyl sulphoxide) has been studied as a function of the basicity of the medium. The regular increase in rate with basicity which is observed at low basicity does not extend beyond the basicity at which spectrophotometric measurements indicate nearly complete conversion of m-dinitrobenzene into a coloured form. The results rule out proton loss from m-dinitrobenzene as the effect responsible for colour formation but are explained by the assumption that the reversibly formed coloured species represents a form of dinitrobenzene which is unreactive in exchange. Comparison between the electronic absorption maxima of the coloured forms of m-dinitrobenzene and of dinitroanisoles suggests that reversible methoxide addition at the 4-position takes place in 1,3-dinitrobenzene. Tritium exchange is shown to occur at a similar rate in a single nuclear position (presumably the 3-position) in 2,4-dinitroanisole. Hydrogen exchange has also been detected in 1,3,5-trinitrobenzene and earlier work on attempted exchange reactions with this compound is re-interpreted.