Some studies of the interactions of aromatic nitro-compounds with nucleophiles

Abstract

The interactions of some aromatic nitro-compounds with nucleophiles in protic solvents and in protic-dipolar aprotic solvent mixtures have been investigated using the techniques of proton magnetic resonance and visible spectroscopy and stopped-flow spectrophotometry. The rates of reaction (nucleophilic reactivities) of a series of substituted thiophenoxide ions with l-chloro-2,4-dinitrobenzene in 95/5 (v/v) ethanol-water, determined by a stopped-flow spectrophotometric method, correlate better with their carbon basicities than with their proton basicities. In methanol 4-methoxy-3,5-dinitrobenzaldehyde is in equilibrium with its hemiacetal formed by solvent addition to the carbonyl function of the aldehyde group, the equilibrium constant for hemiacetal formation (= 8) being obtained from p.m.r. measurements. In the presence of methoxide ions Meisenheimer complex formation occurs by base addition to the parent aldehyde. Kinetic and equilibrium data are reported for complex formation in methanol. In dimethyl sulphoxide there is straightforward formation of Meisenheimer complex. The formation of 1:2 adducts from l-X-2,4,6-trinitroberizenes (X = OMe, OH, NH(_2), NHMe, NMe(_2)) and sodium sulphite in water is characterised by only one relaxation time. P.m.r. measurements also indicate the existence of one isomer of the 1:2 adduct at equilibrium. However when X = H such measurements reveal the presence of both cis- and trans-isomers. Ring-activated glycol ethers cyclise in the presence of aqueous base to give spiro-complexes. Equilibrium and kinetic parameters for complex formation and decomposition are much higher than the corresponding values for their non-cyclic analogues. The spiro-complexes derived from 1-(2-hydroxyethoxy)-2,4,6- trinitrobenzene and -2,4-dinitronaphthalene undergo general acid catalysed decomposition. On going from spiro-complexes which contain 5- to those which contain 6- and 7-membered dioxolan rings there is a dramatic decrease in complex stability.