Some structural, equilibrium and kinetic studies of aromatic nitro-compounds with nucleophiles

Abstract

The interactions of certain aromatic nitre compounds with nucleophiles in protic solvents and in protic dipolar aprotic solvent mixtures have been investigated using the techniques of proton magentic resonance and visible spectroscopy. Picrate ion reacts with concentrated aqueous sodium sulphite to give an unusual ion carrying five negative charges. This results from the covalent addition of two sulphite molecules to the picrate ion, at ring carbon carrying hydrogen. At low sulphite concentration a 1:1 adduct is produced. The equilibrium constants for the formation of these complexes have been determined. Solvent effects on the formation of 1:1 adduct were examined in water-dimethyl sulphoxide mixtures. With hydroxide ion in aqueous solutions, the major of the picrate ion is to give a 1:2 adduct.1,2,3,5-Tetranitrobenzene reacts with hydroxide ion or sulphite ion in water, or ethoxide ion in ethanol to form red complexes whose NMR spectra indicate covalent addition of the nucleophile at C(_4). The complexes are not stable and replacement of the nitro group at occurs quickly. The equilibrium and the rate constants for these processes have been measured. In contrast 1,2,4 ,5-tetranitrobenzene reacts with hydroxide ion to give 2,4,5-trinitrophenol without the production of colour. With ethoxide or sulphite ions coloured species are produced. l,3-Dimethoxy-4,6-dinitrobenzene reacts with methoxide ion in dimethyl sulphoxide-methanol mixtures to form an adduct whose NMR spectrum indicates methoxide addition at C(_2). The equilibrium constants in various solvent compositions have been determined. Solvent effects on the stability of Meisenheimer complexes formed from 1,3,5-trinitrobenzene and thiolate ions have been investigated in alcohol-water mixtures. The equilibrium constants for formation of the1:1 adducts are smaller in water than in alcohol but go through a maximum at about 80% alcohol. The equilibrium constants of the 1:2 adducts increase monotonically as the proportion of water is increased and are ca. 10(^3) greater in water than in pure alcohol. This greater stability is attributed partly to the good solvation by water of a localised negative charge on the nitro group between the positions of addition.