Kinetic and equilibrium studies of the reactions of some aromatic nitro compounds with metal alkoxides

Abstract

Kinetic and equilibrium measurements have been made for σ-adduct formation from propoxide ions in propan-1-ol with 1,3,5-trinitrobenzene and a series of l-propoxy-2,4-dinitro-6-x-benzenes ( x = NO(_2), Cl, CO(_2)Pr, H).The results are compared with those for similar additions of other alkoxide ions. There is evidence for strong association with sodium ions o f the 1,1-dipropoxy adducts, but adducts formed by base additions at unsubstituted ring positions are not strongly associated. In the nucleophilic substitution reaction with l-chloro-2,4-dinitrobenzene free propoxide ions show greater reactivity than sodium propoxide ion pairs. Rate and equilibrium studies are reported for the reactions of three aromatic nitro-compounds with tetramethylarrmonium hydroxide in t-butyl alcohol water mixtures. There is evidence that the reactive nucleophile is t he hydroxide ion rather than the t-butoxide ion. The reaction of 1,3,5-trinitrobenzene in this system leads to a σ-adduct, while l-chloro-2,4-dinitrobenzene gives 2,4-dinitrophenol. That the rates of both these reactions falls rapidly with increasing water content is attributed mainly to the good solvation of the hydroxide ion by water. In contrast the rate of formation of picric acid from picryl chloride increases with water content due to initial formation of the 3-hydroxy adduct. Similar studies have been made using sodium and/or potassium butoxide The interpretation here is complicated by association with anions of the alkali metal cations. In the presence of methoxide ions in methanol 2-methoxy-3,5-dinitrobenzoic acid gives a a-adduct. Using this reaction, acidity functions for lithium, sodium and potassium methoxide solutions have been defined. The observed b a sicity order : sodium methoxide > potassium methoxide > lithium methoxide may derive from specific interactions of the alkali cations with the σ-adduct. Using U.V./visible data the reversible equilibria between 1-x-2,4,6-trinitrobenzenes ( x = NMe(_2), NHMe, NH(_2), OMe or CCH(_2)CH(_2)O) and alkali metal methoxides have been investigated. Usually 1:1, 1:2 and 1:3 interactions are observed. One conclusion is that in general 1:2 and 1:3 adducts associate more strongly with sodium ions than with potassium ions or with lithium ions.