KINETIC STUDIES, MECHANISM AND MICELLAR EFFECTS ON SUBSTITUTION REACTIONS OF SOME O-SUBSTITUTED OXIMES

Abstract

A detailed kinetic investigation on the reactions of 0-(2,4-dinitrophenyl) derivatives of oximes of cyclohexanone (DNPCHOX), acetophenone (DNPAPOX), benzophenone (DNPBPOX) and 4-bromobenzophenone (DNPBRBPOX) with some anionic and neutral nucleophiles viz., Oil", .aethylamine and ethylenediamine has been carried out at various temperature and solvent media. The reactivity of the nucleophiles towards DNPCHOX in 70-30 water-methanol (V/V) decreased in the following order: MeNHp> (CHpNHp)0 > OH**. A reaction pathway initiated by the attack of the nucleophile on the aromatic carbon attached to the oxime oxygen together with the breakdown of the C-C is suggested. Effect of nucleophile concentration, pH, salt, polarity of the medium, presence of cationic, anionic and nonionic micelles has been studied to gain an insight into the mechanism. The cationic micelles exhibit a positive effect. In the presence of cationic micelles, a three fold increase in the base catalyzed rate constant of DNPCHOX is observed. A better way of expressing the efficiency of a micelle in catalyzing the reaction is proposed. The anionic micelles inhibited the reaction while the nonionic micelles showed only a negligible effect. In the case of amines pH effect showed that methylarninolysis is catalyzed by protons, while ethylenediaminolysis is catalyzed by 0H~. However, a strong inhibition of both the V aminolysis by cationic micelles indicated that protonation of the leaving group is an important step in the reaction. Reactions of DNPAPOX, DNPBPOX and DNPBRBPOX in aqueous acetonitrile (50-50 V/V) show that the base catalyzed hydrolysis of all the four substrates proceeds almost at the same rate. This insignificant effect of structural change in the oxime moiety suggests that a rate-limitting attack on the aromatic carbon attached to the oxime oxygen preferentially occurs followed by a faster C-0 bond cleavage. In aminolysis, the rate has been found to be Significantly affected by the structural change in the substrates and the reactivity order, DNPBRBPOX > DNPBPOX > DMPAPOX > DNPCHOX was observed. This is explained by suggesting that amines and substrates establish an equilibrium with a zwitterionic intermediate, which subsequently breakdown to give the products. The results show that the breakdown is often assisted b,y OH"", Based on the kinetic and analytical data,a mechanistic spectrum of these reactions is proposed in the thesis.