STUDIES ON NUCLEOPHILIC REACTIONS OF SOME O-SUBSTITUTED OXIMES AND RELATED COMPOUNDS

Abstract

Results of the kinetics of the reactions of 0-(2,4-dinitrophenyl) N-hydroxy-9,10-dihydroanthracene-9,10-endo- « , 3 - succinimide, 0-(2,4-dinitrophenyl)N-hydroxytricyclo[3.2.2.Cr ' ] non-8-en-endo-6,7-dicarboximide and 0-(2,4-dinitrophenyl)- N-hydroxybicyclo[2.2.1]hepta-5-en-endo-2,3-dicarboximide with hydroxide ion and some neutral amines, viz, hydrazine, morpholine, cyclohexylamine and piperidine in 80-20 acetonitrilewater (v/v) at 35±0.1°C are reported. Kinetics were determined under conditions of excess of nucleophile over substrate and in all cases, excellent pseudo-first order plots were obtained. The second order rate coefficients k or k obtained by dividing the pseudo- -A -r first order rate coefficient by [Nu] or [Nu] are effective almost constant for the reactions of all the three substrates with hydrazine, morpholine and cyclohexylamine. But k or -A k increases with increase in [Pip] __ . or [OH-] indica- -r effective ting a possible catalysis by base. Studies on solvent effect reveal the polar nature of 'the transition state. The effect of pH on rate seems to be negligible for the reactions with hydrazine, morpholine and cyclohexylamine, but in the case of piperidine the rate increases with increase in pH, indica ting a possible catalysis by hydroxide ion. Activation para meters have been calculated in each case. Entropy of activation values are large and negative indicating the congested nature of the transition state. The site of nucleophilic attack has been confirmed to be the nitroactivated aromatic carbon attached to the oxygen atom and the cleavage of C-0 bond occurs. From the analysis of experimental data, rate deter mining formation of zwitterionic a -complex in the case of hydrazine, cyclohexylamine and morpholine is suggested, whereas in the case of piperidine and hydroxide ion the decomposition of intermediate complex is rate determining. The reactions between 0-(2,4-dinitrophenyl)N-hydroxy- 9 ,10-dihydroanthracene-9,10-endo-a,3-succinimide and piperidine has also been studied in various nonaqueous media, viz. acetonitrile, ethylacetate, benzene, dioxane and methanol. Base catalysis has been observed in all the cases except in methanol Better proton acceptor capacity of methanol seems to account for this exceptional behaviour. In all the three substrates mentioned above, the 2,4- dinitrophenyl ring is sterically projected over the rest of the molecule and reactivity in this part of the molecule seems to be influenced by field effect. Hence a few cases of 0-(2,4-dinitrophenyl) derivatives of E-benzaldoximes [XC rftA CH =NOC,. H _ (NO _)_ (2:4) where X=H, p-Me, p-Cl, p-NO„ and 6 4 6322 - — — I o-Cl] have been studied in which the 2,4-dinitrophenyl ring exists in the same plane as the rest of the molecule. Reactions have been carried out with methylamine, cyclohexylamine and piperidine in 50-50 ethanol-water(v/Vj at 35±0.1°C. The reactions are overall second order, first-order with respect to each reactant. Laidler-Eyring and Amis plots are linear. Leffler's isokinetic relationship is applicable. Hammett plots are linear with a p value of ±0.75 for methylamine and ±0.61 for cyclohexylamine and +1.25 for piperidine. These E-0-(2,4-dinitrophenyl)benzaldoximes have three electrophilic sites, the methine hydrogen, the oxime nitrogen and nitroactivated aromatic carbon. Methylamine and cyclohexylamine predominantly give N-(2,4-dinitrophenyl)methylamine and N- (2,4-dinitrophenyl)cyclohexylamine (substitution products) with attack at the aromatic carbon in sharp contrast to earlier observations with hydroxide ion. However, in the case of piperidine both the elimination (abstraction of methine hydrogen and cleavage of the N-0 bond) and substitution (attack at the aromatic carbon with cleavage of C-0 bond) have been obtained. With a strong electron withdrawing substituent such as nitro group in the phenyl ring of the oxime the former pathway is favoured. Probable mechanism for these reactions is proposed.