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Title: KINETICS AND MECHANISM OF AMINOLYSIS OF SOME O-ARYL OXIMES
Authors: Singh, Pritam
Keywords: CHEMISTRY;AMINOLYSIS;O-ARYL OXIMES;AROMATIC NUCLEOPHILIC SUBSTITUTION REACTION
Issue Date: 1991
Abstract: Aromatic nucleophilic substitution reactions with amines as nucleophile have been extensively investigated and the mechanism in partially aqueous medium is firmly established. However, these reactions in apolar and hydrogen bonding dipolar non-aqueous solvents show a different behaviour. For example the order w.r.t. amine changes from two to three in apolar solvents and inverse temperature effect is observed in some dipolar hydrogen bonding solvents. In order to explain these unusual observations three approaches (i) attack of amine dimer, (ii) attack of two amine molecules separately in catalytic step and (iii) catalysis by homoconjugates have been adopted. The experimental information available in these solvents is not detailed enough to allow generalizations to be made with respect to the effect of the nature of amine and solvent on reaction mechanism. Investigations have, therefore, been carried out on aminolysis of O-Aryl oximes in apolar and dipolar solvents to have more experimental data so that reaction mechanism controversy is resolved to the extent possible and some generalizations may emerge out. With this aim in view, aminolysis of four O-Aryl oximes viz. 0-(2,4- dinitrophenyl) substituted cyclohexanone oxime (DNPCHOX), acetophenone oxime (DNPAPOX), benzophenone oxime (DNPBPOX) and 4'-bromobenzophenone oxime (DNPBrBPOX) with cyclic and non-cyclic amines in various apolar and dipolar non-aqueous solvents has been investigated. (ii) SNAr reactions of DNPCHOX,DNPAPOX,DNPBPOX and DNPBrBPOX with cyclic (piperidine and morpholine) and non-cyclic s-butylam'ine) amines in benzene were studied spectrophotometrically under pseudo-first-order condition at 35 ± 0.1°C by keeping amine concentration much larger compared to substrate. All these reactions were found to be wholly base catalysed. The plots of second order rate coefficient (KA) versus [piperidine] were curvilinear and concave towards rate constant axis for all the four substrates. This curvilinear dependence indicates that order with respect to amine is more than two. Further, the plots of kA[Bl_:L (B=amine) against [piperidine] were found to be linear. The analysis of these plots showed that the piperidinolysis proceeds through two routes; (i) route-I involving two amine molecules and (ii) route-II involving three molecules of amine. The results have been explained by electrophilic catalysis by homoconjugates of conjugate acid of nucleophile. Morpholinolysis of DNPCHOX and DNPAPOX could not be studied as the reactions were slow. Substrate DNPBPOX and DNPBrBPOX having better nucleofuge could be subjected to morpholinolysis and behaviour was found to be similar to piperidinolysis. s-Butylaminolysis of DNPCHOX, DNPAPOX, DNPBPOX and DNPBrBPOX showed two types of behaviour. With substrates DNPCHOX,. DNPAPOX & DNPBPOX having poor nucleofuge, behaviour similar to piperidinol ysis and morpholinolysis was observed. However, with the substrate DNPBrBPOX the reaction was found to proceed through only one route involving only two amine molecules. Thermodynamic parameters have also been determined. Reactions showing third (iii) order dependence on amine had negative energy of activation which supports the mechanism adopted to explain the results. The combined results of piperidinolysis, morpholinolysis and s-butylaminolysis indicate that with cyclic amines, the route involving three amine molecules is promoted whereas with noncyclic amine, route involving two amine molecules is favoured. The aminolysis reactions of substrates DNPCHOX and DNPBPOX in a number of good hydrogen bond acceptor solvents viz. dioxane, ethyl acetate and in poor hydrogen bond acceptor solvents viz. chlorobenzene, benzene, were carried out. The results indicate that in solvents of almost the same polarity, the mechanism becomes a function of hydrogen bond acceptor ability of the solvent. The energy of activation in good hydrogen bond acceptor solvent is usually low compared to energy of activation in poor hydrogen bond acceptor solvents. This has been explained by assuming the formation of hydrogen bonded heteroconjugates of conjugate acid of nucleophile with solvent. The aminolysis reactions have also been studied in the presence of small amounts of various additives viz. dimethyl sulphoxide, tetrahydrofuran, acetonitrile and triethyl amine. These additives increase the reaction rate and effect is found related to hydrogen bond acceptor ability. It is reasonable to conclude from these investigations that (1) the reaction course is more a function of cyclic or non cyclic nature of the nucleophile rather than its basicity and (ii) hydrogen bond acceptor ability of the solvent is more important than its polarity in affecting the reaction. CHAPTER - 1 INTRODUCTION 1.1 KINETICS AND MECHANISM Kinetics concerns itself with the study of rate of chemical reaction and the elucidation of its mechanism. The kinetic information about a chemical process is invaluable to chemical technologists who attempt to develop efficient synthetic procedures for practical purposes. However, to a large number of research chemists working in the field of physical organic chemistry, a detailed kinetic study is important as it helps in unraveling the reaction mechanism. The mechanism of a reaction conveys information about all the elementary processes that take place simultaneously or consecutively in producing the observed overall reaction. The study of reaction mechanism is of pivotal importance in chemistry. The advantage of mechanistical approach is optimization of yields, correlation of results and prediction of reactivity. The kinetic study provides answer concerning the individual steps of a reaction. For this purpose the rates of reactions are studied as a function of concentration, temperature, solvent and nature of reacting substances. The experimental results can be interpreted in terms of various conceivable mechanisms. Many a times it is not possible to conclusively suggest only one mechanism by means of kinetic data and in such situations other experimental techniques which may characterise the reaction intermediates are used to sort out the reaction mechanism. Thus elucidation of reaction mechanism is one of the most challenging and fascinating area in chemistry and kinetics is a convenient tool to establish it. 2 A variety of reactions like oxidation, reduction, addition, elimination and substitutions encompassing numerous organic functionalities have been kinetically studied and their mechanisms well established. As the present thesis mainly deals with nucleophilic substitution reactions at aromatic carbon atom, literature dealing with aromatic nucleophilic substitution reactions are reviewed here in detail. 1.2 AROMATIC SUBSTITUTION REACTIONS The reactions wherein the replacement of an atom or a group from the benzene or substituted benzene ring by another atom or group occurs are called Aromatic Substitution Reactions. Depending on the mechanism involved and the nature of attacking substituting reagent, aromatic substitution reactions are broadly classified into three categories. .............
URI: http://hdl.handle.net/123456789/1249
Other Identifiers: Ph.D
Research Supervisor/ Guide: Jain, A. K.
Gupta, V. K.
metadata.dc.type: Doctoral Thesis
Appears in Collections:DOCTORAL THESES (chemistry)

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