KINETIC AND MECHANISTIC STUDIES OF SOME AROMATIC NUCLEOPHILIC SUBSTITUTION REACTIONS

Abstract

Nucleophilic substitution reactions of nitro activated aromatic substrates with amines as nucleophile in nonpolar solvents exhibit third order dependence on amine and generally inverse temperature effect on overall rate. To explain these observations four different mechanisms have been proposed (i) Banjoko's cyclic transition state mechanism, (ii) Nudelman's dimer mechanism, (iii) Forlani's molecular complex mechanism and (iv) Jack Hirst's mechanism involving electrophilic catalysis by conjugate and homoconjugate acids. There are arguments both in favour and against each mechanism. Thus, the reaction mechanism is yet to be finally established and more investigations are needed. As these reactions proceed through various routes, the effect of temperature on the rates of separate routes may provide evidence in support of some mechanism. With this aim in view, aminolysis reactions of O-aryl oximes in benzene have been studied as a function of amine concentration, temperature and nucleofugicity of the substrate. The results support Jack Hirst's mechanism. The effect of additives and hydrogen bond acceptor/donor solvents on aminolysis of O-aryl oximes was also studied to have additional evidence in support of Jack Hirst's mechanism. Finally, the reactions were studied with different amines in order to correlate the reaction mechanism with nature of amines. A brief summary of the work carried out and results obtained are presented here. (i) Three new nitro activated O-aryl oximes viz. 0-(2,4-dinitrophenyl)p-methoxybenzophenone oxime (DNPMeBPOX), 0-(2,4-dinitrophenyl)p-fluorobenzophenone oxime (DNPFBPOX) and 0-(2,4-dinitrophenyl)p-chlorobenzophenone oxime (DNPCIBPOX) have been synthesized and characterized on the basis of C, H, Nanalysis, IR and 1H NMR spectroscopy. The substrates prepared have different nucleofugicity due to the presence of electron donating (-CH30) and electron withdrawing (-F and -CI) groups in the oxime moiety. Aminolysis of these substrates would thus throw light on the relationship between nucleofugicity and reaction mechanism. The reactions of DNPMeBPOX, DNPFBPOX and DNPCIBPOX with two secondary cyclic amines viz. pyrrolidine and piperidine in benzene have been studied spectrophotometrically at A of aminolysis product, under pseudo-first order conditions max as a function of amine concentration at different temperatures. The products of these reactions are N-(2,4-dinitrophenyl) amines which have been identified by TLC and spectral methods. Pseudo-first order rate constants (k0) were calculated numerically from the plots of ln(Aw-At) versus time, where AM and At are the absorbances at infinite and time t, respectively. Second order rate constants (kA) were then obtained by dividing k0 by [amine]. The rates of the three substrates are in the order DNPCIBPOX>DNPFBPOX>DNPMeBPOX which has been explained on the basis of stability of oximate anion formed as a result of breaking of C-0 bond. The plots of kA versus [amine] pass through origin and are curvilinear being concave towards rate axis. This indicates that the reactions are wholly base catalysed and the order in amine is more than two. Further treatment of rate data shows that the plots of kA[B]"1 versus [B], B being amine, are linear. These results are explained by applying Jack Hirst's mechanism which involves reaction proceeding through two routes: in first route, electrophilic catalysis is done by conjugate acid BH+ and in second route electrophilic catalysis is done by homoconjugates BH+B. The intercepts of the plots of kA[B]"1 versus [B] give third order catalytic rate coefficient (k') of the route-I and the slope, fourth order catalytic rate coefficient (k") of route-II. The rate of route-I is found to increase with temperature which is consistent with participation of nonhydrogen bonded species BH+ in catalytic step. On the other hand, the temperature effect on the rate of route-II is inverse, i.e., it decreases with rise in temperature. This inverse temperature effect points out to the participation of hydrogen bonded species BH+...B in the catalytic step. As BH+B breaks down at higher temperatures, the rate of route-II decreases. Further the ratio of k'Vk' increases with decrease in nucleofugicity of the substrate which shows that with substrates having poor leaving group, the reaction proceeds relatively more through the second route. Thus our investigations of temperature effect on the rates of separate routes support the Jack Hirst's mechanism, as all other mechanisms predict the decrease in rate for both the routes. In order to have additional data in support'of the role of

hydrogen bonded species in electrophilic catalysis, pyrrolidinolysis of DNPMeBPOX, DNPFBPOX and DNPCIBPOX at different temperatures was investigated in two pairs of solvents: (i) benzene and dioxane and (ii) chlorobenzene and ethyl acetate which have similar dielectric constant but different hydrogen bond acceptor ability. The results show that the role of dielectric constant is rather limited whereas hydrogen bond acceptor ability plays major role. This is evident from the results in chlorobenzene and ethyl acetate, where the order in amine for pyrrolidinolysis has been found to be three and two, respectively. Different behaviour in ethyl acetate is accounted for due to breaking of BH+B which are present in chlorobenzene but changed to BH+S, S being solvent, in ethyl ac etate as a result of solvent competing with base in the forma tion of hydrogen bonded species. Thus, in chlorobenzene, the catalysis is by BH+ and BH+B whereas in ethyl acetate it is by BH+ and BH+S. The pyrrolidinolysis reactions behave similarly in the other pair of solvents, i.e., benzene and dioxane, as the order is found to be three in amine in both the solvents inspite of sufficient hydrogen bond acceptor ability of dioxane. The similarity in behaviour has been explained due to the presence of two hydrogen bond acceptor sites in dioxane molecule which results in the formation of a mixed heteroconjugate. Thus, these results support the basic assumptions of Jack Hirst's mechanism that hydrogen bonded species participate in catalysis in nonpolar solvents. (iv) In order to elucidate more information about the role of hydrogen bonding in aminolysis reactions in nonpolar solvents, the effect of a number of hydrogen bond acceptor/donor additives viz. 1,4-diazobicyclo[2,2,2]octane (DABCO), pyridine, diethyl ether, triethylamine and n-butanol was also investigated on piperidinolysis of DNPCIBPOX in benzene at 30°C. The rates are found to increase in presence of all the additives except n-butanol, in whose presence, it decreases. These results are explained by assuming electrophilic catalysis by heteroconjugates (BH+A) which are formed as a result of hydrogen bonding between the additives (A) and conjugate acid (BH+). These results have further shown that catalytic effect of additives is parallel to their hydrogen bond acceptor ability. DABCO which has highest hydrogen bond acceptor ability shows maximum catalytic action. Lastly, the reactions of three primary alkylamines viz. n-butylamine, n-hexylamine and n-dodecylamine with DNPMeBPOX, DNPFBPOX and DNPCIBPOX were studied at different temperatures in benzene in order to know the relationship of the reaction mechanism with primary straight chain amines and secondary cyclic amines. The results show that primary alkylaminolysis is significantly different from secondary aminolysis viz. pyrrolidinolysis and piperidinolysis. In contrast to pyrrolidinolysis and piperidinolysis reactions which are wholly base catalysed and third order in amine, the primary alkylaminolysis reactions show an uncatalytic route and second order dependence on amine. The rates of both the catalytic and uncatalytic routes are found to increase with rise in temperature. The uncatalytic route is accounted for due to the presence of second proton in primary alkylamines which forms intramolecular hydrogen bonding with oxygen of leaving group and thus facilitating its expulsion. The second order dependence on amine indicates the absence of BH+B for the amines investigated. This appears due to free rotation about C-N bond in primary amines which hinders the formation of BH+B. In addition to this, the long chains of primary amines are also responsible for instability of BH+B. The results of present investigations support Jack Hirst's mechanism for aminolysis reactions of nitro activated aromatic substrates in nonpolar solvents and show that the mechanism also depends on the nature of amine.