STUDIES ON NUCLEOPHILIC SUBSTITUTION REACTIONS OF SOME NITROACTIVATED AROMATIC SUBSTRATES

Abstract

Mechanism of aromatic nucleophilic substitution reaction in protic and dipolar aprotic- media is still not free from controversy and further data are required for a successful conclusion. A review of the existing literature shows that little attention has been given to the aminolysis reactions of sterically bulky substrates. As base catalysis in aromatic nucleophilic substitution depends on structural as well as steric nature of the nucleophile, the solvent and the nucleofuge, the aminolysis reactions of four sterically bulky substrates, viz., 0-(2,4-dinitrophenyl) indanone oxime (DNPIO), 0-2,4-dinitrophenyl) propanone oxime (DNPPO), 0-(2',4/-dinitrophenyl)-2-naphthol (DNPN), 0-(2,4-dinitrophenyl) camphor oxime (DNPCO) and 3,4-[cyclopenten 3',5'-diyl]-endo- N-(2",4"-dinitrophenoxy) succinimide have been studied with cyclohexylamine (CHA), piperidine (PIP), n-butylamine (n-BuNH ) and ethanolamine (H NCH CH OH), in acetonitrile, benzene, ethyl acetate and methanol. The reactions of DNPCO have also been studied in partially aqueous media with cyclohexylamine and piperidine. All the reactions have been carried out under pseudo first order conditions at 35 ± 0.1 C, and followed spectrophotometerically at the A of the corresponding coloured aminolysis product. Site of nucleophilic attack has been located at the nitroactivated aromatic carbon and a quantitative yield of N-(2,4-dinitrophenyl) amine has been obtained as coloured product in each case. The reactivity of DNPN is higher as compared to DNPIO and DNPPO, which may be attributed to its better leaving group. The (iii) intermediate formed in the case of DNPN is more stable due to derealization of charge and its decomposition is assisted by an additional base molecule. Effect of nucleophile on the reactions of DNPIO have been studied with cyclohexylamine, ethanolamine and piperidine in acetonitrile at 35 ± 0.1 °C. Plots of second order rate constants, k.(= k /[amine]) versus [amine] are linear in the case of A o cyclohexylamine and ethanolamine with positive intercepts and slopes. However, in the case of piperidine a curvilinear response which is concave towards the rate constant axis passing through the origin has been obtained indicating a higher order dependence (order more than two) on piperidine and further the reaction is wholly base catalysed. -1 A plot of k [amine] versus [amine] is linear with positive slope and A intercept, indicating third order dependence on piperidine. A cyclic transition state mechanism has been proposed. The reactions of DNPIO and DNPPO with n-butylamine in different solvents give the following reactivity order : methanol < benzene < ethyl acetate. This is in the order of decreasing n* values of these solvents. The lowest reactivity in methanol has been attributed to hydrogen bonded interaction between methanol and n-butylamine. The various activation parameters have been evaluated for the reactions of DNPIO, DNPN with piperidine, cyclohexylamine, ethanolamine and n-butylamine. Ea and AH values increase with the increase in pK values of these nucleophiles. However, no such regular a trend has been observed in the reactions of DNPPO with the above amines. (iv: Kinetics of aminolysis of N-(2,4-dinitrophenyl) camphor oxime has been carried out with cyclohexylamine (CHA) and piperidine (PIP) in 1:4 (v/v) aqueous - acetonitrile at 35 ± 0. 1°C and the corresponding aminated products have been obtained in quantitative yield. Evidence for base catalysis has been obtained in the case of piperidine. Mechanistic interpretation is provided. Relevant activation parameters have also been calculated. The large negative values of entropy of activation (AS ) observed support the formation of a sterically congested transition state in each case. The higher reactivity of piperidine is supported by the relatively lower energy of activation observed in this reaction.