Effect of Solvent and Nucleophilic Basicity on the Mechanism of Nucleophilic Displacement at Aryl Carbon
The kinetics of the reactions of pyrrolidene, piperidine, n-butylamine, morpholine and benzylamine with 2,4-dinitro-6- methylphenylphenyl ether in acetonitrile, dimethylsuphoxide and benzene have been investigated. All the reactions in acetonitrile and those of pyrrolidine, piperidine and morpholine in dimethyl-sulphoxide are base catalysed. The results confirm that the decomposition of the intermediates to the products takes place by a unimolecular mechanism. In the case of the reactions of n-butylamine and benzylamine, the observed second order rate constants kA are almost insensitive to increases in amine concentrations. The value of k3/k2 which is the index of measuring catalytic efficiency are temperature dependent so that it is possible to minimise the catalysed and the uncatalysed step by temperature variation. Reactivity sequence: n-butylamine > pyrrolidine > piperidine observed runs counter to the normal trend in archtypical SNAr reactions and the formation and decomposition of Jackson-Meisenheimer complexes. In benzene, the reactions of these amines exhibit upward concave curvature typical of reactions involving poor leaving groups in non-polar aprotic solvents. The kinetics of the reactions of 3,5-diniro pyridinephenyl ether with aniline in acetonitrile dimethylsulphoxide, methanol and benzene and of the same substrate with piperidinem n-butylamine morpholine and benzylamine in acetonitrile and benzene have also been studied. The reactions of aniline in aceyonitrile and methanol are a linear function of the amine concentration and the mechanism of the uncatalysed path is the unimolecular decomposition of the intermediate. No catalysis is found in the reaction carried out in dimethylsulphoxide and this records another example of a change from protic to dipolar aprotic solvent. The reactions of the other amines are catalysed in a linear fashion in benzene and acetonitrile. In the latter solvent, the SB-GA mechanism is in operation for the reactions of piperidine and morpholine while the others follow the unimolecular decomposition of the intermediate. K3/k2 values are also temperature variant in acetonitrile and are smaller in the 3,5-dinitro pyridinephenyl ether than in the 2,4,6-trinotrophenyl phenyl ether. The reactions of aniline with 3,5-dinitropyridine phenyl ether, 2,4,6-trinitrophenylphenyl ether and 4-nitrophenyl-2,4,6-trinitrophenyl ether in methanol are subject to general base catalysis. For the reaction of 2,6-dinitrophenylphenyl with piperidine, specific base catalysis is observed. General base catalysis is a consequence of rate-limiting deprotonation of the zwitterionic intermediate complex; specific base catalysis reflects rapid equilibrium deprotonation of the zwitterion followed by a spontaneous non-catalysed leaving group expulsion. The reaction of 3,5-dinitropyridine phenyl ether with aniline in benzene is catalysed by the nucleophile, 2-, 3- and 4-nitroanilines. Mechanism is proposed to rationalise this result. The reactions of 2, 6-dinitrophenylphenyl ether with piperidine n-butylamine, morpholine and benzylamine in acetonitrile, dimethylsulphoxide and benzene and of piperidine and n-butylamine alone in methanol in the presence of 0.1m amine hydrochloride have been investigated. The results are compared with the corresponding reactions of 2, 4-dinitrophenylaphenyl ether, 2,6-dinitrophenylphenyl ether shows greater reactivity with n-butylamine and benzylamine than 2, 4-dinitrophenylphenyl ether in benzene while the order of reactivity is reversed in the reactions of piperidine, n-butylamine, morpholine and benzylamine in aceotnitrile, dimethylsulphoxide and methanol. This is explained in terms of free rotation in conformation of the two ortho nitro groups which facilitate the transfer of a proton to the leaving group. The lower reactivity of 2,6-dinitrophenylphenyl ether with piperidine and morpholine than 2,4-dinitrophenylphenyl ether is a reflection of same sort of steric effect. In the search for electrophilic catalysis, the effects of lithium perchlorate and piperidine hydrochloride on the reaction of 2,6-dinitrophenylphenyl ether with piperidine in diemthylsulphoxide have been investigated. The second order rate coefficients. kA decrease with increase in the concentration of the salts. A tentative explanation based on the differences in the stabilisation of the initial and transition states by lithium and piperidiniumi ions has been suggested.