The Kinetics of Reactions of Amines with Activated Substrates, Different Behaviour of an Orthonitro and a Cyano Substituent as an Activating Group in Aromatic Nucleophilic Substitution

Akpojivi, E. R. (1989-01)

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Arrhenius paramters have been measured for the reactions of picryl chloride with the following anilines in acetonitrile: 2-OMe - 5 - X anilines (X = F, CI, Br, I, Me, COMe, SO2Me or CF3), 2, 4 - (OMe)2 - 5 - CI aniline, 3 - amino acetophenone, 2 - 6 - X2 anilines (X = F, Et), 2, 4, 6 - X3 anilines (X = F, Me), 2, 3, 4, -2, 4, 5, - and 3, 4, 5 - trichloroanilines. With the constant ortho methoxy series, the reactivity of the halogens follows the same sequence as that for 3-halogeno- anilines except that iodide is the most reactive in the latter series. The span of reactivity is small; there is a factor of 1.3 between the most and least reactive amines. In the disubstituted series, the effects of 2 - OMe - 5 - X anilines (X = Me, COMe, So2Me or F) on the free energy of activation are additive. In the case of the other amines, there are negative deviations from the additivity principle. Appropriate linear free energy relationships have been applied to these reactions and the value of p determined. The rate constants for the 2, 6 - series are considerably less than expected due to steric congestion in the transition state. The trichloro-substituted anilines show positive deviations from the additivity principle and this is explained in terms of buttressing effect. All the results have been rationalised in terms of movement of the transition state along the reaction coordinate. A systematic investigation of the reactions of piperidine, n-butylamine, morpholine, benzylamine and aniline with 2 - CN - 4 - NO2PhX (X = F, Oph) in acetonitrile, D.M.S.O. and benzene has been undertaken. The rate constants are compared with those of corresponding reactions of 2, 4 - (NO2)2 PhX (X = F, Oph) obtained from the literature under the same experimental conditions. The kinetic behaviour of the 2 cyano substituent parallels that of 2 - nitro group in most cases in both base catalysed and uncatalysed systems. The ratio KNO2/kCN has been measured and it is found to contain an appreciable contribution from the base assisted departure rate coefficient (k3B) in a catalysed system. A quadratic dependence of the second order rate constant on amine concentration has been found for the reaction of aniline with 2-CN-4-NO2 PhF in acetonitrite and a tenable explanation has been advanced to explain this seemingly anomalous result. The reactions of 4-NO2-2-CF3 PhF with pioeridine, n-butylamine, morpholine and to the theoretical prediction, only the reaction of abnormal behaviour is still unclear; only speculative explanation based on steric effect is adduced for the absence of catalysis observed for the reactions with the other amines.