The n-butylaminolysis of nitro-substituted 4'-nitrophenyl benzoates and cinnamates as well as of phenylacetates in aprotic solvents is governed by a kinetic law implying higher order terms in nucleophile. It is shown that the attacking nucleophile forms a n-a type molecular complex with the substrate before reaching the transition state in the subsequent kinetic step. This molecular complex is a true reaction intermediate as evidenced by the observed negative activation enthalpy. Othernlecules intervene as general base catalysts. Tertiary m i n e d also c a t a l y s 3 h e reaction. Their catalytic activity i s linearly related to their hydrogen bond forming ability and it is not a direct function of their proton basicity. By varying the nucleophile structure, an excellent BrOnsted relationship could be obtained for the first time in aprotic media, the 6 of which confirms that the catalytic collapse of tetrahedral intermediate is the rate determining step. The reaction of cinnamates turns out to be less sensitive to structural changes of th. substrate than is the reaction of benzoates as shown by the corresponding Hammetovalues. The electronic effects of 2-nitro groups are strongly solvent dependent. Once more, it is established that &enever specific solvent effects (n-donor or ndonor ability) are present, they dominate the overall effect. A general reaction mechanism is proposed which not only explains the various roles played by the nucleophile but also accounts successfully for the great variety of kinetic schemes observed in aprotic media. Transacylation reactions are very important in chemistry and biology.Therefore, the study and elucidation of their mechanism present a considerable interest. well understood(2) while a number of well established facts are known in aprotic solvents(3). It seems to be general in aprotic solvents that higher order terms in nucleophile appear in the kinetic eq~ation(~"'~) implying a general base catalysisi this catalytic effect can also be shown by unhindered tertiary a m i n e~(~) r the existence of tetrahedral intermediate is not definitely established but, if it exists, its collapse leading to products can ba considered as rate deter~uining'~).In the present paper, we attempt to elucidate further the mechanism of ester aminolysis reactions. We also show the influence of structural and solvent effects on this mechanism.A series of nitro-substituted benzoates, cinnamateb and acetates has been submitted to n-butylaminolysis in various aprotic solvents (Table 1). The solvents have been chosen in such a way as to represent both general (polarity, The mechanism of ester aminolysis reactions in aqueous media is fairly -1055 -