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Different kinetic behaviors for unimolecular and bimolecular ester hydrolysis reactions in strongly acidic microemulsions.

Replacing the counterion in sodium bis(2-ethylhexyl)sulfosuccinate with H(+) allows strongly acidic microemulsions to be obtained. These systems are the only known colloidal medium in which it is possible to reach local concentrations of acid, expressed as Hammett acidity function (H(0)), lower than H(0) = -0.2, which corresponds to a concentration of acid above 1 M in aqueous solution. In the present work, there has been analyzed the influence of this type of microemulsion on the acid hydrolysis of two esters derived from picolinic acid: 4-nitrophenylpicolinate (NPP) and 2,4-dinitrophenylpicolinate (DNPP). The reaction rate for NPP and DNPP increases up to 16 times on increasing the size of the aqueous nanocore of the microemulsion, which supposes an experimental behavior opposed to the one observed for the hydrolysis of nitrophenylacetate (NPA). The key to this differentiated behavior of NPP and DNPP resides in the fact that the rate-determining step for the acid hydrolysis mechanism is the water addition to the protonated ester. The reaction rate increases on increasing the nucleophilicity of water; that is, on increasing W (W = [H(2)O]/[surfactant]). Therefore, the acid hydrolysis of esters in strongly acidic microemulsions presents an A2 mechanism when reactivity increases with W, and an A1 mechanism if it decreases with W.