Effects of ureas on the kinetics of acid hydrolysis of 1-phenylethyl nitrite in water and in aqueous micellar solutions of sodium dodecyl sulfate

Abstract

The acid ionization constants of urea, methylurea (MEU), ethylurea (ETU), N,N-dimethylurea (DMU) and tetramethylurea (TMU) have been determined in water and in aqueous micellar solutions of sodium dodecyl sulfate (SDS) from the kinetic study of the acid hydrolysis of 1-phenylethyl nitrite (PEN). The acid hydrolysis in water was inhibited by the addition of urea, as a result of its protonation which decreases the effective H⁺ concentration. Analysis of the solvent isotope effects and the activation parameters shows that urea does not modify the solvation of the substrates. MEU and ETU show a similar behaviour to that found with urea, but TMU behaves slightly differently, because TMU decreases the relative permittivity of the water mixtures.

The rate of acid hydrolysis in aqueous SDS goes through a maximum with increasing SDS concentration. The addition of urea and urea derivatives strongly diminishes the value of the attained maximum in such a way that at high concentrations of ureas the observed rate constant continually decreases as the SDS concentration increases. The kinetic data were analysed by using the simple pseudophase ion-exchange (PPIE) model. The results are explained under the assumption that ureas do not modify the structure of the micelles, at least not for the quantities used in this work, except in the case of TMU and DMU. These two ureas increase the solubility of PEN in water and strongly decrease the value of the rate constant achieved in the micellar phase.