Kinetic Study of Thermal Decarboxylation of Substituted Sodium Phenylpropiolates

Abstract

The kinetics of the thermal decarboxylation of a series of substituted sodium phenylpropiolates were studied in detail. The reaction proceeded without formation of significant amounts of side products: its rate was independent of hydrogen ion concentration. The relative rates of decarboxylation are in the order: p-NO2 > m-NO2> m-CF3 > m-Cl > p-Cl > H > m-CH3, and the entropies of activation are large and positive. The results were discussed in terms of the unimolecular (SE1) mechanism. A quantitative study of effects of substituents on the decarboxylation rates was made by the application of the Hammett free energy relationship. The e value of the Hammett plot was 0.886 +/- 0.013. Deviation of para substituted compounds from the Hammett relationship were discussed in terms of para interaction or conjugation effects in the initial or the transition states.