Substituent and solvent effects on the rate of the reaction of 2-methyl-4-phenyl-thiazole ethiodide with substituted benzaldehydes

摘要

Condensation of 2-methyl-4-phenyl-thiazole ethiodide (1) with aromatic aldehydes in presence of piperidine as base catalyst has been studied kinetically at different temperatures. The rate in presence of low concentration of piperidine (<0.5M) is found to be represented by the third order equationv=k′1 aldehydepiperidine. On the other hand the rate in presence of ≥1.013Mpiperidine is represented by the second-order equation:v=k′1 aldehyde. It is concluded from the kinetic results that the dehydration step of the intermediate aldol compound is the rate determining step of the reaction. The dependence of the mechanism of the reaction and the thermodynamic parameters of activation on the molecular structure of the various aromatic aldehydes used is discussed. In various organic solvents, the rate of the reaction increases as the dielectric constant of the medium is increased. The energy of activation and the thermodynamic parameters of activation were calculated and discussed in terms of solvent prope