Thermodynamics and kinetics of indole oligomerization in 0.5 mol L~(-1) aqueous sulfuric acid: evaluation of some temperaturedependant parameters

摘要

Indole is present in awide variety of natural compoundswith physiological activities, aswell as it is a very important intermediate in medicinal and industrial chemistry. For this reason, the evaluation of indole protonation, oligomerization equilibria and the study of the kinetics of dimer and trimer formation in diluted sulfuric acid at various temperatures are of paramount importance because of their practical and scientific implications. Here, we calculate the protonation equilibria by using the literature titration data togetherwith a quantumchemical computational approach, in order to obtain reliable pK_a value of indole from288 to 313 K. Starting from these calculations, we are able to measure the oligomerization equilibrium constants of indole, their kinetic constants, whose values are dependent of indole pK_a, at different temperatures. Enthalpy and entropy of the reactions are calculated by means of Van't Hoff equation, while the activation parameters of Eyring-Evans-Polanyi equation are evaluated for the whole kinetic constants.