Thermodynamics and kinetics of indole oligomerization in 0.5 mol L−1 aqueous sulfuric acid: evaluation of some temperature‐dependant parameters

Indole is present in a wide variety of natural compounds with physiological activities, as well 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 together with a quantum chemical computational approach, in order to obtain reliable pKa value of indole from 288 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 pKa, 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. Copyright © 2014 John Wiley & Sons, Ltd.