The kinetics of esterification of resin acids

Abstract

SummaryThe kinetics of the esterification of resin acids in both concentrated solutions and dilute alcoholic solutions were studied over a wide temperature range. In concentrated solutions the order of the reaction changes with both the temperature and the extent of reaction, but in dilute solutions the kinetics are similar to those for other organic acids. The uncatalyzed esterification is pseudo‐second order while the hydrogen‐ion‐catalyzed esterification is first order and follows the Goldschmidt equation. Thus the esterification mechanism is probably the same as for other organic acids even though the carboxyl group on a resin acid molecule is highly hindered.Catalysts by six metallic resinates—especially calcium and lithium—was also studied. The catalytic rate constants at 260°C. varied quantitatively with the “acidity” of the metallic resinates in rosin at 200°C. As esterification catalysts, the metallic resinates increase the activation energy but also increase the activation entropy sufficiently so that the free energy of activation is decreased, especially at high temperatures. The observed kinetic behavior agrees with a rate expression derived by assuming that the ratecontrolling step is a reaction of metallic resinate with the alcohol to form an ester and a basic metallic resinate.