Single-phase and heterophase solid-state chemical kinetics of thermally induced methyl transfer in tetraglycine methyl ester

Abstract

To better understand the general interrelationships between chemical transformations and physical transformations in solid-state reactions, we have studied the kinetics of methyl transfer in polycrystalline samples of tetraglycine methyl ester [TGME] over the temperature range of 83°C–115°C. Changes in the concentrations of the reactant and various intermediates (sarcosyltriglycine methyl ester METGME, and tetraglycine TG) and products (sarcosyltriglycine METG and N N-dimethyl glycyl triglycine Me2TG) were measured over the entire time course of the reaction using HPLC. Corresponding measurements of physical transformations occurring during the course of the reactions were made using X-ray powder diffractometry and differential scanning calorimetry. Kinetic curves for the loss of TGME in the range of 83°C–115°C have a sigmoldal shape and collapse into one curve when plotted in terms of reduced time. t/t0.5, as do plots of intermediate and product concentration plotted in the same manner. The first 25% of the reaction proceeds homogeneously through what is believed to be the formation of a crystalline solid solution of the intermediates and products in the reactant. The acceleratory character of the kinetic curves in the single-phase portion of the reaction has been described by a kinetic scheme that contains a concentration-dependent rate constant. The apperance of a new crystalline phase beyond 35% of the reaction changes the reaction mechanism from a bulk reaction to an interface-controlled process that causes further acceleration of the methyl transfer. The apparent activation energies for both single-phase and heterophase stages of the reaction are about 100–130 kJ/mole © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 339–348, 1997