Kinetics and thermodynamic studies on chloroformates

Abstract

The purpose of this study is to explore the nature of the reactions of n-propyl and benzyl chloroformates with tetra-n-butylammonium nitrate in actonitrile as a reaction medium. A thermodynamic investigation associated with the enthalpy of formation and vaporization of these chloroformates has also been undertaken. The reaction kinetics were determined at various concentrations of chloroformates and tetra-n-butyl-ammonium nitrate at different temperatures. The reactions have been found to be second-order and to follow pseudo-first-order rate coefficients proportional to the nitrate ion concentration. The changes in entropies, deltaS

+/-, ranged from -59.5 for n-C3H7OCOCl to -14.5J mol-1 deg-1 for C6H5CH2OCOCl, which indicates that the reaction proceeded by a bimolecular SN2 mechanism. The studies give evidence that the rate-determining step is the initial reaction between the chloroformates and NO-3 to give nitratocarbonate via a tetrahedral intermediate. The expected products, nitrate ester and carbon dioxide, are obtained in maximum yields of 18-20% and 61% respectively. Addition of pyridine or tetra-n-butylammonium perchlorate, has little effect on the yield of nitrate ester product. The tetra-n-butylammonium chloride product is more reactive towards both n-propyl and benzyl chloroformates than the corresponding nitrate. The enthalpies of solution of n-propyl and benzyl chloroformates in sodium hydroxide (1 mol dm -3 ) have been measured over a range of concentrations at 298. 15 K, using a solution reaction calorimeter. The variation with the concentration of the enthalpy of hydrolysis was not significant over the concentration ranges studied deltaHthetaf (C3H7OCOCl, 1) = -524.396 +/- 0.767 kj mol-1 for 5791