Kinetics and thermodynamic studies on chloroformates

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 <n < 19593 and deltaHtheta(C6H5CH2OCOCl, 1) = -375. 244 +/- 1.088 kJ mol-1 for 36804 < n < 52495. Enthalpies of vaporization have been obtained from measurements of vapour pressure over the temperature range 293. 13 to 306. 30 K, deltaHthetav (n-C3H7OCOCl, 298. 15 K) = 40. 676 +/- 0. 450, and 293. 770 to 302. 34 K, deltaHthetav (C6H5CH2 OCOC1, 298. 15K) = 38. 527 +/- 0. 1 10 Kj mol -1. Enthalpies of vaporization were also obtained using a vaporization calorimeter, giving deltaHthetav (n-C3H 7OCOCl, 298. 15 K ) = 40. 742 + 0. 403 kj mol -1. These values of deltaHthetav, obtained from the vapour pressure measurements, are combined with the enthalpies of formation of deltaHthetaf (1) values to give enthalpies of formation in the gaseous state at 298. 15 K viz., deltaHthetaf (n-C3H7OCOCl, g) = -483.720 +/- 0. 864 kj mol deltaHthetaf (C6H5CH2OCOCl, g) = -336.717 + 1.094 kJ mol