Kinetics and mechanisms of the elimination of ethyl andtert-butyl esters of carbazic acid in the gas phase: experimental and theoretical studies

Abstract

The kinetics of the gas-phase elimination of ethyl and tert-butyl carbazates were studied in a static system over the temperature range 220.3–341.7°C and pressure range 21.1–70.0 Torr (1 Torr = 133.3 Pa). The reactions in seasoned vessels are homogeneous, unimolecular and obey a first-order rate law. The variation of the rate coefficients with temperature is given by the following Arrhenius equations: for ethyl carbazate log[k1(s−1)] = (11.84±0.22)−[(176.2±2.5) kJ mol−1](2.303 RT)−1 and for tert-butyl carbazate log[k1(s−1)] = (12.34±0.29)−[(153.6±2.9) kJ mol−1](2.303 RT)−1. The theoretical examination indicates that the molecular mechanism corresponds to a concerted non-synchronous reaction giving the products. Bond order analysis and natural charges imply that polarization of the O(alkyl)C(alkyl) bond of the ester is rate determining in these reactions. The rate coefficients from the experiments are in good agreement with the theoretical calculations. The mechanisms of these reactions and the role of the hydrazo group at the acid side of the ester are discussed. Copyright © 2005 John Wiley & Sons, Ltd.