Gas-phase elimination kinetics of ethyl, isopropyl and tert-butyl N,N-diethylcarbamates.: Application of Taft-Topsom correlation for substituents other than carbon at the acid side of organic ethyl esters

The elimination kinetics of ethyl, isopropyl and tert-butyl N,N-diethylcarbamates were investigated in a static reaction vessel over the temperature range 220-400 degrees C and pressure range 17-160 Torr. These reactions are homogeneous, unimolecular and follow a first-order rate law. The temperature dependance of the rate coefficients is given by the following equations: for ethyl N,N-diethylcarbamate, log k(1) (s(-1)) = (11.47 +/- 0.25) - (178.4 +/- 3.1) kJ mol(-1) (2.303 RT)(-1), for isopropyl N,N-diethylcarbamate, log k(1) (s(-1)) = (12.83 +/- 0.70) - (179.8 +/- 7.9) kJ mol(-1) (2.303 RT)(-1); and for tert-butyl N,N-diethylcarbamate, log k(1) (s(-1)) = (12.87 +/- 0.62) - (158.6 +/- 6.2) kJ mol(-1) (2.303 RT)(-1). The branching of the alkyl groups at the alcohol side of the ester exerts a significant effect on the rates in the order tert-butyl > isopropyl > ethyl, in addition, the presence of different substituents other than carbon at the acid side of organic ethyl esters gives the best correlation when using the Taft-Topsom equation: log k/k(H) = -(0.68 +/- 0.12)sigma(alpha) + (2.57 +/- 0.12)sigma(F) - (1.18 +/- 0.27)sigma(R) (r = 0.984 +/- 0.119 at 400 degrees C). According to this relationship, the field (inductive) effect of the substituent has the greatest influence on rate enhancement, while the polarizability (steric) and resonance factors, although small in effect, favour the elimination process. Copyright (C) 1999 John Wiley & Sons, Ltd.