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-diethylcarb amates were investigated in a static reaction vessel over the temperature r ange 220-400 degrees C and pressure range 17-160 Torr. These reactions are homogeneous, unimolecular and follow a first-order rate law. The temperatur e 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, l og 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 al kyl 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 pre sence of different substituents other than carbon at the acid side of organ ic ethyl esters gives the best correlation when using the Taft-Topsom equat ion: 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 t o this relationship, the field (inductive) effect of the substituent has th e greatest influence on rate enhancement, while the polarizability (steric) and resonance factors, although small in effect, favour the elimination pr ocess. Copyright (C) 1999 John Wiley & Sons, Ltd.