A structure-reactivity correlation with three slopes in the elimination kinetics of 2-substituted ethyl N,N-dimethyl-carbamates in the gas phase

The elimination kinetics of 17 2-substituted ethyl N,N-dimethylcarbamates i n the gas phase were determined in the temperature range of 269.5-420.2 deg reesC and the pressure range of 24-186 Torr. The reactions in a static syst em and in the presence of a free radical inhibitor are homogeneous and unim olecular and follow a first-order rate law. The kinetics and thermodynamic parameters are described. The use of several structure-reactivity relations hip methods meaningless results, except for Taft sigma* values. Three good slopes are originated at sigma*(CH3) = 0.00. Slope a: the 2-substituted alk yl groups gave a good straight line when log (k/kCH(3)) vs sigma* values (r ho* = -1.94 +/- 0.30, r = 0.977 at 360 degreesC) were plotted. Slope b: Pol ar(2) substituents gave an approximate straight line with rho* = -0.12 +/- 0.02, r = 0.936 at 360 degreesC. Slope c: the correlation of multiple bonde d and electron-withdrawing substituents interposed by a methylene group at the 2-position of ethyl N,N-dimethylcarbamate was found to give a vent good straight line with rho* = 0.49 +/- 0.02, r = 0.991 at 360 degreesC. Mechan isms are suggested on the basis of these relationships. The point position of the substituents phenyl (C6H5) and isopropenyl [CH2=C(CH3)] at the 2-pos ition was found to fall far above the three slopes of the lines. These resu lts are interpreted in terms of neighboring group participation of these su bstituents in the elimination process of the carbamates. However, the acidi ty of the benzylic and allylic C-beta-H bond for a six-membered cyclic tran sition state may not be ignored. Copyright (C) 2001 John Wiley & Sons, Ltd.