A structure–reactivity correlation with three slopes in the elimination kinetics of 2–substituted ethyl N,N‐dimethylcarbamates in the gas phase

Abstract

AbstractThe elimination kinetics of 17 2‐substituted ethyl N,N‐dimethylcarbamates in the gas phase were determined in the temperature range of 269.5–420.2 °C and the pressure range of 24–186 Torr. The reactions in a static system and in the presence of a free radical inhibitor are homogeneous and unimolecular and follow a first‐order rate law. The kinetics and thermodynamic parameters are described. The use of several structure‐reactivity relationship methods meaningless results, except for Taft σ* values. Three good slopes are originated at σ*(CH3) = 0.00. Slope a: the 2‐substituted alkyl groups gave a good straight line when log (k/kCH3) vs σ* values (ρ* = − 1.94 ± 0.30, r = 0.977 at 360 °C) were plotted. Slope b: Polar2 substituents gave an approximate straight line with ρ* = − 0.12 ± 0.02, r = 0.936 at 360 °C. Slope c : the correlation of multiple bonded and electron‐withdrawing substituents interposed by a methylene group at the 2‐position of ethyl N,N‐dimethylcarbamate was found to give a very good straight line wirh ρ* = 0.49 ± 0.02, r = 0.991 at 360 °C. Mechanisms are suggested on the basis of these relationships. The point position of the substituents phenyl (C6H5) and isopropenyl [CH2C(CH3)] at the 2‐position was found to fall far above the three slopes of the lines. These results are interpreted in terms of neighboring group participation of these substituents in the elimination process of the carbamates. However, the acidity of the benzylic and allylic Cβ‐H bond for a six‐membered cyclic transition state may not be ignored. Copyright © 2001 John Wiley & Sons, Ltd.