ChemInform Abstract: THE EFFECT OF ALKYL GROUP SUBSTITUTION ON THE RATE OF THERMAL ISOMERIZATION OF SMALL RING COMPOUNDS

Abstract

The thermal isomerization of 1-chloro-4-methylbicyclo[2.2.0]hexane and 1-chloro-4-lthylbicyclo[2.2.0]hexane has been studied in the gas phase and in tetrachloroethylene and o-dichlorobenzene solvents over the temperature range of 409–512°K. Good first-order kinetics yield the following Arrhenius equations: log kg/(sec−1) = (13.86 ± 0.03) − (36.90 ± 0.07)/middot; log ka/(sec−1) = (13.55 ± 0.31) − (35.50 ± 0.13)/θ log kb/(sec−1) = (13.55 ± 0.15) − (35.87 ± 0.06)/θ for the 4-methyl derivative, and log ka/(sec−1) = (13.68 ± 0.29) − (36.19 ± 0.13)/· log kb/(sec−1) = (13.55 ± 0.15) − (35.87 ± 0.06)/θ for the 4-ethyl derivative. The subscripts g, a, and b refer to the gas phase, tetrachloroethylene, and o-dichlorobenzene as solvents, respectively, θ = 2.303 RT kcal/mole, and the error limits are least squares deviations. The activation energy is increased by 1.2 ± 0.5 kcal/mole for a methyl group and 1.5 ± 0.5 kcal/mole for an ethyl group with respect to the unsubstituted bridgehead compound. With reference to the reported Arrhenius parameters for all bicyclo[2.2.0]hexane isomerizations, it is shown that this increase in activation energy with alkyl substitution is not due to the steric interaction in the transition complex. It is postulated that the ground-state reactant bicyclo[2.2.0]hexane molecules are stabilized by alkyl substitution. This ground-state alkyl stabilization of small ring compounds may also account for an increase in the activation energy on pyrolysis of alkyl substituted cyclopropanes and cyclobutanes.