KINETICS OF THE REACTIONS OF CYCLOPROPANE DERIVATIVES .5. GAS-PHASE UNIMOLECULAR REACTIONS OF 1-ALPHA-CHLORO-2-ALPHA,3-ALPHA-DIMETHYLCYCLOPROPANE AND 1-ALPHA-CHLORO-2-ALPHA,3-BETA-DIMETHYLCYCLOPROPANE

Thermolysis of the “all‐cis” compound 1α‐chloro‐2α,3α‐dimethylcyclopropane (A) at 550–607 K and 6–115 torr is a first‐order homogeneous non‐radical‐chain process giving penta‐1,3‐diene (PD) and HCl as products. The Arrhenius parameters are log10A(sec−1) = 13.92 ± 0.08 and E = 199.6 ± 0.9 kJ/mol. The isomer with trans‐methyl groups, 1α‐chloro‐2α,3β‐dimethylcyclopropane (B) reacts by two parallel first‐order processes giving as observed products trans‐4‐chloropent‐2‐ene (4CP) and PD + HCl, with log10A(sec−1) = 14.6 and 13.8, respectively, and E = 199.5 and 190.2 kJ/mol, respectively. The 4CP undergoes secondary decomposition to PD + HCl (as investigated previously). Comparison of the results for compounds (A) and (B) with those for other gas‐phase and solution reactions leads to the conclusion that the gas‐phase thermolyses proceed by rate‐determining ring opening to form olefins which may decompose further by thermal or chemically activated reactions, and that the ring opening is a semiionic electrocyclic reaction in which alkyl groups in the 2,3‐positions trans to the migrating chlorine semianion move apart, with appropriate consequences for the rate of reaction and the stereochemistry of the products. Copyright © 1979 John Wiley & Sons, Inc.