The thermal isomerization of 1.2-dimethyl cyclo propane I. Cis-trans isomerization

Abstract

This isomerization has been investigated between 380 and 453 °C. In this temperature range, in an ' aged ’ reaction vessel, cis-trans isomerization occurs as a reversible homogeneous unimolecular reaction. The rates of this geometrical isomerization have been measured and the Arrhenius parameters evaluated. In the case of cis 1. 2 -dimethyl cyclo propane, the rate equation obtained is k ∞ = 10 15.25 exp ( - 59 420 ± 500/ RT ) sec -1 . The values of the equili­brium constant for the reaction have also been determined at various temperatures between 380 and 476 °C, and the value of the enthalpy change calculated. The first-order rate constant begins to decrease at about 16 mm, and has fallen to approximately 70 % of the high-pressure value at 0.1 mm. Occurring simultaneously with the geometrical isomerization is a structural isomerization to yield cis and trans pent-2-ene, 2-methylbut-l-ene and 2-methylbut-2-ene. The geometrical isomerization is considerably faster than the structural isomerization. Approximate values for the Arrhenius parameters relating to the various products of the structural isomerization have been determined. Gas chromatography, by means of a katharometer detector with electronic integration, has been used throughout and has been developed to give a precision of ± 0.5 % in the analysis of the reaction mixtures.