THE HYDROGEN ISOTOPE EFFECT IN THE PYROLYSIS OF CYCLOPROPANE

Abstract

The hydrogen isotope effect in the thermal rearrangement of cyclopropane has been determined by the copyrolysis of cyclopropane and cyclopropane-d6. The isotope effect in the temperature range 407–514 °C and at pressures of about 60 cm Hg may be expressed by[Formula: see text]The isotope effect is pressure dependent, decreasing from 1.98 at 76 cm Hg to 1.35 at 0.0178 cm Hg at 482 °C.The isotope effect and its temperature and pressure dependence are discussed in relation to the mechanism. The conclusion is reached that though trimethylene may be formed as an intermediate, the isotope effect is consistent only with an activated complex where a hydrogen atom is weakly bonded both to its original and ultimate carbon atoms, a situation which might also lead directly to propylene. A similar argument is presented to account for the previously described isotope effect in the decomposition of cyclobutane.