Interferometric Determination of the Rate Constant of the Isomerization of Cyclopropane to Propene in the Temperature Range between 690 K and 750 K

The thermally induced isomerization of cyclopropane (C) to propene (P) was studied for the first time with an interferometric technique, using HeNe‐laser radiation of vacuum‐wavelength λ = 632.99 nm. The investigations have been performed in the temperature and pressure ranges between 690 K and 750 K and between 7 · 104 Pa and 9 · 104 Pa, respectively. By recording the isothermal and nearly isobaric increase of the refractive index nM (t) and therefore of the polarizability αM (t) = xC(t)αC + xP(t)αP of the mixture with increasing reaction time t, one can obtain the mole fraction xC(t) = (αM(t) – αP)/(αC – αP) = exp(–k ∞ t) of cyclopropane. Assuming that the high pressure limit of this reaction is already achieved, a usual Arrhenius plot ln(kx/s−1) = ln(A/s−1) – EA/RT with the high‐pressure rate constant kx yields for the frequency factor ln(A/s−1) = 34.32(60) and for the activation energy EA = 267.5(3.6) kJmol−1. These values, also somewhat smaller, are in fair agreement with data published in the literature.