Fluorine unimolecular reaction rates

The m-(trifluoromethyl)phenyl groups are lost in pairs, stepwise, in a unimolecular reaction for which the ratio of the rate constants, x/ 2> is 28 at 73.7°C. Cairncross and Sheppard believe that this behavior may be typical of other arylcopper compounds which are not highly substituted by fluorine (37). 

In contrast to the cyclobutane systems, fluorinated cyclopropanes show a lesser thermal stability than the parent hydrocarbon. Trotman-Dickenson et al. (refs. ° )have studied thethermally induced unimolecular isomerisations of partly fluorinated cyclopropanes to mixtures of the corresponding fluoro-propenes. At 450 °C the rate of decomposition of monofluorocyclopropane is about three times that of cyclopropane products formed are 1-fluoropropene (79 %) cis-trans mixture), 2-fluoropropene (9 %) and 3-fluoropropene (11 %). The substitution of further fluorine atoms in the cyclopropane ring further increases the rate of isomerisation ° (see Table 2). Unlike the partly fluorinated cyclopropanes, perfluorocyclopropane does not isomerise to propene compounds but decomposes at 250-300 °C by a first-order reaction to perfluoroethylene . The rate coefficient is expressible as, k = 1.78 x 10 exp (-38,600//JjT) sec , and the decomposition is consistent with the mechanism... 

The chemically activated molecules are formed by reaction of CH with the appropriate fluorinated alkene. In all these cases apparent non-RRKM behaviour was observed. As displayed in figure A3.12.il the measured unimolecular rate constants are strongly dependent on pressure. The large rate constant at high pressure reflects an initial excitation of only a fraction of the total number of vibrational modes, i.e. initially the molecule behaves smaller than its total size. However, as the pressure is decreased, there is time for IVR to compete with dissociation and energy is distributed between a larger fraction of the vibrational modes and the rate constant decreases. At low pressures each rate constant approaches the RRKM value. 

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