Isomerisations of cyclopropane

Fig. 1.3. The fall-off in rate of the isomerisation of cyclopropane in an infinite volume (dotted line), in a 11 sphere (solid line), and in a 16 ml sphere (dashed line), at 765 K.

Fig. 5.1. Comparison of the theoretical fall-off curve for the thermal isomerisation of cyclopropane at 765 K with the experimental results. The theoretical curve is calculated from the parameters listed in [78.Y2], except that the infinite pressure rate constant is taken to be 3.57 x 10 s rather than 3.41 x lO s as recommended by Falconer, Hunter Trotman-Dickenson [61.F1], see Footnote 4 the internal relaxation rate constant is r, = 5.50x10 Torr s. The experimental data are those of Chambers Kistiakowsky [34.C] (diamonds). Corner Pease [45.C] (crosses), and of Pritchard, Sowden Trotman-Dickenson [53.P2], also Appendix 2 (circles) the dotted line shows the high pressure limit of [61.F1],...

Fig. 5.2. Comparison of the theoretical fall-off curve for the thermal isomerisation of cyclopropane in a 11 sphere at 765 K with the experimental results from [63.K2], also Appendix 2 (circles). The dotted curve is that corresponding to an infinitely large vessel, as shown in Fig. 5.1, and the much fainter dotted curve shows by how much the rate would be increased by the water vapour which is known to be present. The crosses are experimental results for a packed reaction vessel, and the dashed line is a fit to these data, which corresponds to a sphere of about 16 ml.

Rate constants for the thermal isomerisation of cyclopropane and for the thermal decomposition of cyclobutane... 

Because of an editorial policy discouraging the presentation of experimental results in both graphical and tabular form, the primary rate constant data for the thermal isomerisation of cyclopropane in the fall-off region [53.P2] are only available in thesis form. One might have expected these results to have been superseded by now, but that has not happened, and Sowden s rather inaccessible thesis [54.S] remains the only source of these key data. In view of their continuing importance in the testing of unimolecular reaction theories, I am reproducing those results here (and also those of the cyclobutane reaction) for the convenience of future users. 

Schobert, M.A., and Y.H. Ma, "Isomerisation of Cyclopropane on Synthetic Faujasite by Pulse Technique -1, Mathematical Model", J. Catal. 70,102,1981a... 

Rate of reaction usually changes as the reaction proceeds. This is because the concentration of reactants is decreasing. Taking the isomerisation of cyclopropane to propene as an example ... 

The analyses of statistical factors may provide important information on the symmetry of the transition state. For example, the isomerisation of cyclopropane to propene may proceed via a symmetrical or an asymmetrical transition state (Figure 6.4). In the first case, (Tf = 2, whereas in the second case cif = 1. The symmetrical transition state corresponds to... 

Figure 6.4 Symmetrical and asymmetrical transition states in the isomerisation of cyclopropane to propene. The symmetrical transition state does not have any physical meaning because it requires the splitting of the reaction path into two paths at the transition state.

When 8 is sufficiently high, every energised molecule A is essentially an activated species A, but this conditions depends on the value of s as shown in Figure 8.4. Figure 8.5 illustrates the apphcation of Lindemann-Christiansen-Hinshelwood, RRK mechanisms to the isomerisation of cyclopropane as a function of pressure [4]. 

Under such conditions, pre-exponential factors greater than 10 sec can be estimated. This is usually interpreted in terms of weakly bonded activated molecules, which have a density of states much higher than the reagents. Figure 8.5 also illustrates the application of RRKM theory to the isomerisation of cyclopropane as a function of pressnre. Data were reproduced with a pre-exponential factor of A" = 2.82 X 10 sec and an activation energy = 274 kJ moF. ... 

ISM can also be employed to provide some insight for unimolecular reactions. We will consider the case of the isomerisation of cyclopropane to propene. 

We will assume the harmonic approximation for the stretching reactive modes for the isomerisation of cyclopropane. Additionally, we assume that the CCC angle at the transition state is intermediate (0 = 90°) between the value in the reactants (60°) and that in the products (120°), as illustrated in Scheme 8.VI. If such modes have a local character, with force constants % the effective force constant is just the result of a vectorial addition as shown in Figure 8.8. Generalising for three bonds, in order to encompass the CH bond perpendicular to the CC bonds, one obtains the following expression ... 

The following first-order rate constants fcj were obtained for the isomerisation of cyclopropane ... 

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