Methyl isocyanide, isomerisation

Any reaction may be chosen as the subject for each calculation, but the methyl isocyanide isomerisation provides a fairly realistic example of a strong collision reaction without consuming too much computer time data sets are given for both methyl isocyanide and cyclopropane. 

Two thermal unimolecular reactions may be regarded as the benchmarks against which to test the theory these are both isomerisation reactions, of cyclopropane to give propylene and of methyl isocyanide to give methyl cyanide. It turns out, in fact, that methyl isocyanide is a borderline strong collision case, a point that we will discuss much more fully later, and so the comparisons made in this chapter will be limited to the cyclopropane reaction, wherever possible. 

Fig. 5.9. Variation of the activation energy for the thermal isomerisation of methyl isocyanide as a function of pressure. Experimental data squares from Schneider Rabinovitch [62.S], triangles from Collister Pritchard [76.C]. The solid line assumes that ii is independent of temperature the dotted line assumes that i increases slightly with temperature, with an Arrhenius temperature coeffident of 0.25 kcalmoP .

Fig. 6.5. Comparison of the synthetic specific rate function (dashed line) for the thermal isomerisation of methyl isocyanide with that derived from the inverse Laplace transform (solid line).

There have been several examples recently of experiments from which it is possible to deduce approximate values of the specific rate constant for reaction at certain fixed values of the energy E. If methyl isocyanide vapour is subjected to intense laser radiation at 7265 A, corresponding to an excitation energy of 39.3 kcal mol , then those molecules which absorb radiation are raised to an energy which is about 1 kcal mol above the thermal threshold for isomerisation. In its most primitive form, the reaction sequence can be written as... 

Fig. 7.1. Fall-off curves for a model calculation on the thermal isomerisation of methyl isocyanide considering only first order randomisation processes. In descending order, the curves correspond to values of /

It is quite clear from an inspection of Figures 7.1 and 7.2 that the kind of agreement found between theory and experiment in Chapter 5 could not have occurred unless Pr was effectively infinite in these strong collision systems, because as soon as the randomisation processes become rate determining, there are severe departures from the simple strong collision, i.e. p, = oo, behaviour. We must then ask whether there are any known experimental results from which we can demonstrate a departure from infinitely rapid randomisation, and it appears that the well-studied thermal isomerisation of methyl isocyanide may be just such an example. 

Schneider Rabinovitch [63.S1] also performed another series of experiments in which they measured the relative rates of isomerisation of the two isotopic species of methyl isocyanide Figure 7.5 demonstrates the remarkable internal consistency of their data and shows that one does not expect to find the simple S-shaped dependence on pressure which is predicted by strong collision theory. 

Fig. 7.3. Fall-off curves for the thermal isomerisation of methyl isocyanide at 230.4°C. The points represent the experimental results of Schneider Rabinovitch [62.S]. The dashed line is the standard strong collision fall-off calculation. The solid line includes both first order and second order randomisation effects, with r,= 1.20 x 10 Torr s fii =4.0 x 10 s and r, = 1.25 X 10 Torr s .

Fig. 7.6. Fall-off curves for the thermal isomerisation of methyl isocyanide at 0.01 Torr in the presence of helium, at 245°C. The dashed line represents the strong collision fall-off curve, whereas the solid line is calculated with r = 22 X 10 Torr s , /i, = 5.0 x 10 s" ,andrj = 1.0 x 10" Torr s . The points are the measurements of Wang Rabinovitch [74.W2 75.W], but they do not extrapolate well towards the recommended value of [62.S], and it has been assumed in constructing this diagram that the temperature was 243°C.

Fig. 8.3. Model calculation for the isomerisation of methyl isocyanide using a two-component generalised weak collision matrix as shown in Fig. 8.2. In ascending order, thecurves are for = = =0isthe...

Unsaturated isocyanides, too, have been the focus of considerable attention. Vinyl isocyanides have been prepared either by Cu20-catalysed isomerisation of allylic analogues or by the Schollkopf method of reacting aldehydes with situ metallated methyl isocyanide followed by dehydration of the adducts.The... 

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