The thermal isomerisation of methyl isocyanide in the temperature range 120–320 °C

Abstract

Can. J. Chem. 54,2380(1976. The thermal isomerisation of methyl isocyanide has been measured in the rigorous absence of self-heating over a temperature range from 120-320 "C and at pressures from 2-100 torr. Expressions are given for the rate constant as functions of both temperature and pressure in these ranges of the two variables. The infinite-pressure rate constant is accurately represented by an Arrhenius line with no curvature having E , = 38.2 + 0.2 kcal mol-1 and loglo A , (s-1) = 13.35 + 0.11, although at all other pressures, E is a function of temperature.A number of tests on the cleanliness of the reaction were made, and some previously expressed concerns about side reactions are alleviated considerably. On a effectuC un certain nombre d'essais pour vCrifier la propretC de la reaction; on considere que quelques unes des inquietudes exprimks antkrieurement concernant les rkctions secondaires avaient Ct C grossierement surestimks. JOHN LEWIS COLLISTER et[Traduit par le journal] B Introductiontemperature range was made as wide as possible It has been suggested recently that the explosive isomerisation of methyl isocyanide may prove to be a sensitive and reliable standard against which to test theories of gaseous thermal explosions (1); the required kinetic and thermodynamic data on methyl isocyanide and its iiomerisation are, however, incdmplete for this purpose. As far as the kinetics of the isomerisation are concerned, the rate of the reaction has been studied extensively as a function of pressure at three temperatures only, 200, 230, and 260 OC (2), and extrapolation of these rates into the explosion region (310-360 OC) is very sensitive to small imperfections in the data. The uncertainty of this extrapolation is aggravated by the fact that in some experiments a correction had to be ma8e for self-heating in the reaction, and this correction although small, is itself rather uncertain, giving rise to possible errors in the required extrapolated data. In the work described in this paper, self-heating was eliminated by the appropriate choice of reaction-vessel size for the temperature and pressure in question, and the

A reformulation of the theory of unimolecular reactions

“…Whether or not experimental rate data for thermal unimolecular reactions obey [34] rigorously is a matter which we will discuss further below, cf. Appendix 1; deviations from [34], o n the other hand, are clearly small, so that we may write a generalised form of the Arrhenius equation, i.e. 'It should be remembered, however, that k,@) 'nay not be defined in the form of eq.…”

“…Figure 11 shows the experimental data of Holbrook and Marsh (64) at 521°C, compared with our calculated form of kuni; the agreement is equally satisfactory at other temperatures also, but unfortunately, because of the scatter inherent in the experimental measurements, does not provide a very stringent test of this reformulation. The thermal isomerisation of methyl isocyanide t o methyl cyanide represents, to date, the most thoroughly studied unimolecular reaction (5,9,34). The fall-off behaviour was first studied by Schneider and Rabinovitch (9) a t 200, 230, and 260°C; they also derived the infinite-pressure Arrhenius parameters as A , = 10'3.6 S-' and E , = 38.4 kcal mol-'.…”

The kinetics of internal energy randomisation in thermal unimolecular reactions

“…also footnote 23 of ref. 8) that, despite superlicial appearances, the infinite-pressure limit extrapolated from our own experiments (25) is not the correct one. pressure limit (cf.…”

“…The discrepancies are similar (20) at each of the three temperatures studied by Schneider and Rabinovitch, and since the data for the temperature of 230.4"C are the most extensive, I will confine my attention to this set of data. The situation was confused recently by the emergence of another set of data from this laboratory (25): these experiments were not undertaken for the purposes of testing unimolecular reaction theory, and were confined to a very limited pressure range (2-100 Torr); their comparison with unimolecular reaction theory was only suggested later (1 1) in order to try to throw some more light on the nature of these generally agreed discrepancies. It turns out that over their limited pressure range, not only do the data of Collister and myself (25) agree very well with those of Schneider and Rabinovitch, but they also fit superbly to standard unimolecular fall-off curves corresponding to a somewhat lower high-pressure limit than that of Schneider and Rabinovitch (cf.…”

Failure of the scaling laws in the thermal explosion of methyl isocyanide

“…The experimental technique is simple and well known (1)(2)(3)(4): methyl isocyanide is admitted to a spherical vessel held in a constant temperature air bath by momentarily opening an electrically operated valve; the temperature at the centre of the vessel is monitored using a fine thermocouple, and the pressure using a pressure transducer attached to the neck. If the pressure of isocyanide admitted is below the critical explosion pressure, a temperature rise of less than 40°C is recorded at the centre of the vessel, Can.…”