The thermal isomerisation of allyl isocyanide

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MARSHAT. J. GUONNAand Huw O. PRITCHARD.Can. J. Chern. 57,2482 (1979). The thermal isomerisation of allyl isocyanide to allyl cyanide has been studied ...
2482

The thermal isomerisation of allyl isocyanide MARSHA

T. J.

GLIONNA AND

Huw O.

PRITCHARD

Chemistry Department, York University, Downsview, Ont., Canada M3J lP3 Received March 23, 1979 MARSHAT. J. GUONNAand Huw O. PRITCHARD.Can. J. Chern. 57,2482 (1979). The thermal isomerisation of allyl isocyanide to allyl cyanide has been studied in the gas phase over the temperature range 130-200°C. The reaction is homogeneous and first order, and at high pressure (20 Torr) has an activation energy of 40.8 ± 0.6 (2sdm) kcal mol-1 ; the corresponding range of frequency factor is 1014.77±o.3o S-1. MARSHAT. J. GUONNAet Huw O. PRITCHARD.Can. J. Chern. 57,2482 (1979). Operant en phase gaze use, it des temperatures allant de 130 a 200°C, on a etudie I'isomerisation thermique de l'isocyanure d'allyle en cyanure d'allyle. La reaction est homogene et du premier ordre; it pression elevee (20 Torr), l'energie d'activation est 40.8 ± 0.6 (2sdm) kcal mol-1 et Ie facteur de frequence correspondant est 10'4.77±O.30 S-1. [Traduit par Ie journal]

Introduction

In our recent exploratory study of the reactions of methylene radicals with isocyanides (1), we noticed that allyl isocyanide appeared to be somewhat less readily isomerised than were methyl or ethyl isocyanides: since this isomerisation appears to take place through the insertion of the methylene radical into the R-NC bond, followed by its elimination, this might suggest that the R-NC bond in allyl isocyanide is slightly stronger than a normal aliphatic R-NC bond. The origin of such a strengthening could be rationalised in terms of some residual conjugation between the 1t-electrons in the C=C and N=C bonds and if this were the case, the activation energy for isomerisation to cyanide would be expected to be larger than normal: the activation energies for isomerisation at infinite pressure for methyl and ethyl isocyanides are both 38.4 kcal mol-1 (2, 3) whereas for vinyl isocyanide where there is full conjugation between the two 1t-electron systems, the activation energy for isomerisation is estimated to be about 70 or 80 kcal mol-1 (4). On the other hand, allyl isocyanide could conceivably isomerise by a head-to-tail intramolecular mechanism, which would almost certainly have a lower activation energy than normal. Consequently, it is of some interest to determine the activation energy for the thermal isomerisation of allyl isocyanide to allyl cyanide. E

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Allyl isocyanide was prepared from allylamine by standard methods (1) and was purified by preparative-scale gas chromatography. Allyl cyanide, required for calibration purposes, was prepared by carrying the thermal decomposition of a sample of our allyl isocyanide to completion: a commercial

sample (Aldrich) appeared to be a mixture of two isomeric substances which we were unable to resolve sufficiently well to provide an analytical standard. Reaction was carried out in a 100 mL spherical Pyrex bulb immersed in an oil bath; reaction was initiated by admitting a (nominal) pressure of 20 Torr of allyl isocyanide vapour into the bulb, already immersed in the bath, and was terminated by removing the bulb from the bath and quenching it. Auxiliary tests using fine thermocouples inside the reaction vessel showed that the maximum uncertainty in the measured reaction time was ± 3 s; this is negligible, since the reaction times ranged from 3-4 days near 130°C to 15 min near 200°C, Reaction temperature was measured by chromel-alumel thermocouple with a Digitemp digital display: the instrument was calibrated at 0, 100, and 327°C, and its linearity in the experimental interval was checked by comparison against a mercury in glass thermometer; the precision of the temperature control during any experiment was ± 0.1 °C, with an absolute accuracy of ± 1°C. Analysis of the reaction products was carried out by gas chromatography using a Pennwalt 223 column at 70°C and thermal conductivity detectors: due to the tailing of the isocyanide peak, the precision of these analyses was about ± 5%, and this represents the major source of uncertainty in these experiments.

Results

The cleanliness of the reaction was demonstrated by the experiment, noted above, to prepare allyl cyanide from allyl isocyanide: apart from a trace of HCN, which is typical in these isomerisations (5, 6), there was about 1% of an unidentified product of retention time intermediate between that of the isocyanide and the cyanide; we cannot tell whether this was present in the original isocyanide, or whether it was a byproduct. Also, a separate series of experiments was undertaken at 175°C to show h h I d h' h h t at t e ca cu ate rate constant,. w l~ we ave assumed to be first order and at Its hIgh-pressure limit, was independent of the initial pressure, the

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0008-4042/79/182482-02$01.00/0

© 1979National Research Council of Canada/Conseil national de recherches du Canada

GLIONNA TABLE 1. The thermal isomerisation of allyl isocyanide at 18 Torr in the temperature range 128.7-201.8°C

% T(K)

Time (s)

401. 9 404.2 415.3 419.4 422.5 431.1 440.3 449.9 458.2 467.5 475.0

2.55 x 10s 3.55x 10s 7.75 X 104 1.53x10S 6.36x 104 2.88x 104 1.35x104 3.74x103 3.73x 103 1.73x103 9.43 X 102

reaction

0.96 1. 76 1. 51 5.06 2.85

3.11 3.87 3.07 7.63 8.60 8.69

2483

AND PRITCHARD

---

k",(S-l)

3.8x 10-8 5.0x 10-8 2.0x10-7

3.4x 10-7 4.6x 10-7 1.1 x 10-6 2.9x 10-6 8.3x10-6

2.1x10-s 5.2x 10-5 9.6x 10-5

tion of methyl isocyanide (2, 8) or than that for ethyl isocyanide (3). This supports the supposition that there may be a small enhancement of the strength of the R-NC bond in the allyl compound, and that the alternative head-to-tail intramolecular mechanism is not important: the latter conclusion is also supported by the relatively high value we find for the frequency factor, since the head-to-tail mechanism would require a cyclic transition state, and would probably have a rather lower frequency factor on this account. A definitive determination of the mechanism of this reaction could be made by using isotopically labelled allyl isocyanide; unfortunately, suitably deuteriated allyl compounds are not readily available. Acknowledgement

reaction time and the surface-to-volume ratio: all the rate constants fell within a range of ± 6%, with no apparent trends, as would be expected for a well behaved reaction with the analytical precision attainable. The results of a series of experiments to determine the activation energy, all made at the same pressure (18 Torr), are shown in Table 1; assigning' equal weight to each determination, the least-squares treatment yields an activation energy of 40.81 kcal mol-1 with a standard deviation (7) of ± 0.29 kcal mol-1; the corresponding range in the frequency factor is 1014.77±O.15 S-l. The activation energy for the isomerisation of allyl isocyanide appears to be about 2.4 ± 1 kcal mol-1 greater than that for the analogous isomerisa-

This research is supported Research Council of Canada.

by the National

1. M. T. J. GLIONNA and H. O. PRITCHARD. Can. J. Chern. 57,

1229(1979). 2. F. W. SCHNEIDER and B. S. RABINOYITCH. J. Am. Chern. Soc. 84,4215 (1962). 3. K. M. MALONEY and B. S. RABINOYITCH. J. Phys. Chern.

73, 1652(1969). 4. J. B. MOFFAT. J. Phys. Chern. 81. 82(1977). 5. C. K. YIP and H. O. PRITCHARD. Can. J. Chern. 48. 2942 (1970). 6. T. FUJIMOTO. F. M. WANG, and B. S. RABINOYITCH. Can. J. Chern. 50. 3251 (1972). 7. N. C. BARFORD. Experimental measurements: precision. error and truth. Addison-Wesley, London. 1967. p. 62. 8. J. L. COLLISTER and H. O. PRITCHARD. Can. J. Chern. 54.

2380 (1976).