The reaction of methylene radicals with methyl ...

The reaction of methylene radicals with methyl isocyanide MARSHAT. J. GLIONNA A N D HUW0. PRITCHARD

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Chemistry Department, York University, Downsview, Ont., Canada M3J IP3 Received October 10,1978

MARSHA T. J. GLIONNA and Huw 0. PRITCHARD. Can. J. Chem. 57, 1229 (1979). An exploratory study has been made of the gas-phase reactions of methylene radicals, generated by the photolysis of ketene near 3000 A, with methyl, ethyl, and allyl isocyanides at room temperature. With methyl isocyanide, the principal product at low pressure is ethyl cyanide, together with a few percent of methyl cyanide; ethyl isocyanide is also formed, increasingly so as the total pressure is increased. Reaction appears to take place through a vibrationally excited ethyl isocyanide intermediate, and approximate rate constants for each reaction pathway are derived. Isotopic studies suggest that the methylene radicals insert in the H,C-NC bond of the methyl isocyanide. MARSHA T. J. GLIONNA et HUW0.PRITCHARD. Can. J. Chem. 57, 1229 (1979). On a effectuk une etude prdiminaire des rkactions en phase gazeuse des radicaux mkthylenes, produits par photolyse d'un cktene pres de 3000 A, avec des isocyanures de mdthyle, d'ethyle ou d'allyle a temperature ambiante. Avec I'isocyanure de methyle, a basse pression, le produit principal est le cyanure d'kthyle avec un faible pourcentage de cyanure de mkthyle; il se forme aussi de I'isocyanure d'ethyle dont la quantite augrnente avec I'augmentation de la pression. I1 semble que la rkaction se produise gr2ce un intermkdiaire vibrationnellement excite de l'isocyanure d'ethyle; on a kvalud les constantes de vitesse approximatives de chacune des voies de reaction. Des Btudes isotopiques suggerent que les radicaux mkthylenes s'inskrent dans la liaison H3C-NC de l'isocyanure de mkthyle. [Traduit par le journal] I

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I

1

I I

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Introduction relative proportions varied with the total pressure, In a recent series of chemical activation studies, detailed identification of each product was not Rabinovitch and co-workers (1-3) have generated pursued because, unexpectedly, the major ~ r o d u c t vibrationally excited molecules in which there are of the reaction was allyl cyanide. Since allyl cyanide two (almost isoenergetic) pathways for decomposi- Was not formed photochemically from allyl isocytion, in with collisional stabilisation. anide under our conditions, we undertook the study ~t would be interesting to extend this kind of ap- reported below to try to elucidate the nature of this preach to the study of a vibrationally kxcited mole- catalysed isomerisation. In view of the tentative cule which could decompose by two reaction paths conclusion reached, that the principal reaction of having very different critical energies. F~~example, methylene radicals with saturated isocyanide molevibrationally excited cyclopropyl isocyanide could cule~is to insert in the R-NC bond, the proposed isomerise to ally1 isocyanide at excitation energies study of methylene radicals with allyl isoc~anide greater than about 60 kcal rnol-l and to cyc~opropy~appears to be rather intractable because some of the cyanide at excitation energies greater than about Same reaction products may arise both from the 40 kcal mol- 1; isomerisation of both functional addition to the double bond and from the insertion groups is also possible. Unfortunately, the producthe all~l-NC We proceed present tion of vibrationally excited cyc~opropy~ isocyanide evidence for this insertion of methylene radicals into molecules by addition of methylene radicals to the R-NC bond, where R is methyl, and also some vinyl isocyanide is not feasible because vinyl iso- circumstantial evidence for the same process when R cyanide is rather unstable, both to visible light and is with respect to spontaneous polymerisation (4). Experimental Consequently, despite the fact that more isomerisaMethyl, ethyl, n-propyl, s-propyl, and allyl isocyanides were tion pathways exist, we undertook to explore the homologous reaction, methylene radicals plus allyl prepared from the appropriate N-alkyl formamide by the method of Casanova, Schuster, and Werner (5); except in the isocyanide9in search of the same kind of informa- ca, of N-rnethyl formami& which was obtained commercially, tion. However, although the expected range of mass the other N-alkyl formamides were prepared from ethyl 81 cyanides and isocyanides was observed, and their formate and the corresponding amine by standard methods 0008-40421791lo 1229-04$01.oo/o 01979 National Research Council of CanadalConseil national de recherches du Canada


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CAN. J. CHEM. VOL. 57. 1979

(6). Each isocyanide was purified by vapour-phase chromatography. Methyl, ethyl, n-propyl, s-propyl, and ally1 cyanides, used for identification and calibration purposes, were obtained commercially. Ketene was prepared by the pyrolysis of acetic anhydride vapour at 400°C (7), and after separation by trap-to-trap distillation, was stored at liquid nitrogen temperature; the liquid was degassed each time before use. Dideuterioketene (CD2CO) was prepared in an analogous manner from 99% acetic anhydride-d,, obtained from Merck, Sharp and Dohme. Photolysis experiments were conducted at room temperature in a 500 mL Pyrex glass vessel, using the unfiltered light from a 100 W high-pressure mercury lamp; during the filling of the reaction vessel and the analysis of the products, it was necessary to avoid condensing isocyanide and ketene together in liquid nitrogen, as they react immediately under these conditions. Products were identified by gas chromatography (using a Pennwalt 223 column at 70°C)coupled with mass spectrometry.

Results and Discussion Our initial supposition was that methylene radicals, which commonly insert into C-H bonds in organic molecules (a), would react with methyl isocyanide to form a vibrationally hot ethyl isocyanide molecule. This excited molecule would then isomerise to ethyl cyanide [21

kl [C2H5NC]*+ C2H5CN

at a rate which can be estimated according to unimolecular reaction theory (9), or be stabilised collisionally [31

k2

[C2H5NC]*

+ M + C2HSNC+ M

Thus, the relative rates of formation of cyanide and isocyanide should obey the simple form REtCN[M]/REtNC= kl/k2 = constant. [4] At low pressures (5-10 Torr of methyl isocyanide, 1-5 Torr of ketene), the reaction product was almost exclusively ethyl cyanide, with small amounts of methyl cyanide and hydrogen cyanide amounting, respectively, to about 10% and 1% of the total product. Increasing the total pressure of the system by addition of up to 600 Torr of argon or nitrogen led to small but increasing yields of ethyl isocyanide; qualitatively, at least, this is consistent with the reaction scheme proposed above. Likewise, the reaction of methylene radicals with ethyl isocyanide gives significant quantities of n-propyl cyanide and n-propyl isocyanide, but only minute traces of s-propyl cyanide and s-propyl isocyanide. Neither methyl cyanide nor ethyl cyanide reacts with methylene radicals, nor are any of the cyanides or isocyanides affected by the photolysis lamp in the absence of ketene (cf. also

refs. 10 and 11 for more extensive information, on the photochemical reactions of methyl isocyanide). Returning to the reaction of methylene radicals with methyl isocyanide, the relative variations of the yields of ethyl cyanide and ethyl isocyanide can be shown to be consistent with the reaction scheme [I]-[3], at least semiquantitatively. Because of the need to avoid condensing reactant or product mixtures in liquid nitrogen, and because of the relatively high pressures of argon required to produce significant quantities of ethyl isocyanide, we chose simply to study the variation of product yields with varying ketene pressure at fixed isocyanide pressure, realising that this procedure is somewhat unconventional and very inefficient in the use of methylene radicals. Table 1 gives a summary of the results of four experiments carried out at different ketene pressures, but otherwise performed under the same conditions. It is clear that, in the penultimate column of this table, the relationship [4] holds quite well. If we assume collision diameters of 4.5 and 5.5 A respectively for CH,CO and [C,H,NC]*, the appropriate collision number Z = 4 x lo-'' cm3 molecule-' s-' at room tem~erature.We mav also assume (9) that the rate constant for deactivation of vibrationally excited ethyl isocyanide, i.e. k,, is 0.1 times the collision rate constant Z, whence the data in Table 1 give k, as approximately 2.2, x 10" s-I. By using the following thermochemical data (all relating to-the gas phase a t room temperature) AH,(CH,NC) AH,(CH,) AHf(C2H,CN) the overall reaction

--

41 kcal mol-

'

(12)

92 kcal mol-

'

(13)

12 kcal mol-'

(14)

is approximately 121 kcal mol-' exothermic, and since the AH for isomerisation of ethyl isocyanide is about -22 kcal mol-' (12, 15), this indicates that the newly formed ethyl isocyanide molecule in reaction [I] contains about 99 kcal mol-' of internal energy. We may then estimate the rate constant k t for the isomerisation of such an excited molecule (16), treated as a collection of Morse oscillators' and rigid rotors with kinetic and spectroscopic properties as tabulated in ref. 9, to be approximately 4.1 x 10tOs-'. We conclude that in view of both the uncertainties in the theoretical model and in the experimental method, the proximity of this figure to IAlternatively, in the harmonic-oscillator rigid-rotor approximation, theestimated value would be about 6 x 10''s-' (16).

GLIONNA AND PRITCHARD

TABLE1. Relative rates of formation of products in the reaction of methylene radicals with methyl isocyanide at room temperature


PCH~COP t o t a ~ (Torr) (Torr)

RE~CNIRB~NC RM~CNIREINC P t o t a ~x R (= R) (ER') ( Xlo-3)

x R' ( x lo-3)

Ptota~

our enperid7ntally derived value of about 2 x 101° s-' lends sppport to our interpretation of the experimentdliprocess. / The last column in Table 1 may suggest that the is also constant, if one ratio RMeCN[M]/REtNc acknowledges the fact that at high pressures, the yield of methyl cyanide is too small to be measured accurately on the tail of the methyl isocyanide peak: thus, it appears that at these energies, there could be another reaction channel available

pressed the formation of all the products noted above. However, in view of the high reactivity of isocyanides in general (19) and the incipient freeradical nature of the proposed adduct [C2H,NC]*, we do not feel that this observation is decisive, one way or the other. Thus, although we have been unable to demonstrate conclusively that we observed the reactions of singlet methylene radicals, the high pressures used in Table 1 would tend to support that assumption. However, from the mass-spectroscopic evidence we now describe, it appears to be reasonably certain that the overall addition process [l] may be regarded as an insertion into the R-NC bond rather with k3 -- 1.7 x lo9 s-'. This would imply that at than an insertion into a C-H bond, as is usually very high temperatures, the thermal decomposition observed (8). of ethyl isocyanide should yield small amounts of Using dideuterioketene as the source of radicals, methyl cyanide, as well as the normal product ethyl samples of ethyl cyanide and methyl cyanide taken cyanide; methyl isocyanide would also be formed by from low-pressure runs, and a sample of ethyl the reverse of reaction [I], but would not live long isocyanide taken from a high-pressure run, were subenough to be detected by conventional kinetic tech- jected to mass-spectroscopic examination. These n i q u e ~ At . ~ the same time, at these high tempera- experiments show that (i) the ethyl cyanide and ethyl tures, one would expect the formation of large isocyanide formed both contain two D atoms as quantities of hydrogen cyanide, since this is a major expected; (ii) the cracking patterns of these moleproduct in the conventional pyrolysis of ethyl cules show fairly unambiguously that the CD2 cyanide (17, 18); however, in our reaction, the yield radical inserts into the H3C-NC bond: in the first of hydrogen cyanide was only of the order of 10-20% place, mass peaks are present at m/e = 42 (CD2CN') of that of the methyl cyanide, and we were unable to and 40 (CDCN' or CH2CN+), but there is no peak estimate a rate constant for the postulated third at m/e = 41 (CHDCN'); moreover, there is a strong channel peak at m/e = 15 (CH,'), but nothing above background at mass values of 16 (CH2Df) or [C2HsNC]* -+ HCN + C2H, [71 17 (CD2Hf). This conclusion is also supported by The observations described so far are not incon- the absence of s-propyl products in the reaction of sistent with the proposed reaction scheme [I]-[3], methylene radicals with ethyl isocyanide, as noted but they do not shed any light on the nature of the above; (iii) the methyl cyanide formed in the CD2addition process [I], neither in respect of the site of labelled reaction [6] contains no deuterium atoms. attack nor in respect of the degree of excitation of Acknowledgements the methvlene radical involved. The addition of This work was supported by the National Research small quantities of air or oxygen to our reaction Council of Canada, and we would also like to mixtures, commonly used (1, 2) to ensure that only singlet methylene radicals undergo reaction, sup- acknowledge the benefit of considerable assistance from Drs. J. L. Collister, B. H. Khouw, and M. H. B. 21n this respect, it is interesting to note that the simple Vayjooee.

photolysis, under our conditions, of an unpurified commercial sample of ethyl cyanide gave small amounts of methyl cyanide, methyl isocyanide, and ethyl isocyanide, presumably through photosensitisation by an unknown impurity to form a hot ethyl cyanide molecule.

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3. A. N. KO, B. S. RABINOVITCH, and K. J. CHAO.J. hem. Phys. 66, 1374 (1977). 4. D. S. MATTESON and R. A. BAILEY.J . Am. Chem. Soc. 90, 3761 (1968). 5. J . CASANOVA, R. E. SCHUSTER,and N. D. WERNER.J . Chem. Soc. 4280 (1963). 6. H. E. BAUMGARTEN (Editor). Organic syntheses, collected volume 5. John Wiley, New York. 1973. p. 301. 7. P. G. BLAKEand A. SPEIS.J . Chem. Soc. Perkin 11, 1879 (1974). 8. H. M. FREY.Prog. Reaction Kinet. 2, 131 (1964). 9. A. W. YAUand H. 0. PRITCHARD. Can. J . Chem. 56. 1389 (1978). J . Phys. Chem. 70,1230 10. D. H. SHAWand H. 0. PRITCHARD. (1966). 11. J . T. KNUDSTON and M. J. BERRY.J. Chem. Phys. 68,4419 (1978).

J. L. COLLISTER, and H. 0. PRITCH12. M. H. B. VAYJOOEE, ARD. Can. J . Chem. 55,2634(1977). 13. JANAF Thermochemical Tables, Second Edition, National Bureau of Standards, Washington, DC. 1971. 14. D. R. STULL,E. F. WESTRUM,and G. C. SINKE.The chemical thermodynamics of organic compounds. John Wiley, New York. 1969. 15. P. LEMOULT.C. R. Acad. Sci. Paris, 149, 1602 (1909). 16. A. W. YAU.Unpublished results. 17. M. HUNT,J. A. KERR,and A. F. TROTMAN-DICKENSON. J. Chem. Soc. 5074 (1965). 18. P. N. DASTOOR and E. U. EMOVON. Can. J. Chem. 51,366 (1973). D . J . Chem. 45,2749 19. D. H. SHAWand H. 0. ~ I T C H A R Can. (1967).