KOTES
Jan., 1961 FORMATION OF METHYL HYDROPEROXIDE I N THE PHOTOOXIDATION OF AZOMETHANE' BY M. S H ~ A NAND I N ~K. 0. K U T S C H ~ Diuiaton of Pure Chemistru, National Research Council, Ottawa d ,
Canada Received J u l y 8.9. 1960
Various radicals have been suggested as precursors of the secondary products which are commonly investigated in the reaction of methyl radicals with oxygen. The "third-body" nature of reaction 1 was demonstrated by Hoare and Walsh3 CH,.
+ + M +CHaOO. + M 0 2
(1)
Sleppy and Calvert4 and Christie,6 but little is known of the subsequent reactions of methylperoxy radicals so formed. It is anticipated that reaction 1 will be exothermic (between 20-30 kcal./mole) and thus will result in a vibrationally excited species which could be stabilized by collision with a thirdbody (M). Subsequent to this process, methylperoxy radical is expected to undergo familiar abstraction reactions which are observed with other alkylperoxy radicals. Gray6 studied the mercury photosensitized oxidation of methane below 50' in a fast flow system (ca. 1 l./min.), and reported that methyl hydroperoxide, which was determined iodometrically, was the major product. The mechanism suggested w a s CH302.
and also CH302.
CHI. f 0 2 +CH202. CHa --+CHSOOH CII?.
(2) (3)
+ CH4 +CHaOOCHz + H
(4)
+
+
Methyl hydroperoxide, however, has not been identified unambiguously in the azomethane and acetone photooxidation systems. The products chiefly were methanol, formaldehyde and formic acid. 'To account for the production of methanol, the predominant radical in the system has been assumed to be the methoxy radical and several suggestions have been made whereby the conversion of me thylperoxy radical to methoxy radical is accomplished. In order to clarify these contradictory conclusions and also to obtain further insight into the reactions in azomethane photooxidation, a flow system mas set up in which the contents of the reaction cell (azomethane, 6.6 mm.; oxygen 11 mm.; carbon dioxide 48 cm.) were circulated by means of an allglass piston pump through a multiple trap attached to the system by ground glass joints and kept at -77'. The pump operated at about 3 l./min., and the time taken to circulate gases from the reaction cell to the cold trap was about 3 4 seconds. 6107.
(1) Iesued as N.R.C. No. (2) National Research Council Post-doctorate, 1958-1960. (3) D. E. Hoare and A . D. Walsh, Trans. Faradag sot., 63, 1102
(1957). (4) 'X. C. Sleppy and J. G. Calvert. J. A m . Chem. Soc., 81, 769 (1959). (5) M. I. Christie, Proc R o g . Soc. (London), 8244, 411 (1958). (6) J. A. Gray, J . Chsm. Soc., 3105 (1952). (7) P. L. Hanst and J. G. Calvert, THISJOURNAL, 63, 7 1 (1959). (8) R. L. Strong and K, 0. Kutschke. Can. J . Chem., S7, 1456 (1958).
(9) F. Wenger and K. 0.Kutwhke. %bid..37, 1546
(1959).
189
In order to maintain this rate no stopcocks were used in the circulating system. The upper part of the trap was warmed by a small electric fan to avoid mist formation. No metal valves were used. After long periods of irradiation (corresponding to the decomposition of about 60 pmole of azomethane, whose partial pressure was maintained constant by excess liquid in the cold traplo while the trap was held at - 7 7 O , the gases were expanded slowly into larger volumes and pumped until the pressure in the system reached 10-' mm. Air was admitted to the system and the trap removed. Stopcocks were attached to the trap at the joints and, after removing the air, the contents were analyzed mass spectrometrically. It was hoped that m/e 48 (CHBOOH) would be prominent if methyl hydroperoxide were present in quantity; the contribution by any residual azomethane to this mass number is negligible. Initial experiments showed no trace of a peak at m/e 47,48 or 49. However, on a sudden and direct introduction of the gases from the multiple trap to the ionization chamber, very large peaks at m/e 47 and 48 were recorded. The heights of these peaks were found to decrease sharply and they disappeared after a few minutes with a half-life of approximately 25 seconds. It was thought that metal valves and connecting tubing (brass and stainless steel a t room temperature) in the inlet system of the mass spectrometer might have caused surface catalyzed decomposition of the methyl hydroperoxide. The observation was repeated many times by pumping out the gases already in the mass spectrometer and introducing a fresh supply of the vapor from the multiple trap into the ionization chamber. Two other mass spectrometers, each with different lengths of metal tubing in the inlet system, were employed. The rate of decrease in the peak heights of m/e 48 and 47, and hence the decomposition of the hydroperoxide, was found to vary depending on the surface area exposed. A synthetic sample of methyl hydroperoxide prepared by Dr. L. C. Leitch of these laboratories was then examined similarly and found to behave in an exactly identical fashion; the peak height of m/e 48 exhibited an identical half-life. To observe the possible products of the decomposition of methyl hydroperoxide, observations were made on two other mass numbers, 30 (formaldehyde) and 32 (methanol). The peak height at m/e 32 was found to remain unaffected, while that at m/e 30 (formaldehyde) was observed to increase at a rate, almost equal to the decrease in the peak height of m/e 48. It is concluded, therefore, that reaction 5 takes place very rapidly on the surface of the metal. CHaOOH +HCHO
+ Ha0
(5)
Following these results, the observation of the mass number of the hydroperoxide, together with its rapid decomposition, was taken as a criterion to identify the presence of methyl hydroperoxide in subsequent experiments. The results summarized in Table I, indicate threepimportant features. (i) Methyl hydroperoxide is formed by reaction 6 at temperatures as high &6 1 5 8 O (10) G.R. Hoey and K. 0. Kutsohke, Can. J . Chdm., 33,496 (1955).
COMMUNICATIONS TO THE EDITOR
190
CILOO. + R H +CHzOOH + R * (6) RH might be aaomethane although it might also
be formaldehyde, since the experiments were carried to a high conversion a t constant azomethane concentration. (ii) Experiment 7 was done under conditions similar to those reported by Hanst and Calvert,' except, for lower intensity. While they did not find methyl hydroperoxide among their reaction products, under our conditions, this substance was readily detected. (iii) Abetraction of a deuterium atom from CD4 by CI130z., even at temperatures as high as 158' is not rapid. The results of Gray6 and Nalbandyan" on methane, might then be reinterpreted in terms of the mechanism suggested by Watson and DarwentlZ for ethane oxidation. A preliminary estimate of the quantum yield of the hydroperoxide (iodometric titration), indicated (11) A. B. Nalbandyan, Zhur. F?z.Khzm., 32, 1443 (1948). (12) J. S. Wrttson and B. deB. Darnent, THIBJOURNAL, 61, 577
(1957).
Vol. 65
TABLE I CONSTANT PRESSURE OF AZOMETHANE = 6.6 MM. PRESSURE OF OXYQEN(AT ROOMTEMPERATURE) = 11 MM.
Expt. no.
1 2 3 4 5 6 7
Other gases present in em.
COz; 48 Cot; 48 COz; 48 CDI; 48 CDI; 48 Propylene; 12 Oxygen; 70
Teyp.,
C.
27 105 158 27 158 27 27
CHsOOH mle 48
Present Present Present Present Present Present Present
CHsOOD, m/e 49 CDsOOH. m / e 51 CDsOOD, m/e 52
Absent A4bsent
that it lay between 0.5 and unity for conditions as in experiments 4 and 5, but wit,h CHI and at 100'. Thus it was an important product of the reaction. More quantitative work on the formation of methyl hydroperoxide is now in progress. The authors wish to acknowledge the generous assistance of Dr. F. P. Lossing and his associates for the mass spectrometric analyses.
COMMUNICATIONS TO THE EDITOR GAS
CHROMATOGRAPHY OF PARAHYDROGEN, ORTHOHYDROGEN, HYDROGEN DEUTERIDE AND DEUTERIUM
worthwhile to separate ortho-Hz and HD or to obtain four separate peaks, para-Hz, ortho-Hz, HD and Dz, on a single chromatogram. I n this communication we wish to present such a method and analytical data found with it. The separation of this kind would be realized if Sir : The gas chromatographic separation of hydrogen one employs two sorts of columns in series; the isotopes HI) and Dzland nuclear spin isomers para- first contains alumina or Molecular Sieves and the Hz and ortho-H22 on suitable adsorbents kept a t second paramagnetic substances in addition. Para-195" was demonstrated in 1958. Since then it Hz, eluted out of the first column, would undergo has become of interest to suppress the separation of rapid para-ortho conversion during passage through para-ortho isomers and hence to give three peaks the second column presenting a peak. Ortho-Hz, corresponding to HS, HD, Dz on absingle chromato- with a certain time delay, will follow a similar begram. Smjt h and Hunta used chromia-alumina havior. Now, if the second column be efficient columns and Moore and Ward4used alumina pack- enough also for the separation of three isotopic ings coated with ferric oxide, both making rapid forms, ortho-Hz and H D ought to be separated on para-ortho interconversion and thereby giving a this column. Consequently, we may expect the unified peak for these isomers. I n the absence of four peaks mentioned above. Experiments were made with the apparatus and such pararnagnetic substances on the columns, ortho-Hz and H D overlap and the three apparent procedure reported previously.6 An actire alumina peaks are para-HZ, ortho-Hz plus H D and DO, column 200 X 0.4 em. was employed as the first respectively, as already reported by Van Hook and column. The second one was filled with active alumina coated with ferric oxide after Moore and Emmett5 and by I
