PHOTOCHEMICAL STUDIES OF METHYL RADICALS WITH OXYGEN
Sept, 5, 1953
[CONTRIBUTION FROM THE
DEPARTMENT OF CHEMISTRY,
4183
UNIVERSITY OF ROCHESTER]
Photochemical Studies. XLVIII. The Reactions of Methyl Radicals with Oxygen1 BY R. BRUCEMARTINAND
w.ALBERTNOYES,JR.
RECEIVED FEBRUARY 2@ 1953 Exposure of mixtures of mercury dimethyl and oxygen and of methyl iodide and oxygen to radiation from the Alpine burner leads t o different relative amounts of products than exposure of mixtures of acetone and oxygen. Since methyl radicals formed photochemically in these systems start chains of events which lead to secondary production of different radicals, it is difficult t o isolate uniquely the reactions of methyl radicals. It is concluded that both carbon monoxide and carbon dioxide result ultimately from reaction of methyl radicals with oxygen. The reaction of methyl radicals with mercury dimethyl t o give ethane may not proceed as a simple one-step process. The question of "hot" radicals in these systems is discussed.
I. Introduction The photochemical reaction of acetone with oxygen a t low oxygen pressures (less than 2 mm.) has already been studieda2 A mechanism consistent with most of the facts was presented, but proof for the various steps in the mechanism was not conclusive. According to this mechanism the reaction CHB 0 2 = HCO HzO was followed either by HCO = H CO or HCO f 0 2 = COZ OH. The photochemical oxidation of formaldehyde, which presumably proceeds wia the formyl radical,a leads mainly to carbon monoxide. However, formaldehyde seems to act as an inhibitor in the acetone-oxygen system4so that formaldehyde oxidation may not afford conclusive proof against the previously presented mechanisms. Other mechanisms will undoubtedly account also for the facts in the acetone-oxygen system. In any of these oxidation reactions secondary radicals, such as acetonyl, are produced by hydrogen abstraction. The reactions of these radicals with oxygen lead to products which complicate the elucidation of the methyl radicahxygen reaction. The present studies with mercury dimethyl and with methyl iodide were undertaken to attempt to obtain additional information about the reactions of methyl radicals with oxygen.
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11. Experimental The preparation of mercury dimethyl has already been described.6 The methyl iodide was that used in a previous study. 6 Oxygen was prepared by heating solid potassium permanganate and dried by passage through a trap immersed in liquid nitrogen. A Hanovia Type SH "Alpine" burner was used as a source of radiation. The radiation was made parallel with a quartz lens and passed through a thickness of 3.0 mm. of Corning No. 9863 Purple Corex glass. The beam was approximately 2.0 cm. in diameter and passed centrally down a quartz reaction cell 5.0 cm. in diameter and 8.5 cm. long with fused quartz windows. The main wave lengths absorbed by both gases are 2537 and 2654 A. Since absorptions for these wave lengths for both gases are high, wall effects were not completely obviated, especially with mer(1) This work was supported in part by a contract between the Office of Naval Research, United States Navy, and the Department of Chemistry, University of Rochester. (2) F. B. Marcotte and W. A. Noyes, J?., Discs. Faraday Soc., 10, 236 (1951); THISJOURNAL, 1 4 , 783 (1952). (3) E. C. A. Homer, Ph.D. thesis, King's College, London, 1951. (Work performed under the direction of Dr. D. W. G. Style.) (4) Results obtained by Dr. D. E. Hoare at t h e University of Rochester and to be published in the near future. (5) R . Gamer and W. A. Noyes, Jr., THISJOURNAL, 1 1 , 3 3 9 0 (1949); see H. Gilman and R. E. Brown, ibid., 52, 3314 (1930). (6) J. T. Dubois and W.A. hToyes, Jr., J . Chcm. Phys., 19, 1512 (1951).
cury dimethyl which could not be used at high pressures because of its low vapor pressure at robm temperature. Since oxygen was not added during the course of the runs, the partial pressure of oxygen did not remain constant. The gases were circulated with a magnetically driven stirrer to avoid depletion of oxygen in the reaction zone. Gaseous products were separated and determined by methods previously described .z A fraction determined to
TABLE I MERCURY DIMETHYL-OXYGEN Cell diameter, 5.0 cm.; cell length, 8.5 cm.; Hanovia "Alpine" burner; diameter of light beam, 2 cm.; Corning 9863 filter. Products are given in molecules formed per second. Time of runs, 100 min. unless otherwise stated; pressure of mercury dimethyl, 5.0 mm. unless otherwise stated; volume of reaction cell plus tubing to cut-offs and stirrer, 486 ml. Av. Pea, mm.
- AO2,
molecules sec. - 1 x 10-18
con
CO, CHI, molecules molecules sec. - 1 sec. - 1 x 10-18 x 10-18
molecules sec. - 1 x 10-18
1.12 2.07" 0. 55b 6BC 12d 14d 5gd
31 x 28.5 34.9 29.8 33.2 17.9 26.2 29.5 31.8 79.4 74.6 8.7 8.3 3.5
Temp. = 200" ti.07 7.15 .... 5.67 8.68 7 .(18 .... 5.15 5.44 4.75 2.89 1.43 9.74 5.02 .... 4.38 5.71 7.01 (7.97) 11.11 11.95 5.42 2.93 .... 3.18 .... 1.25
0.19 .28 . 51a . G9# .55f
(9.9 17.6 13.3 11.9 76.0
Temp. = 150" 1.96 3 , 66 3,15 3.15 2.24 2.52 2.02 1.74 7.11 8.04
0.22 .27 .45 .58 .64
.76 .77
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It. 57 .47
coz/co 0 x5
1.23
. ti0
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.60 .43 .63 .60 .53 1.07 4 25 0.52 .58 27
1.14 2.02 1.94
0.28
,27 .'t7 .32 .72
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0.82 (0.72) 2.20
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.. 55 I . on
0
u
8'3
1.16 0.88
Temp. = 100' 0 . 6 4 ' ~ ~ 38 2 0.41 3.24 3.46 0.93 a A run without oxygen immediately following this run and under otherwise identical conditions gave N c ~ H = ~5.64 and N C H ,= 0.80 X 1013molecules/sec. To a first approximation CO CO, = 2CzHs CH4. *P(Hg(CH&) = 20 mm., COZ probably low. "P(Hg(CH8)s) = 58 mm.; time = 60 min. Cell, 20.0 cm. long; 1.8 cm. diameter; P(Hg(CHa),) = 5.3 cm.; time = 300 min. Light beam filled cell. e 120 min. f P(Hg(CH8)z) = 55.0 mm.; time = 120 min. 0 P(Hg(CH3)z)= 51.0 mni.; time = 120 min. A run without oxygen immediately following this run and under otherwise identical conditions gave N C ~ H=) 10.2, N C H , = 0.33 X 1013 molecules/sec. In this case CO COz < 2CzH6 CHI.
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R.BRUCEMARTINAND W. ALBERTNoms, JR.
4184
be formaldehyde by mass spectrographic analysis' was separated in some instances by the Ward still at -130'. Prepurified tank nitrogen was passed through a liquid nitrogen trap and over heated copper turnings. Tank carbon dioxide was degassed a t - 160". The reaction cell was placed in a furnace and the temperature maintained and read by customary procedures.
111. Results Table I presents results obtained in mixtures of mercury dimethyl and oxygen. Ethane was not found among the products when oxygen was present. The oxygen was always determined at the end of a run, and in no case was it completely used up. The amount of methane was too small t o permit good accuracy in its determination. It was always found even when oxygen was pre ent. Table I1 shows r&ts obtained with methyl iodide. In this case it was found impossible t o use temperatures higher than about 120" because of thermal reaction with the oxygen.
TABLE I1 METHYLIODIDE-OXYGEN Conditions as in Table I except P(CHa1) = 141 mm.
-moleAOa,
Av.
Time, mtn.
120 120 120
Po,,
mm.
0.0 .68 .99
cules
set.-' X 10-11
CO, molecules
CH4, molecules sec. -1 sec. - 1 COz/ X 10-lg X 10-11 CO
cot,
molecules s e c - 1 X 10-13
Temp. = 35" (CzHe 19.1) 18.2 2.37 2.75 12.4
0.0
2.82 0.94 2.48 2.5 1.42 1.72 1.9
Temp. = 120" 60 0.35 64.7 5.58 7.07 1.16 0.79 120a 0.68 66.0 7.70 9.91 2.54 .78 .67 120 1.10 72.6 6.90 10.30 1.96 120* 1.18 68.7 6.07 9.85 1.44 .63 1.86 128.2 13.78 9.70 3.39 1.42 90 a Immediately preceding this run a run was made without oxygen but under otherwise identical conditions. NQH, = 20.5; N C H = ~ 2.85 X 10'8 molecules/sec. Hence CO COZ
