NOTES
178
dose and independent of oxygen concentration within the range studied (0.1 to 10.0 mole %). TABLEI FORMATION OF PEROXIDES FROM METHANE-OXYGEN MIXTURES (‘rOTAL PRESSURE -760 MM.) IN PYREX GLASSVE& WITH co-60 T-RAYS SELB (VOL. = 130 ML.), IRRADIATED e.v. ml.-l Dose rate in the gas at N.T.P. = 1.9 X min. -l
02 in gas mixture (mole %)
0.1
1.0
10.0
Tobal dose (e.v./ml.) X 1O-le
0.92 I. .43 l.92 0.63 0.96 1.24 :i .36 I .46 1.79 I .95 2.61 0.63 0.96 1.62 1.95 3.78
MeOOH (measured by iodide method) Yield (moles/ G ml.) (molecules/ X 10@ 100 e.v.)
0.44 0.72 1.05 0.37 0.50 1.00
2.9 3.0 3.3 3.5 3.1 4.2
..
.. ..
1.10
3.7
1.40 0.35 0.48 1.03
3.2 3.3 3.0 3.8
2.18
3.5
..
..
..
Vol. 65
tures suggesting, in agreement with Watson and Darwent,15that the reaction CH3OO
+ CH,
= CH300H
+ CH3
(2)
does not occur at room temperature. If oxygen acts merely as a radical scavenger in the irradiated methane, the observation that the yield of methyl hydroperoxide is independent of the concentration of oxygen over the range 0.1 to 10.0 mole yo implies that 0.1 mole yo of oxygen is suffiTotal peroxide (measured by cient to scavenge all of the available radicals at the ferrous total pressures employed in these experiments thiocyanate) Yield (-760 mm.). It is of some interest that methyl (moles/ml.) X 108 G nydroperoxide was the only peroxide found, and .. . . it must be assumed that H and CPHS, if formed, do .. . . not give peroxides under these conditions. .. .. Methyl hydroperoxide formation in the radiolysis .. .. has been attributed to reactions of methyl radicals .. .. but an alternative mode of formation cannot, at .. . . present, be excluded. Thus, in addition to the sca1.00 4 . 2 venging of radicals by oxygen, it is possible that elec0.80 3 . 3 tron capture by oxygen may take place.16 Even 1.00 3 . 3 at oxygen concentrations below 0.1 mole %, the 1.20 3 . 7 processes
.. .. ..
1.01 1.08 2.14
..
..
.. 3.7 3.3 3.4
The yields of total peroxide are also given in Table I. Within experimental error, G (total peroxide) was found to be equal to G (methyl hydroperoxide) showing that dimethyl peroxide was not formed. Discussion Radiolysis of methane may lead to the formation of free-radicals as a result of excitation, of ion-molecule reactions, and of ion-neutralization. The formation of hydrogen atoms and of methyl and ethyl radicals in pure methane during irradiation has been poz;t~lated.~~.~3 Oxygen, when present a t small concentrations in irradiated methane, may be expected to act as a free-radical scavenger. It is probable that the methyl hydroperoxide formed in methane-oxygen mixtures on irradiation arises from methyl radicals via methyl peroxy radicals, which are generated in the reaction
+
OZ e - = 0 2 (3) O2 e - = 0- 0 (4) may be important and capable of competing with electron capture by positive ions. Neutralization of positive ions by the negative ions 0 2 - and 0-, which would occur as three-body recombinations at the pressures considered here, l7 may lead directly to stable oxygen-containing radicals.
+
+
(16) G. S. Hurst and T. E. Bortner, Radiation Research, Suppl. 1, 547 (1359). (17) H. S. W. Massey and E. H. S. Burhop, “Electronic and Ionic Impact Phenomena,” Oxford Univ. Press, 1956, P. 630.
THE HEAT OF CHLORINATION OF DIBORON TETRAFLUORIDE’ BY STUART R. GUNNAND LEROYG. GREEN Laurence Radiation Laboratory, University of Califoinia, Livermore, Cahfornia Received June BO, 1960
The preparation and properties of BZF4 have been described by Finch and Schlesinger2 who noted that its chemical properties were similar to those of B2C14. It thus appeared probable that the compound would add chlorine nearly quantitaCHI. Oz (+ M) = CHOO (+ M) (1) tively and that this reaction could be performed Formation of methyl hydroperoxide and ethyl calorimetrically, as was done with BzC14,3 to hydroperoxide in the mercury photosensitized oxi- determine the heat of formation of the compound dation of methane and ethane, r e s p e c t i ~ e l y ~and ~ J ~ and hence an additional value for the boronformation of methyl hydroperoxide in the radiolysis boron single-bond energy. of aqueous solutions of methane and oxygeng has The thermal stability of B2F4 is greater than that been interpreted in terms of interactions of the alkyl of BzC14; however, when the chlorination was hydroperoxy radicals. performed under conditions similar to those used The value found for G(CH300H) (- 3.5) in the with B2C14, similar amounts of solid products present work indicates that a chain-oxidation does were formed. It was then found that addition of not occur in the radiolysis of methane-oxygen mix- a small amount of helium to the reaction tube
+
(12) F. W.Lampe, J . Am. Chem. Sac., 79,1055 (1357). (13) G. G.Meisels, W. H. Hamill and R. R. Williams, THISJOURNAL, 61, 1456 (1357). (14) J. A. Gray, J . A m . Chem. Sac., 74, 3150 (1952). (15) J. S. Watson and B. de B. Darwent, THIS JOURNAL, 61, 577 (1957).
(1) This work was performed under the auspices of the U. S. Atomic Energy Commission. (2) A. Finch and H. I. Schlesinger, J . .4m. Chem. Soc., 80, 3573 (1958). (3) S . R. Gunn, L. G. Green and A . I . von Egidy, THISJOURNAL, 63, 1787 (1959).
NOTES
Jan., 1961 reduced this solid formation to a negligible amount, presumably due to the more rapid transfer of heat from the reaction zone. Materials.-BzF4 was prepared by the method of Finch and Schlesinger in a vacuum line equipped with mercury float valves. The uamples, ranging from 1.2 to 1.5mmoles, were measured in a volume of 270 ml. and condensed in glass ampoules of about 7 ml. Chlorine was purified by fractional bulb-to-bulb distillation. Experimental Procedure .-The reaction vessel, calorimeter and erformarice of the runs were as previously dea run, the gas in the reaction vessel scribed. was sealed in a tube with mercury, shaken overnight to remove chlorine, and measured in the same calibrated bulb used for the sample measurement.
%'allowing,
Results.-Results in Table I. HEATOF Temp., Run "C.
5 5 25 25 25
1
2 3 4 5
BCh, -97.1', AH? (B2F4)is calculated to be -342.0 kcal. mole-'. Using $18.3 for AHfo(F,g)*and +135.2 f 4 for AHrO (B, g)6the thermochemical B-F bond energy in BF, is 153.3; assuming this to be unchanged in B2F4, E(B-B) is 72.4. This is to be compared with a value of 79.0 derived for E(B-B) in B2C14 by the same method. This calculation is probably less valid for B2F4 than for B2C14; in B2C14 the B-CI distance is nearly the same as in BC13 whereas the B-F distance in B2F4is roughly 0.03 A. longer than in Also the B-B distance in B2F4 is roughly 0.08 A. shorter in B2F4than in BzC14. (7) W. H. Johnson, R. G. Miller and E. J. Prosen, J . Research N B S .
(1959).
62, 213 (8) F. D. Rossini, et al., Circular of the National Bureau of Standarde 500, 1952, (9) L. Trefonas and W. N. Lipscomb, J. Chena. Phys., 28, 54
TABLE I REACTION OF BtF4 WITH Clz
Helium, cm.
0 10 10 8 10
of the measurements are given
179
Mol. wt.
98.1
< 99.0
