K. E. VARNERINAND J. S. DOOLING
2012 [CONTRIBUTION F R O M
THE
1701. 78
DEPARTMENT O F CIIEMISTRY, THEC A T H 3 L I C LNIVERSITY
OF
AMERICA]
The Thermal Decomposition of Ethane1 BY ROBEKT E. 'I-ARNERIN AND JOHN S. DGCNLING RECEIVED SOVEMBER 7 , 1955
It is shown that in the thermal decomposition of 50-50 mixtures of C?H, and CD4, the C D ~ H / C D A ratio is approximately independent of the concentration of nitric oxide. In the thermal decomposition of 50-50 mixtures of C2Dsand CH:, the D2/CHa ratio is approximately independent of t h e concentration of nitric oxide, but the CD3H/CH( ratio increases strongly with increasing concentration of nitric oxide. Mechanisms for these decompositions are proposed and discussed.
recent paper by Rice and Yarnerin? reported the thermal decomposition of ethane.de. \Ye have studied the thermal decomposition of ethane and have also performed additional experiments on ethane-& and are now able to report the CD," CH? and the Dz/CH4 ratios in the decomposition of .50-30 mixtures of C2D6 2nd CH4 and the CD3H/ CD4 ratio in the decomposition of 50-50 mixtures of C2Hs and CD4. These experimental results, as well as some remarks on proposed mechanisms for these reactions form the subject matter of this paper. Results The apparatus, the mass spectrometer and the gases have been described previously.? The thermal decomposition of 30-30 mixtures of CzH8 and CD4 were studied by decomposing them alone and in the presence of nitric oxide to different fractional decompositions a t an initial ethane pressure of 80 mm. and a t 3%". I n Fig. 1 the CDIH CD? ratio is plotted against percentage of C2Hedecomposed. From these results it is seen that the amount of isotopic mixing, as measured by the CDdH CD3 ratio, is approximately proportional to the percentage of ethane decomposed and is approximately independent of the presence of nitric oxide.
the CD3H'CHI and the D2ICH4 ratios can now be reported. These data are given in Figs. 2 and 3.
B
0.1 0.2 0.3 0.4 CL):jH/CH, X 100. decomposition of C:Ds in presence o f CII,.
0
Fig. 2.-Therrrial
S o . absent a t A 600" 0 621° 0 579O
3
tD @
+.
A o
1
599O 621'
S o . present 4 , 0 t r i m . a t 621" 1 4 . 6 m m .:it 624' 30.0 tnm. a t 624' 9 . 0 nini. a t 601" 6 . 7 nmi. a t A O l o
-=-1
NO ABSENT NOABSEN7
624' 4 m m NO bot0 6 7 m m N O 0 624O 14.7mm NO @
A
601° 9 m m N O 4
26
-
624'
1
3 0 m m NO
"
I
c
-
,
M
0.2 0.3 0.4 CD.jH/CDi X 100. Fig. 1.-Thermal decomposition of equiinolsr mixtures of C2Hs and CD, alone and in presence o f nitric oxide ;it 582' and an initial ethane pressure o f 80 mm.; ni:iis spec*trimeter measurement5
0
0.1
Reinvestigation of the tlicrmal decomposition o f 50-50 mixtures of C2D6and CH4 has been carried out. In addition to what was reported previously, ( 1 ) T h i s nrlrk w a s supp
4G 4
-
I
-
~
f h
Li
I i
1
0
1
->
,
4
... 3
~
~
I .
)
L):,CIi, x l i l i i . Fig. 3 . -Therinal decomposition of mixtures of C2IIsCHI a t an initial pressure of 80 IIIITI. of C Z I ) ~ .
May 20, 1956
T13E
THERMAL DECOMPOSITION OF
The D2/CH4 ratio, like the CH3D/CH4 and H D / CH4 ratios reported previously, are approximately independent of the concentration of N O , but the CDFH/CH4ratio increases sharply with increasing concentration of NO. This is not surprising since CD3radicals can be formed only by the initial split of C2Ds. X comparison of the proposed mechanism of the C2Hs-CD4 decomposition with the mechanism of the C2De-CH4decomposition studied by Rice and Varnerin follows. The proposed reactions for the decomposition of C2Hsin the presence of CD4are given in Table I. TABLE I THERMAL DECOMPOSITIOSS OF 50-50 MIXTURES OF CsH6 A N D CD, REACTIOSS k , mole-' cc. -1 sec. -1
+
CzHs + CH3 CHa (2) CH3 CQH6+CH4 C2Hj (3) CH3 CD,+ CH3D CDa (4) CDs C.Hs--;t CD3H CQH~ CD, + G H j D CD3 (5) C2Hj (6) CzH6 ----f CrH, H (7) H CrH6HI C2Hj (8) H CD4 +H D CD3 a Set.-'. (1)
+ + + +
+ +
+
+ + + +
+ +
7.3 7.3 7.3 7 3 2.4 I .o
X X X X 10-"
x x 1.0x
IO@ 10-13
lo-"
The values of the rate constants in Table I were calculated from data in Steacie3 and TrotmanD i ~ k e n s o n . ~It was assumed that the activation energy of deuterium abstraction from a deuterated molecule is 1.7 kca1.j greater than for hydrogen abstraction from the corresponding ordinary molecule and t h a t the -4factors are the same. I t was also assumed, since tdata are lacking, t h a t ethyl and methyl radicals react with the same molecule a t the same rate. From this it can be seen t h a t the reaction ( 2 ) is about 10 times as fast as reaction (3); reaction (5) is negligible compared t o reaction (6) ; reaction (7) is ahout 10 times as fast as reaction (8). Since reactions ( 3 ) , ( 5 ) and (8) are relatively inactive, the concentration of CD3 must be low and one would not expect reaction (4) t o produce CD3H in any great quantity. This is borne out experimentally by the CD3H/CD4ratio which is only 0.0035 for 57' decomposition of C2Hs. The reaction scheme proposed here may be considered as follows. Reaction (1) is the initial split of ethane into two methyl radicals. This is followed by a series of reactions between various radicals and C2Hsor CD,. I t should be noted t h a t reaction of a radical with a molecule must always result in the generation of another radical which may or may not be the same as the original radical. I n our system we define a "chain radical reaction" as a member of a series of reactions initiated by a radical and ending with the same radical. Reaction ( 5 ) is an example o f a chain radical reaction. Ai"non-chain radical reaction" is defined as one in which the initiating free radical is not sub(3) E W. K Steacie. "Atomic and Free Radical Reactions," 2nd E d . , Reinhold Pub1 Corp., New York, N. Y . 14) A. F. Trotman-Dickenson, Q x a v l Rev., V I I , 210 (1953) ( 3 ) J . R . hIcNesby, el ai , THISJ O U R K A I . . 76, 823, 1 1 1 A (1954).
'043
ETHAXE
sequently regenerated. Reactions ( 2 ) and (3) are examples of this type of reaction. X reaction such as (4)is of a mixed type and may be classed either as a "chain radical reaction" or a "non-chain radical reaction" depending on which particular sequence is considered. The three chains (see Table I) are considered as follows: Chain A formed by reactions ( 5 ) and (4) is insignificant. This is predicted from the values of the rate constants of reactions ( 5 ) and ( 6 ) . Chain B, formed by reactions ( 0 ) and ( i ) ,is the most important one in this reaction. Xgain this is predicted by the values of the rate constants and is confirmed experimentally. Chain C, formed by reactions (6), (8) and (4), should be about onetenth as effective as Chain B. A similar mechanism for the C2D6-CH4 decomposition studied by Rice and Varnerin2is shown in Table 11, following the scheme of Table I. The rate constants are calculated in the same way. Here reaction (3) is of comparable importance t o reaction (2); in a similar manner, reaction (8) is of importance to reaction ( i ) . This means that reaction (4) should be more important in this scheme than in the case of the decomposition of C2H6. This is confirmed by the CH3D/CH4 ratio which is 0.025 for 5% decomposition of CzD6. This ratio is seven times as great as the corresponding ratio in the case of 5yodecomposition of C2H6 in the presence of CD4. TABLE I1 THERMAL DECOMPOSITION O F m-50 A S D CHI
CrD6
hfIXTKRES O F
k mole-' cc
+
C ~ D+ G CD3 CD, CD3 C ~ D G + CDa CrD, 3 CD3 CH, +CDaH CH3 3 CHa CzDs + CHBD C Q D ~ 3 3 CH,+ CiD,H + CHI ( 5 ) C2D5 (6) C?D,+ C2D4 D 2 4 C ? D G + D? CzD, (7) D (8) D CH,+ HD CHa 4 (1) (2) (3) (4)
+ + + +
+
+ +
+ + +
+
+
-1
sec
'
7 X 3 X 7 X 3 X IO-'' 4 x 104~ 7 x 10- 14 1 x 10 l 4
Sec
The results are expressed in a different manner in Table I11 which gives the number of moles of certain of the products formed for every 100 moles of ethane-de which decomposes initially for various additions of nitric oxide. The amount of CHBD TABLE I11 MOLESOF PRODUCTS PER 100 MOLESOF C2D6 DECOMPOSED IN PRESENCE OF I-ARIOUS .IDDITIONS OF SITRIC CXIDE S O , mm CH3D HD D2 CiD ,H CD,H (CHSD DJ CH,D/CD,H
+
4 0 6 7 9 0 146 47 45 4') 11 32 34 34 29 28 63 75 83 72 87 11 16 11 19 18 6 3 6 3 3 9 0 0 2 1 131 130 117 103 119 25 12 7 6 6 3 6 9 0
14
30 17 27 64 G 7 8 111
A 0
formed remains essentially constant, the amount of CzDsH formed decreases and the amount of CD3H formed increases appreciably as the N O concentra-
C. C . SCHUBERT, S.J., AND K. N. PEASE
2034
tion increases. The CD3H concentration increases with increasing concentration of NO because CDaH can be formed only in a non-chain radical reaction, it depends only on the initial split of the ethane-& molecule and is independent of any chain. As the chain shortens with increasing concentration of NO, the initial split becomes more important and the concentration of CD,H increases. On the other hand, C2D5H is formed only in a chain-radical reaction. With addition of NO the chain becomes less important compared with the original split, and the concentration of C2D5H should fall. CH3D can be formed either in a chain or non-chain radical reaction and its constancy with respect t o increasing concentration of NO may be ascribed t o a cancelling of two effects. The (CH3D Dt) row is a measure of the accuracy of the experimental data. For every molecule of C2D6 decomposing b y a chain, a molecule of either Dz or CH3D is formed. Since the (CHID D?) values differ considerably from 100, our results are only semi-quantitative.6
+
+
+ +
It is a consequence of this mechanism t h a t
+
[CONTRIBUTIOS FROM
THE
[CzD;Hl
[CHsD]
-
[HD]
I t is seen from Table 111 t h a t t h e d a t a fit t h i s relation a t intermediate nitric oxide concentrations b u t deviate markedly a t low a n d high concentrations.
A : C2D6 CHa +C2D6H f CHBD B : CzDs +C2D4 f Dp C : CzDs CHI +C2D4 HD CHID
+
7s
As pointed out previously the CD3H is a measure of the initial split of the CZD6. The CH3D can be formed in either a "non-chain" or a "chain radical" reaction. If we neglect the CH3D formed outside of the chain, the ratio of CH3D to CD3H (Table 111) is essentially the length of the chain consisting of reactions (6), (8) and (4). The experimental results indicate that, as nitric oxide is added, the chain length decreases from 25 t o about 6 links. As the chain length decreases the initial split of the C2D6 and the "non-chain radical" reactions become relatively more important than in the uninhibited reaction. This seems to be confirmed by the fact that the amount of CD8H formed increases as the nitric oxide concentration increases. These results support the previously published conclusion that the thermal decomposition of C2H6 or C2D6 is a chain reaction in which there is negligible direct molecular separation of Hz or Dz. Acknowledgment.-The authors wish to thank Dr. F. 0. Rice who gave generously of his time and knowledge during the course of this work.
(C) I t was pointed o u t b y t h e reviewer t h a t t h e reactions designated
a s A , B and C are
Vol.
WASHISCTOS,D. C.
FRICK CHEMICAL LABORATORY, P R I S C E T O N VSIVERSITY]
The Oxidation of Lower Paraffin Hydrocarbons. I. Room Temperature Reaction of Methane, Propane, n-Butane and Isobutane with Ozonized Oxygen1 BY CLARENCE C. SCHGBERT, S.J., .IND ROBERT N. PEASE RECEIVED JASUARY 20, 1956 Reaction rates of ozonized oxygen (ce. 3 mole % 0 8 ) with methane, propane, n-butane and isobutane were measured, in s i b , in a temperature-controlled infrared absorption cell by observing the decrease of ozone Concentration as measured by the absorption a t 1055 cm.-'. The activation energies calculated on the basis of a reaction first-order with respect t o ozone were 14.9, 12.1, 11.1 and 10 3 kcal., respectively. The pre-exponential term for the methane-ozone reaction was found t o be 7.2 X 1010, for propane-ozone 3.1 X lo1, for n-butane-ozone 8.2 X IO8 and for isobutane-ozone 4.4 X 108 cc. mole-' see.-' estimated on the basis of a reaction rate 0: IO,] [HC]. The isobutane-ozone reaction was predominantly homogeneous; n-butane-ozone showed some acceleration in rate due to increased Pyrex surface, little dependence on sodium chloride surface. The main gaseous product of the isobutane-ozone reaction was t-butyl alcohol with smaller amounts of acetone and 1-carbon products approximately equal to molar amounts of acetone. A mechanism based on the postulation of a low lying triplet state of ozone is presented.
Relatively little work has been done on the kinetics of the action of ozone on saturated hydrocarbons. It was reported by Otto2 in 1S9S that ozone reahted with methane to yield aldehyde a t temperatures as low as 13'. In 1922 \\'heeler and Blair8 observed that a t 100" methane reacted with ozone a t such speed t h a t 5370 of the introduced (1) T a k e n f r o m a thesis submitted b y C. C. Schubert. S.J.,in partial fulfillment of t h e requirements for t h e P h . D . degree. T h e work d e scribed in this paper w a s supported jointly by Contract NOrd-7920 with t h e U. S. Kava1 Bureau of Ordnance as codrdinated by t h e Applied Physics Laboratory, T h e Johns Hopkins University and by Contract NO-ori-10: with t h e Office of S a v a l Research as coordinated b y Project Squid, Princeton Uni\.ersitj-. Reproduction. translation, publication, use and disposal in whole or in p a r t b y or ior t h e United S t a t e s Government is pertnittetl ( 2 ) R I . hf. O t t o , A11n. c h i m . phyr , [ V I I ] 13, 109 (1898). (3) T. S. Wheeler and E. \ V Blair, J . Cliem. Soc. I n d . , 41, 303 (192").
ozone disappeared within 2 minutes. Also in 1922 Elworthy4 concluded that the reaction of methane with ozone gave promise as a means t o produce methanol and formaldehyde. I n 1928 Iiiesenfeld and Gurian5 re-investigated this reaction. Ethanol was reported6 to result from the action of ozone on ethane and this reaction was noted t o proceed with greater rapidity than the reaction of ozone with methane. Some patents' have been issued to cover the use of ozone as a stimulator for the combustion of gaseous, liquid or solid fuels. (1) R . T. Elworthy, T i a n s . R o y . Sor. Can.. I1111 1 6 , 98 (1922). f.5) E . H. Riesenfeld and D. Gurian, Z . p h y s i k . C h e v . , 8139, 169 (1928).
W. A . Bone and J. Drugman, Proc. Chem. S O L . 2, 0 , 127 (1904). fi) C . E . T h o r p e , "Bibliography of Ozone 'Technology," Val. I, Armour Research Corp., Ill. Inst. of T e c h . , Tech Center, Chicagu, 1954. (6)
