789
HIGH-TEMPERATURE VACUUMULTRAVIOLET PHOTOLYSIS OF %-BUTANE
High-Temperature Vacuum Ultraviolet Photolysis of n-Butane
by James R. McNesby and Rudolph V. Kelley Institute for Materials Research, National Bureau of Standards, Washington, D . C. $0234
(Received May $4, 1968)
Mixtures of n-butane and n-butane-dlo have been photolyzed at 1470 A over the temperature range 308-671'K. The mechanism of methane formation is shown to be largely intramolecular a t 308OK and becomes increasingly dominated by a chain reaction carried by methyl radicals as the temperature is raised. At 671°K, the mechanism of methane formation is almost entirely a free-radical process. Estimates have been made of average chain lengths as R function of temperature and from this, the chain length in the pyrolysis of n-butane is calculated to be about 2000. It is suggested that none of the methods of detecting molecular elimination of methane in the pyrolysis of butane has been sufficiently sensitive. Molecular elimination of methane in pyrolysis may be a t least as important as methyl radical formation and still escape detection.
Among the products of the photolysis of mixtures of n-butane and n-butane-& in the presence of NO a t 1470 A is methane of which more than 80% is CH4 CDI. Nevertheless, appreciable amounts of CH3Dand CDsH are present. Their presence is even more apparent in the absence of NO.' These results suggest that methane is formed in two ways, molecular elimination and abstraction of hydrogen by methyl radicals. Moreover, in the presence of NO where the molecular elimination of methane is dominant,2 photolysis of CH,CD2CD&H, gives primarily CH4 and CHID. The photolysis of CD3CD2CH2CH3 at 1236 A gives mainly CHd and CDda2The dominant mechanism for molecular elimination of methane is, therefore
+
CHgCH2CH2CHa +CH4
+ CHCH&H3*
(1)
The excited propylidene rcarranges rapidly to excited propylene which may be stabilized by collision or decompose further as
CHCH&HS* *CHZ=CHCHI* Similarly, n-butane-& in
(2)
photolysis produces CD4 as
CD~CD~CDZCD~ ----+ CD4
+ CDCD2CD3*
(3)
Methyl mdicals are also formed in the primary process n-CdHio +CHI n-C3H7 (4)
+ CD3 + n-C3D7
n-C4Dlo --f
(5)
The methyl radicals may be expected to abstract hydrogen from the parent increasingly as the temperature rises as shown by
CH3
+ n-C4Hl0
-+CH4
+ C4He
+ n-C4Dlo +CH3D + CsDB CD3 + n-C4H10 +CD3H + C4H9 CD, + n-C4Dm +CD4 + C4D9
CH3
(6) (7) @)
(9)
At low temperatures most methyl radicals disappear by association, e.g.
2CH3
+M
CZH,
+M
(10) and the methanes are dominated by molecular elimination reactions 1 and 3. As the temperature is raised and abstraction reactions 6-9 assume greater importance, additional methyls are formed by decomposition of the s-butyl radical which is formed preferentially over the n-butyl radicaks Thus, all of the elements of a chain reaction are present. This presentation is considerably oversimplified because (a) propyl radicals formed in the primary process decompose to give methyl radicals, (b) H atoms and ethyl radicals formed in primary photochemical processes abstract hydrogen to produce s-butyl radicals, and (c) those abstractions which occur at the primary CH or CD bonds produce n-butyl radicals which decompose thermally to give ethyl radicals which in turn can carry a chain. I n spite of all of these complexities it is of interest to understand the nature of the competition between the modes of metJhane formation, molecular and free radical, and to obtain an estimate of the length of the chains in the photolysis of n-butane as a function of temperature. Two studies of chain reactions in hydrocarbon photolysis have already been reported for ethane and pro~ a n e . ~ The J propagation of the chain reactions in these cases involved the necessity of abstracting the primary hydrogen atom. It was felt that the chains might be propagated at a somewhat lower temperature in the n-butane photolysis (or be longer at the same temperature) since the propagation step involves the abstraction of the more labile secondary hydrogen atom. ----+
(1) H. Okabe and D. A. Becker, J. Chem. Phys., 39,2549 (1963). (2) R. P.Borkowski and P. J. Ausloos, ibid., 39, 818 (1963). (3) J. R. MoNefiby and A. S. Gordon, J. Am. Chem. SOC.,78, 3570 (1956). (4) R. F. Hanipson, Jr., and J. R. McNesby, J. Chem. Phys., 42, 2200 (1965). (5) A. 11. Laufer and J. R. McNesby, J. Phys. Chem., 70, 4094 (1966). Volume 78,Number 4
April 2069
JAMES 11. MCNESBYAND RUDOLPH V. KELLEY
790
Table I: Relative Amounts of Products of Photolysis of Mixture of %-Butane and n-Butane-dlp Temp, OC
35 70 75 125 157 189 206 239 272 314 349 397 398
CH4
100.0
...
100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
CHaD
3.27
...
3.64 4.49 4.Q3 4.25 4.32 4.32 5.84 5.25 5.85 8.20 8.30
CHzDz
0.64 .,.
0.0 0.0 0.74 0.0 0.32 0.33 0.0 0.0 1.08 0.0 0.0
CDaH
11.10 11.85 11,71 13.70 14.38 13.22 14.56 14.33 15.41 14.68 12.66 10.94 9.83
Experimental Method The details of the apparatus have already been described4v5and involve the use of a lithium fluoride-silver chloride-silver-Pyrex graded seal to facilitate working at temperatures up to about 400”. A conventional xenon resonance lamp powered by a 2450-AIHz microwave generator was used as the light source. About 2% of the intensity of the 1470-A line appeared at 1295 A and no other radiation absorbed by n-butane was present. The n-butane was research grade material and the isotopic purity of the n-C4Dlois characterized by D:(D H) > 0.99. Thc n-C4Dlo was purified by gas chromatography, dried over P205, and purged of C02 with Ascarite. Blank experiments showed no detectable methane prior to photolysis. After photolysis, the fraction of the reaction mixture (methane and hydiogen) volatile at 77°K was sampled and analyzed on an analytical mass spectrometer of resolution 1/4000. Thus, HzO+, OH+, and N + presented no problem in the methane analysis. Cracking patterns were measured on authentic isotopic methanes. Most experiments were done on a master mixture with the ratio n-C4Dlo:n-C4Hlo = 0.27.
+
Results Table 1 shows the isotopic methane analyses measured in experiments done in the temperature rango 308-871°K. Figure 1 shows the Arrhenius plotJsof the ratios [CH4]/[CH3D] and [CDsH]/ [CD,]. Two experiments not reported in the table were done with a 1 : 1 mixture of n-C4Dloat the same total butane pressure as those experiments reported in Table I and at 308°K. These experiments were done to test for kinetically “hot” methyl radicals. A pressure of 66,500 N/m2 (500 Torr) of helium was added prior to photolysis and pumped away after photolysis by means of a liquid hydrogen cold trap. It was found that the presence of helium had no effect on either ratio [CH4]/[CHsD] or The Journal of Physical Chemistry
CD4
5.75 100.0 3.70 3.06 3.69 3.72 3.35 3.11 3.00 1.87 1.40 1.24 1.18
Hz
HD
DZ
1015 283 573 340 421.7
62.7 93.0 31.3 22.7 27.8
126.0 1415.0 70.5 39.6 42.1
406.6 365 182.3
33.8 30.9 27.4
39.3 35.0 27.3
123.8 48.3 58.1
6.3 4.5 3.5
...
...
...
...
...
... 6.1 3.2 2.5
[CD3H]/[CD4]. All reactions were carried out to less than 1%conversion.
tions 1, 3, 6-9. In eq I and I1 [BuH] and [RudD] are concentrations of n-butane and n-butane-dlo, respectively, cp1 and p3 are quantum yields of reactions 1 and 3, respectively, and I , is the rate of light absorption. Inspection of eq I and I1 reveals that when the temperature is so low that reactions 6 and 7 are negligible, the ratio JCH4]/CH3D]approaches infinity. On the other hand, a t low temperature where reactions 8 and 9 are also negligible, [CD,H]/ [CD4]approaches zero. Consequently an increasing divergence in the two ratios should occur as the temperature decreases. AS the temperature increases, (6) dominates (1) and (9) dominates (3) with the result that eq I and I1 reduce t!o eq 111 and IV. [CH4]/ [CH3D] = k’6[BuH]/(le7[Bu~D]) (111) [CDiH]/ [CDd]
ks[BuH]/(ks[BudD])
(IV)
It is well documented6 that k6/k7 = k8/lcg and it follows that the ratios [CH3D]/[CH~I and [CDaHl/[CD41 should approach asymptotically the same Arrhenius line defined by the kinetic isotope effect measured for n-butane by McNesby and GordonS3 The data in Figure 1 show that this is the case for the highest temperatures a t which experiments were possible. (6) W. M. Jackson, J. R. McNesby, and R. deB. Darwent, J. Chem. Phys., 37, 1610 (1962).
791
HIGH-TEMPERATURE VACUUMULTRAVIOLET PHOTOLYSIS OF %-BUTANE I /
J/
I
I
iog R 0.8
0.4
1,4
0
1.6
2,2
2.6
3.0
3.4
10~1~
Figure 1. Effect of temperature on isotopic methanes.
A most striking feature of Figure 1 is that the divergence expected on the basis of the proposed mechanism becomes markedly less severe below about 523”K, with the ratio [CH4]/[CH3D]actually falling to a value well below that expected from a pure free-radical abstraction mechanism. There are two explanations for this observation that may be advanced. (1) Some of the methyl radicals are “hot” and those that are should abstract hydrogen and deuterium approximately in the ratio’ 4: 1 to give ([CH4]/ [CHaD])h,t = ([CDsH]/ [CD,]),,,, = 4. The observed ratios log { [CH4]/ [CH3D]] and log { [CD3H]/[CU,] ] should tend toward the value log 4 = 0.60 as observed in Figure 1. The addition of inert gas in sufficient quantity to quench “hot” methyl radicals should cause a very large increase in [CHI]/ [CH3D] and a corresponding decrease in [CD3H]/[CD4]. The observation that large amounts of helium have no effect on these ratios proves the point that their depressed divergence at low temperatures cannot be due to hot radicals. An alternative explanation is (2) disproportionation of methyl with other radicals such as n-propyl and butyl. It is apCH3
+ n-C3H7 +CH, + C3Hs
(11)
proximately true that at temperatures too low for efficient decomposition of butyl and n-propyl the numbers of CxHzx+lradicals are 3.7 X 1.6 or 5.9 times as abun-
dant as CzDzz+1radicals, where the factor 1.6 is the ratio of absorption coefficients8 of C4HIO and C4D10present in the ratio 3.7: 1. Since disproportionation reactions have very low activation energies, the methanes formed will be approximately in the ratio [CH4]/[CH3D]= [CD,H]/ [CD,] = 5.9. The disproportionation hypothesis, therefore, predicts a result in the same direction as that predicted by the ‘‘hot” methyl hypothesis with the exception that no effect of inert gas is expected. Disproportionation of methyl and other free radicals as an explanation of the depressed divergence of [CH,]/ [CH3D] and [CD3H]/[CD,] is in good agreement with all experimental observations. A further possible explanation that wall reactions are involved may be dismissed since the walls of thevessel are certainly vastly richer in H than in D. Abstraction of H from the walls could not, therefore, cause the observed drop in the [CH4]/[CHSD] ratio. A rough estimate of the chain length may be made as follows. Assume all D2to be formed by molecular elimination, Okabe and Recker’ have estimated that methyl radicals are produced at a rate 17% of that at which molecular hydrogen is formed. However 12% of the molecular hydrogen in our system is Dz. Thus, 17:12 = 1.4 is the ratio of production of methyl radicals t o production of Dz. At 671”1
