D. A. BECKER,H. OKABE, AND J. R. MCNESBY
538
Vacuum Ultraviolet Photochemistry. VIII.
Propylene
by D. A. Becker, H. Okabe, and J. R. McNesby National Bureau of Standards, Washington, D. C. 220,234 (Received September
6, 136.&
The direct photolysis of propylene was carried out below (at 1470 8.)and above (at 1236 8.) the ionization potential. Major products were acetylene, ethylene, hydrogen, methane, ethane, propane, propyne, allene, isobutane, and Gunsaturated hydrocarbons. No appreciable energy dependence was observed in the distribution of products. Isotopic analysis of photolytic products, hydrogen, methane, ethane, and ethylene, from the mixture of C 3 H r C3D6 and from CH~CHCDZ, was made to obtain information on the reaction mechanism. The isotopic composition of the ethylene product, which must at least partially be produced at 1236 A. from the ion-molecule reaction, C3H6+ C3H6 C4H8+ cZH4, was not sufficientlywell defined to confirm the occurrence of this reaction. The following primary processes reasonably explain the isotopic distribution of products: C3Hs CH3 H C2Hz; C3He --t CH4 CzH2; C3H6 --t Hz C3H4; C3He -t H CaH6; CZHs -+ CHB C2H4; and C3H6 --t CH3 CZft3.
+
+
+
Introduction Since the ionization potential of propylene is 9.7 e.v.,lP2 it is possible to study its photochemistry above and bzlow the ionization potential with xenon (mainly 1470 A., 8.4 e.v.) and krypton (mainly 1236 A., 10.0 e.v.) resonance lamps. Thus, it is of interest to examine the product distribution with these light sources. Isotopic analysis of products might reveal that certain of them are formed from ion-molecule reactions. Since photons of an energy of 10.0 e.v. prothe interpretation of the reduce only the parent sults should be simpWed. In this work, attempts were made to clarify the energy dependencies of the reaction mechanisms. Experimental The light sources were gettered xenon and krypton resonance lamps of high chromatic purity.4 A lithium fluoride window was sealed to the Pyrex body of the reaction vessel with Apiezon W wax. The lamps were external to the reaction vessel and a l-mm. space between the lamp window and reaction vessel window was flushed with nitrogen to prevent atmospheric absorption. In order to distribute the reaction products throughout the reaction vessel so as to prevent their own photolysis, a circulating pump, similar to that described by Watson: was employed. Figure l is a schematic representation of the apparatus. The Journal of Physical Chemistrz,
+
+
-f
+
--f
+ + +
The volume of the sampling chamber, A, is only about 1% of the volume containing the reacting gas. In order to sample the reaction mixture during photolysis, valve B is opened, allowing the mixture to expand into the evacuated sampling chamber, A. After closing valve B, the sample is taken by inserting a l-cc. gastight hypodermic syringe, which had been flushed with helium, into A through a self-sealing rubber septum, C. This technique allows analysis to be made as a function of time or per cmt decomposition. Gas chromatographic calibrations (squalane) were done on all major products and it was found that peak areas (peak height X peak width at the half-height) were proportional to the amounts of products. Flame ionization detection was used throughout the work. Materials. Phillips research grade propylene and Merck C3D6 and CH3CH=CDz were purified by gas chromatography on a silica gel column. After this treatment, the only detectable impurity in the propylene was 0.02% n-butane. By mass spectrometry, it was found that 3.3% propylene-& was present as an impurity in propylene-d6. (1) K. Watanabe, J. Chem. Phys., 26, 542 (1957). (2) J. A. R. Samson, F. F. Marmo, and K. Watanabe, ibid., 36, 783 (1962). (3) B. Steiner, C. F. Giese, and M. G. Inghram, ibid., 34, 189 (1961). (4) H. Okabe, J. Opt. SOC.Am., 54, 478 (1964). (5) J. S. Watson, Can. J . Technol., 34, 373 (1956).
VACUUM ULTRAVIOLET PHOTOCHEMISTRY OF PROPYLENE
t o gas handling
\ \
Figure 1. Schematic representation of the photolysis apparatus.
It is more diflCicult to detect a propylene-d4 impurity. The number of CD2Hgroups in the nominal propylenede was determined by near-ultraviolet photolysis of DzS in its presence. The D atoms sometimes add nontenninally to propylene and the excited n-propyl (but not isopropyl) decomposes to give CD2H and CD3 approximately in the ratio of CD2H groups:CDa groups.6 The abstraction of D from D2S by methyl is D
+ CD2HCD=CD2 +CD2HCD&Dz* +
+ COD* (1) +CD&D2CD2* + CDs + CzD4 (2)
539
provided a material balance measurement which is an indication of the reliability of the analytical method. Hydrogen and methane were not determined in experiments done with the circulating pump. The material balance in the static system experiments is indicated by the cumulative formula of the reaction products of C3H5.,and CaHc.l at 1470 and 1236 8.)respectively. There is the possibility that, even at conversions of less than 1%) the photolysis of primary reaction products can contribute in an important way to the observed chemistry. Such a situation can arise if reaction products accumulate near the window of the reaction vessel and if they have very much higher absorption coefficients than the parent molecule. Table I shows that the absorption coefficients of two of the most important products are smaller than those of the parent and therefore it is unlikely that secondary photolysis is important. In the static system, measurable amounts of higher hydrocarbons were formed while none were produced in the circulating system.
Table I : Absorption Coefficients of Propylene and Some Important Reaction Products
CDzH
D
+ CD&D=CDz
a fast reaction and the relative abundance of CDzH and CDa radicals and, therefore, of CDzH and CDs groups is given by the reaction product ratio CDaH/ CD4. It was found that 5.5% of the methyl groups are CD2H and 94.5% are CD,. Mass spectrometer analysis reveals 3.3% propylene-ds in the propylene-de. It was not possible to assess the integrity of CHaCH2= CD2.
Results Some preliminary photolyses at 1470 and 1236 8. were done in a static, noncirculating system. I n these experiments the total number of moles of hydrogen and methane produced were determined absolutely by means of a Toepler pump-gas buret assembly and the relative amounts of these by mass spectrometry. The relationship between the amounts of methane and other hydrocarbons formed was determined b r gas chromatography. It was found that a t 1470 A., H2/CH4 = 2.2 and, a t 1236 A,, H2/CH4 = 3.8. Further, the Hz/C2H2ratios were 0.20 and 0.42 at 1470 and 1236 b.,respectively. While the product analyses for these experiments were only in fair agreement with those obtained in the more reliable circulating system, they
1470 8.
Propyleneb Ethylene' Acetylened
-P
1236 I.
1000 -600 -200
530 400 -530
'
a Definition of E : I / & = exp( --Patmxom). See ref. 2 in text. "M. Zelikoff and K. Watanabe, J. Opt. SOC.Am., 43, 756 (1953). T. ru'akayama and K. Watanabe, J . Chem. Phys., 40, 558 (1964).
In order to learn if the reaction products are involved in the reaction to a significant extent, product analyses, as a function of conversion, were performed. The observed rate of formation of a reaction product, which is itself being consumed in proportion to its rate of accumulation, cannot be independent of the per cent decomposition of the parent. The slopes of the curves in Figure 2 represent the rates of formation of ethylene. The lack of dependence of the rate on the per cent decomposition at either 1470 or 1236 b.shows that secondary reactions of ethylene are not involved. Similarly, Figures 3 and 4 show that acetylene, allene, ethane, and isobutane are primary products whose rates of formation are independent of conversion at both wave lengths. The analyses for hydrocarbon products other (6) T. Yokota and B. deB. Darwent, unpublished results.
Volume 69, Number 8 February 1966
D. A. BECKER,H. OKABE,AND J. R. MCNESBY
540
,6b
/'i
ETHYLENE PQODUCTION
Kr Lines
4
Table I1 : Percentage Composition of Products in Propylene Photolysis" a t 0.2% Conversion
z W
X, A.-
J
1470
Hydrogen Methane Acetylene Ethylene Ethane Allene Isobutane butene-1 Isobutene trans-Butene-2 cis-Butene-2 % DECOMPOSITION
t
Lz
"/
(3.8)b
(3.4)b 38.0 12.2 7.4 7.4 11.4
34.9
18.2 4.1 6.8 8.6 4.8 4.1 0.0
3.5 7.2 1.9
a Propylene pressure a t 10 rnm. These figures are from sepsr rate experiments which showed that, a t 1470 d., H2/CH4 = 2.2 and Hz/CZH~ = 0.20; at 1236 d., Hg/CH( = 3.8 and H2/CzHP= 0.42. Experiments with a different chromatographic columm revealed propyne and propane each approximately equal to allene.
J
2.81
(14. 7)b
(7.6)*
+
Figure 2. Dependence of ethylene production on per cent decomposition.
1236
LL
,ISOBUTANE
/
\
v)
W
0
J
0
5 0
=' z
' 6 ETHANE
02
0.4
06
0.8
~.
. _
1.0
'10 DECOMPOSITION
Figure 4. Dependence of isobutane and ethane on per cent decomposition: 0, xenon lines; 0 and 8, krypton lines.
Photolysis of Labeled Propylenes. The analysis of the ethane fraction from the photolysis of C3Hs C3D6at both 1470 and 1236 A. was not done with great
+
+ C3Ds
Table 111: Photolysis of 1: 1 Mixture" of C3H6
x, A.
Figure 3. Dependence of acetylene and allene production on per cent decomposition: 0, xenon lines; 0, krypton linea.
than CHI for photolysis at low conversion (-0.2%) are presented in Table 11. The Journal of Physk4-d C h m k t r y
1470 (1295)
1236(1165)
Isotopic analysis of ethane and ethylene (percentages)
CiDe
CiDaH
CzDiHi
CiDsHs
25 22
16 19
2 5
57 54
" Total pressure a t 10 mm. addition of 15% NO.
-
(CzDsH/ CzDdcor.
0.88* 0.60
'The ratio is unchanged by the
VACUTJM ULTRAVIOLET PHOTOCHEMISTRY OF PROPYLENE
541
Table IV: Photolysis of Propylene“
A,
1: 1 Propylene :propylene-d6 1: 1 Propylene :propylene-& CHaCH=CDz 1:1 Propylene :propylene-&
+ 15% NO
A.
1470 1470 1470 1236
Hn
HD
57 62 64 59
llb
3b 27 18
Isotopic analysis of hydrogen and methane, % Dn CH4 CHaD CHzDz
32 34 8 23
44 49 38 40
8
(15)” (8)c (34)c (8)”
NO
23 12
CDaH
CD4
8” 0” 5 13
25 43 0 27
” Total pressure a t 10 mm. Corrected for blank experiment with nominal CaD6. These values are uncertain due to the inability of the mass spectrometer to distinguish between the small amounts of CHzDz and background HzO. precision because of the uncertainty of the cracking patterns and the interference of air at masses 28 and 32. However, it was possible to analyze the ethanes having at least three deuterium atoms using the cracking patterns of Bell and Kistiakowsky.7 The results are given in Table 111. Included are the ratios C2D3H/ C2D4 after correction for C2D3Hobtained in the photolysis of the nominal C3D6. These results show that an important source of ethylene is molecular elimination. Nevertheless, substantial mixing is evident in the ethylenes, and abstraction of H and D by the vinyl radical, which is not scavenged by NO, is undoubtedly involved. Acetylene produced in the photolysis of C3Hs$- C3D6 in the presence and absence of NO is more than 90% unmixed.
cause ethylene is overwhelmingly the major hydrocarbon product in cyclopropane photolysis at 1470 8.,8 while acetylene dominates the products of propylene photolysis. Since the amount of acetylene produced is so much greater than that of methane, the molecular elimination of methane, while it is a major source of methane, is only a minor source of acetylene.
Discussion I . Photolysis at the Xenon Lines. A . Molecular Elimination of Hydrogen and Allene Formation. Table
The sum of the sequence (6), (7) is
IV shows that the hydrogen produced in the photolysis of mixtures of propylene and propylene-ds is nearly 90% H2 and D2, which indicates that molecular elimination is an important process. A small amount of atomic hydrogen is prodaced which is eliminated almost entirely in the presence of NO. Since nearly two-thirds of the hydrogen in the photolysis of CHaCH=CD2 is H2, it is certain that the methyl group is involved in the elimination of Hz. There are two possible mechanisms
+ CHCH=CD2* + H2 + CH-C=CDz +Hz + CHFC=CD~
CH,CH=CDZ* +H2 CH&H=CD2*
(3) (4)
Table I1 shows that the allene is approximately equal to the hydrogen. B. 14cetyZene Formation. The fact that (Table 11) hydrogen and allene are not the major products indicates that modes of decomposition of the excited state, other than molecular hydrogen elimination, are predominant. The possibility that the excited states of cyclopropane and propylene decompose through intermediates of similar configuration is entirely discounted be-
CHsCH=CH2 +CH4
+ CHGCH
(5)
As is the case in the photolysis of eth~lene,~Jo the rapid, consecutive rupture of two bonds is probably responsible for the production of acetylene. CH~CHECH~ -3 CH3
+ CH=CHz*
CH=CH2* +CHGCH
+H
(6)
(7)
+ +
CHaCH=CHZ +CH3 H CHGCH AH = 140 kca1.l’ (6.1e.v.)
(8) Thus, the energy of the 1470-8. photon (8.4 e.v.) is more than enough to cause reactions 6 and 7 to occur. C. H Atom Formation. From the observation recorded in Table IV that HD is produced, to some extent, in the photolysis of c3H6 C~DS, it is obvious that H atoms are not totally scavenged by propylene as they are by ethylene in ethylene photolysis. Rather, there is some contribution to H atom disappearance made by the abstraction reaction
+
H
+ CHaCH=CH2
--t
H2
+ CHFCH-CH~
(9)
It is evident (Table IV) that NO scavenges most of the H atoms that would otherwise abstract H from propyl(7) J. A. Bell and G. B. Kistiakowsky, J . Am. Chem. Soc., 84, 3417 (1962). (8) C. L. Currie, H. Okabe, and J. R. McNesby, J . Phys. Chem., 67, 1494 (1963). (9) M. Sauer and L. M. Dorfman, J . Chem. Phys., 35, 497 (1961). (10) H.Okabe and J. R. McNesby, ibid., 36, 601 (1962). (11) The heat of formation of CHs is 32.6 kcal. (ref. 7) and the heat of formation of propylene is -1.9 kcal. (“Handbook of Chemistry and Physics”). Other data from National Bureau of Standards Circular 500, U. S. Government Printing O5ce, Washington, D. C.
Volunte 69,Number 8 February 1966
D. A. BECKER, H. OKABE,AND J. R. MCNESBY
542
ene. It is possible that H atoms associate rather than abstract to form H2. D. Ethijlene and Butene Formation. The fact that C2D4 > C2D3Hin the photolysis of a mixture of C3H6 and C3D6 (Table 111) shows that, in the photolysis of propylene, ethylene is formed partially by a molecular elimination reaction. C&CH=CH2 +CH2
+ CHyCH2
(10)
The CH2 species released in the ethylene elimination may be expected to react with propylene and this reaction accounts for the appearance of the various butenes. CH2-CH-CH3*
CH2
+ CH2=CHCH3 + \/
---f
CH2 butenes (11) The question of whether methylcyclopropane is formed has not been resolved because of analytical difficulties. The observation that a substantial amount of isotopic mixing occurs in the ethylene formation at 1470 8. suggests that, in addition to the molecular elimination of ethylene, vinyl radicals, which are not scavenged by NO, are involved in ethylene formation.
+ CD=CD2 + C3H6 +CDFCHD + C3H5 hv
CD~CDECD~ +CDs CD2=CD
(12) (13)
It is also possible to form ethylene from the reaction H $- C3Ha +C3H7 +CH3
+ CzH4
(14)
E. Ethane Formation. Judging by the dominance of CH3CD3 in the ethanes resulting from the photolysis of C3H6 C&D6(Table 111), ethane formation is largely attributed to association of methyl radicals. The appearance of appreciable amounts of C2DsHis not easily understood but it may involve atomic cracking reactions of propyl and isopropyl radicals. However, with the information a t hand, further comment on the mechanism of C2DsHformation is purely speculative. F. Methane and Isobutane Formution. According to Table IV, the methane produced in the photolysis of C3H6 C3D6 is mainly CH4and CD4and, in the presence of a scavenger, NO, it is almost entirely CH4 and CD4. The photolysis of CH3CH=CD2 gives CH4 and CH3D. The mechanisms of molecular elimination of methane are, therefore
+
+
The Journal of Physical Chemistry
CH3CHeCH2 +CH4
+ C=CH2*
+
CH=CH* CH&H=CH2 +CH4
+ CH=CH*
(15) (16)
I n the absence of NO, the methanes show some isotopic mixing and the abstraction or association reaction is responsible for this observation. CH3
+ CH&H=CH2 CH3
----+
CH4
+ CHtCHzCH2 (17)
+ H +CH4
(18)
Isobutane undoubtedly comes from the addition of H atoms to propylene followed by association of the isopropyl radicals so formed with methyl radicals. II. Photolysis ut the Krypton Lines. The ionization efficiency of propylene at 1236 measured by Samson, Marmo, and Watanabe,2is 0.32 and only the parent ion, C3H6+, is formed at this wave length.3 The only significant ion-molecule reaction to be expected a t this wave length is the condensation reaction
s.,
C3H6 C3&+
hu = 10 e.v.
+ CaHs
C3H6+
+C4&+
+e
4- CzH4
(20)
which has a large reaction cross section of 74 X cm.2.13114If a mixture of C3H8-C3D6 is photolyzed a t 1236 8.,a large isotopic mixing in the ethylene would be expected to occur from this reaction. Therefore, if there were no isotopic mixing in ethylene a t 1470 8., where no iots are formed, and a large amount of mixing a t 1236 A., this would suggest the occurrence of reaction 20. Unfortunately, for the purpose of the projected experbe$, isotopic mixing in ethylene is appreciable a t 1470 A. even in the presence of 15% NO. In addition, a large air background, at masses 28 and 32, made it practically impossible to measure an accurate isotopic distribution of ethylene. This and other ionmolecule reactions or neutralization reactions involving C3H6+, C4H8+,etc., appear to have an insignificant effect upon the distribution of products. Acknowledgment. This research was supported by the U. S. Atomic Energy Commission. (12) J. N. Butler and G. B. Kistiakowsky, J . Am. Chem. Soc., 82, 759 (1960). (13) D. 0. Schissler and D. P. Stevenson, J . Chem. Phys., 24, 926 (1956). (14) D. P. Stevenson, J. Phys. Chem., 61, 1453 (1957).
