2598
(17) (18) (19) (20)
Timothy Rose and Colin MacKay
D.Rapp and D. Briglia, J . Chem. Phys., 43, 1480 (1965). C. E. Melton and P. S. Rudolph, J. Chem. Phys., 47, 1771 (1967). G. J. Schulz, Phys. Rev., 128, 178 (1962). J. A. D. Stockdale, R. N. Compton, and P. W. Reinhardt, Phys. Rev. Lett., 21, 664 (1968); Phys. Rev., 184, 81 (1969). C. E. Melton and G. A. Neece, J. Amer. Chem. Soc., 93, 6757 (1971). C. E. Melton, W. Massey, and B. N. Abeis. 2. Nalurforsch. A, 26, 1241 (1971). P. Eberhardt, 0. Eugster, and K. Marti, Z. Nafurforsch. A, 20, 623 (1965). J. R . Waton and A. E. Cameron, Z. Naturforsch. A, 21, 115 (1966). J. P. Chittum and V . K. Lamer, J. Amer. Chem. Soc., 59, 2424 (1937). C. E. Meiton, "Mass Spectra of Organic Ions," F. W. McLafferty, Ed., Academic Press, New York, N. Y.. 1963, p 82. D. C. Frost and C. A. McDowell, J. Chem. Phys., 29, 1424 (1958). L. M. Branscomb and S. J. Smith, Phys. Rev., 98, 1127 (1955). H. D. Hagstrum, J. Chem. Phys., 23, 1178 (1955).
F. R . Gilmore, J. Quanta Spectrosc. Radium. Transfer, 5, 369 (1969). J. A. D. Stockdale, R. N. Compton, G. S. Hurst, and P. W. Reinhardt, J. Chem. Phys., 50, 2176 (1969). A. L. Farragher, F. M. Page, and R . C. Wheeler. Discuss. Faraday SOC.,37, 203 (1964), J. Jortner and U. Sokolov, Nature (London), 190, 1003 (1961). J. D. Weisner and 8. H. Armstrong, Proc. Phys. Soc., 83, 13 (1964). G. Giumousis and D. P. Stevenson, J. Chem. Phys., 29, 294 (1958). H. A. Landolt and R. Bornstein, "Zahlenwerte und Functionen," Voi. 1, Part 1 , 6th ed, Springer, Berlin, 1951, p 514. "Handbook of Chemistry and Physics," 50th ed, The Chemical Rubber Publishing Company, Cleveland, Ohio, 1969, p E-72. S. K. Gupta, E. G. Jones, A. G. Harrison, and J. J. Myher, Can. J. Chem., 45, 3107 (1967). C. E. Melton. "Principles of Mass Spectrometry and Negative Ions," Marcel Dekker, New York, N . Y., 1970, p 233.
On Abstraction or Stripping of CH2 as a Route to Acetylene Formation in Hot Carbon Atom Reactions Timothy Rose* Department of Chemistry, Texas
A
& M University, Coiiege Station. Texas 77843
and Colin MacKay Departmen! of Chemistry, Haverford College, Haverford, Pennsylvania 19041 (Received May 21, 1973)
The yields of acetylene and the C4 fragmentation products resulting from the reaction of carbon atoms with cyclopentene and cyclopentane show very different effects with changes in the experimental parameters. Analysis of the results shows that more than half of the acetylene is formed by a direct reaction of a hot atom rather than through an energy equilibrated intermediate which can lead to both acetylene and the C4 products. These conclusions are consistent with the acetylene double tracer work reported previously.
Introduction Studies of the hot atom chemistry of the monovalent tritium and halogen atoms have revealed completely new modes of reaction unavailable to species limited to thermal energies.l-3 By contrast, for some time the multivalent carbon atom was thought to undergo the same primary modes of reaction as the thermal atom, with only the relative importance of these being altered.4 However, recently Lemmon, et al., have pointed out that the experimental observations on acetylene formation in hot C atom systems do not rule out a contribution from a highenergy stripping mechanism,5 and Wolf, e t al., have discussed formation of C2 and CzH, analogs by a stripping process as a possible source of the isotopically mixed acetylene fraction found in reactions with mixtures of hydrogenated and deuterated hydrocarbons.6 We wish to report evidence indicating that hot C atoms remove groups such as CH2 as a single unit in a rapid reaction which does not involve formation of an equilibrated intermediate, and may well be a direct reaction such as stripping.? Acetylene is produced from hydrocarbons by both hot and thermal atoms.6.839 Flash photolytic experiments proThe Journai of Physical Chemistry, Vol. 77, No. 22, 7973
vide strong evidence that it is produced by the reaction of a C(lD) atom with methane,g and the production of acetylene in hot systems by two paths, one of which involves a single molecule, is well documented.4.6 This latter path has usually been explained as involving insertion of a C atom into a C-H bond followed by unimolecular decay of the resulting a d d ~ c t . ~
H
H
R
R
I
I
CH=CH
+
R
+
R'
(1)
Evidence that the reaction may be more complex than indicated by this simple equation has been presented by Welch and Wolf.lO Reactions 2-4 represent some of the unimolecular decay processes expected to follow insertion of a C atom into C-H bonds of cyclopentene, and reaction 5 represents the sequence initiated by the 8-bond attack which is also expected.*Jl Reactions 2, 3, and 4 are particularly signifi-
On Abstraction or Stripping of CH?
2599
N
H~~C=CH-CH,-CH, H-'C=CH
+
+
CH=CH
(2)
CH~-CH,-CH=CH
The separation and analysis of the labeled products of interest were carried out by standard techniques of radio gas chromatography using columns described previously.12 Absolute yields of the gas-phase irradiations were determined using ethane as a monitor on the EAL13 and oxygen on the HILAC .I4 Results
H'~C=CH
+
CH,=CH-CH=CH,
(3)
i'CH~C-CH=CH,
+
CH2=CH2
(4)
CH="C-CH=CH2
+
CH,=CHI
(5)
cant since they yield both 11C-acetylene and labeled C4 compounds from a set of very similar primary intermediates. Consequently in experiments such as those involving comparison of gas and condensed phase results in which the energy distribution of reacting 11C atoms is unchanged, it seems required that with such energy equilibrated intermediates the yields of both I1C-acetylene and labeled C4 molecules be affected to about the same degree. For example, a reduction in the yield of lcbeled C4 molecules from reaction 2 by a given factor should be accompanied by a reduction in the llCH=CH yield from this reaction by about the same factor. Similarly, since (3) and (4) are quite similar, a reduction in the yield of labeled vinylacetylene from (4) should be accompanied by a roughly comparable reduction of llC-acetylene from (3). Cyclopentane represents an even more straightforward situation. Here there is only a single primary intermediate.11 Quenching of this intermediate should affect both
+
H ~ ~ C = C H CH,-CH~--CH~-CH~ (6) E~~C=CH-CH~---~H~ CH?=CH. yields to about the same degree. Thus the relative yields of acetylene and (24's from (6) should not be greatly different in the gas and condensed phases.
+
Experimental Section Standard nuclear recoil methods were used for generation of the radioactive carbon-11 atoms (20.5 min halflife).4J2 The details of the procedure are described elsewhere. All irradiations were done on the Yale electron accelerator (EAL) except those samples containing neon which were done on the Yale heavy ion accelerator (HILAC). The radiation damage on both accelerators has been determined to be less than 10- eV/molecule. Cyclopentene and cyclopentane were obtained commercially and had specified purities of 95-99%. Gas chromatographic analysis showed the principal impurity in the cyclopentane was cyclopentene (2-4%). Both hydrocarbons were degassed on the vacuum line before use. Matheson research grade neon and extra dry grade oxygen were used directly from the cyclinders.
In Table I we show our results for gas-phase reactions of nucleogenic 11C with cyclopentane and cyclopentene and compare them to the liquid-phase results of Jewett and Voigt.15 Since the addition of oxygen as scavenger has no effect on any yield,l6 these data reflect primary processes which are not significantly modified by secondary reactions such as loss of C4 radicals by reaction with radicals produced by the general radiation field. First of all we call attention to the 95% neon moderator results. These represent a lower average energy of reaction than in the unmoderated system as is illustrated by the reduction in the yield of diacetylene relative to the unmoderated system. This is expected to be a high-energy product presumably formed by elimination of Hz either from excited C4H4 formed in reactions 2, 4, and 5 , or from the corresponding primary intermediates. More significant for the purposes of this paper is the marked decrease in the acetylene yield in the moderated samples relative to the unmoderated ones, indicating that most of the acetylene yield is formed in a hot reaction. Comparison of the gas- and liquid-phase results is striking, particularly for cyclopentane. Contrary to our expectation based on formation of the energy equilibrated intermediate of (6) that in the gas and liquid phases the ratio of acetylene to butadiene should be about the same, we find it changing by a factor of about 10 in favor of acetylene. Examination of the cyclopentene data reveals the same general pattern. The yield of acetylene is barely quenched in the liquid, while that of butadiene is noticeably reduced. Discussion Qualitatively it seems clear from our results that most of the acetylene comes from a process or processes other than those involving the energy equilibrated intermediates of (2)-(6)? Moreover, the process leading to acetylene must be sufficiently different from those leading to the (24's that it is barely quenched in the liquid phase whereas C4 pathways are very effectively quenched. The intermediates of reactions 2-6 do not seem to meet these requirements. The most plausible explanation of these results is that there are at least two important paths to acetylene formation. One involves unimolecular decay of energy equilibrated intermediates to either acetylene or C4 compounds depending upon which is favored by the structure of a given intermediate. This route is effectively quenched in the liquid phase. The second type leads exclusively to acetylene formation via a reaction occurring on a time scale comparable to or faster than the time between collisec) and, therefore, is almost tosions in solution ( tally unaffected on going from gas to condensed phase. Such a reaction might better be described as a hot abstraction of CHZ or as a stripping reaction1x3,5,6 rather than as an insertion into a C-H bond followed by unimolecular decay.lg As such, the mechanism resembles the type of rapid reaction a t a single site which is such a The Journal of Physical Chemistry, Vol. 77, No. 22, 1973
Timothy Rose and Colin MacKay
2600
TABLE I : Yields of " C Labeled Fragmentation Products from "C Reactions with Cyclopentene and Cyclopentane Cyclopentene
-
Product
30 cma
Ethylene Acetylene Methylacetylene Allene
1 . 6 f 0.2 15.2 f 2 . 6
1,3-Butadiene
Vinylacetylene Diacetylene
4 cm, 76 c m Nen
nd 5.8
} 0.9 f 0.1
10.6
2.9 f 0.2
1.5
11.6 3.4
f 0.8 f 0.5
Cyclopentane Liquidb 1.2 14.3
f 0.07 f 0.3
0.56 f 0 . 0 2 f 0.01
0.86
f 1.0
15.3 f 1 . 4
1.4
1.79
f 0.02
nr nr
30 cm" 3.8 f 1.3 22.0 i 2.5
} 1.6 f 0 . 6
Liquidh
~ _ _ _ _ _ 3.22 f 0 . 1 3 1 6 . 4 f 0.5
1.15 f 0.05 1 . 5 9 f 0.08 0.53 f 0.07 ~
f 0.3 5.7 f 0 . 2
7.4
0.8 f 0.2
~~
nr nr
a Yields are per cent total "C atoms available for reaction. nd means not determined. Errors indicate the standard deviation of several measurements. Where no error is indicated only a single measurement'was made and the estimated error is less than f20%. Data from ref 15. Yields are per cent of total "C atoms available for reactlon. nr means not reported.
prominent feature of the chemistry of hot hydrogen atoms and which Wolfgang has classified as a direct reaction.1 Relative Importance of the Two Routes A natural question involves the relative importance of the two routes leading to acetylene formation. A rough estimate can be made for cyclopentane on the basis of two reasonable assumptions: (1) the yield, D,of acetylene from the fast, direct reaction is the same in the gas and liquid phases; (2) for the energy equilibrated intermediates the ratio of the yield of llC-acetylene to labeled butadiene, Y A / Y B is , the same in the two phases, which allows us to calculate from the butadiene yield the acetylene yield for this route. The first assumption seems justified by the small phase effect on the acetylene yield. The second seems consistent with the nature of reaction 6 as discussed above. These assumptions and the data lead to the following two equations Y, = D (7.4 f 0.3)tY Y , ) = 22.0 f 2.5 ( 7 )
+
where Yc,and YI. are the total gas- and liquid-phase yields of acetylene, and 7.4 f 0.3 and 0.53 f 0.7 are the butadiene yields in the two phases. These equations yield the values D = 16.0 f 1.4 and Y,/YB = 0.8 f 0.4 which means that 73 f 10% of the acetylene comes from direct reaction in the gas phase.20 It thus appears that in the hot reaction with cyclopentane most of the acetylene formed arises from nonequilibrated intermediates. The cyclopentene system is more complex than cyclopentane, and the data are not as tractable. Yevertheless we can use the above method in calculating the direct yield if we make an additional assumption. In addition, by using a previously developed empirical equation for correlating acetylene production in the C5 molecules to the number and types of bonds, we will use the calculated direct yield from cyclopentane to estimate a value for cyclopentene. We will find that the results of the two calculations taken together imply strongly that most of the acetylene formed from cyclopentene also comes from the direct reaction. In order to apply the cyclopentane method to cyclopentene we must make the additional assumption that the relative importance of reactions 2, 3, and 4 is not altered on going from gas to condensed phase. With this assumption we calculate D = 12.9 f 4.2, and 85 f 30% of the acetylene is formed directly in the gas phase. While the error limits are large, this calculation does indicate that The Journal of Physical Chemistry, Voi. 77, No. 22, 7973
direct production of acetylene is important in cyclopentene. That the cyclopentane and cyclopentene results are consistent is shown by a different computation based on cyclopentane. The direct yield of acetylene from cyclopentane is 3.3 f 0.3% per CHz group. Rose16 has shown for cyclopentane, cyclopentene, cyclopentadiene, and benzene that the yield of acetylene can be directly related to the number of CH2 and -HC=CH- groups, with the yield from the CH2 group being 1.8 times from the -HC=CHgroup. The predicted direct yield of acetylene from cyclopentene on this basis is 11.7 f 0.870,in good agreement with the value of D calculated above. In summary. while we cannot make as reliable an estimate of the importance of the direct reaction from cyclopentene as we can for cyclopentane, estimates by two methods indicate strongly that for cyclopentene as for cyclopentane the direct reaction should account for a t least half of the acetylene in the unmoderated systems. Nature of the Direct Reaction It seems reasonable to speculate on the identity of the group or groups removed in the direct reaction. Work with deuterium-labeled hydrocarbons indicates that characteristically l5-20% of the acetylene observed comes from an intermolecular reaction.6 For cyclopropane and benzene this rises to 35-40%, but no higher percentage of intermolecular reaction has been reported. The fact that more than half of the acetylene from cyclopentane and cyclopentene is formed directly implies that the direct reaction must have a significant intramolecular component. In other words, the CH2 group must be removed as an entity a t least part of the time. Conversion of translational energy to internal energy on subsequent collision by this translationally hot C-CHz entity can lead to bond rupture and account, a t least in part, for formation of Cz and C2H fragments of the type and Wolf, e f If a CHZ discussed by Lemmon, e t group can be removed in a direct reaction it seems likely that a CH3 group can be as well, and this provides an additional route to be considered in hot formation of ethylene from these group~.~1-23
Acknouledgment. This work was done a t Yale University using the facilities of the Yale HILAC and electron accelerators. We thank the directors and staff for their cooperation. The support of this work by the Atomic Energy Commission and the Robert A. Welch Foundation is gratefully acknowledged.
Heterogeneous Loss Reaction of Carbon Monosulfide
2601
References and Notes R . L. Wolfgang, Progr. React. Kinet., 3, 99 (1965). F. S. Rowland in "Chemical Kinetics," Vol. 9, J. C. Polanyi, Ed., Butterworths University Park Press, London, 1972, Chapter 4. J. W. Dubrin. Ann. Rev. Phys. Chem., 24, in press, R. F. Peterson. Jr., and R. L. Wolfgang, Advan. High Temp. Chem., 4, 43 (1971). H. M. Pohlit, T.-H. Lin, and R . M. Lemmon, J. Amer. Chem. SOC., 91, 5425 (1969): H . M. Pohlit, W. Erwin, H. T. Lin, and R. M . Lemmon, J. Pbys. Chem., 75,2555 (1971). R. N. Lambrecht, N. Furukawa, and A. P. Wolf, J . Phys. Chem., 74, 4605 (1970) Evidence that high-energy stripping is a general reaction of hot species i s accumulating both from studies of ion moiecule react i o n ~ and , ~ from theoretical studies. For a recent trajectory study on F Hz seeJ. T. Muckerman, J , Chem. Phys., 57,3388 (1972). J. Villaume and P. S. Skell, J. Amer. Chem. SOC.,94, 3455 (1972) W. Braun, A. M. Bass, D. D. Davis, and J. D. Simons, Proc. Roy. SOC., Ser. A , 417 (1969); D. Husain and L. J, Kirsch, Trans. FaradaySoc., 67,2025 (1971). M. J. Welch and A. P. Wolf, J. Chem. SOC. D, 117 (1968); J. Amer. Chem. SOC.,91,6584 (1969). These representations are not meant to exclude the possibility of direct decomposition of the primary intermediate to a given product. J. Dubrin, C. MacKay, and R. Woifgang, J . Amer. Chem. SOC.. 86, 4747 119641. G. Stocklin'and A. P. Wolf, 3. Amer. Chem. Soc., 85, 229 (1963). J. Dubrin, C. MacKay, M . L. Pandow. and R. Wolfgang, J. inorg. Nucl. C h e m , 26, 21 13 (1964). G. L. Jewett and A. F. Voigt, J , Phys. Chem., 75, 3201 (1971). T. Rose, P1i.D. Thesis, Yale University, 1967. A possible explanation of these results is that we lose CC compounds by bimolecular reaction in the condensed phase, but do not do so in the gas.This seems unlikely. As already mentioned, gasphase experiments with O2 show no reduction in Cc yield. This indicates that the processes which convert the radicals of (2) and (6) to molecules, which are probabiy primarily H atom shifts, are fast compared to the rate of addition of these radicals to 0 2 . in condensed phase, Cd radicals can be removed by coupling to radiation produced radicals, or, in the cyclopentene, by addition to the double bond. However, under our conditions radical concentrations are too low to compete with the indicated rapid intramolecular process
(18) (19)
+
(13) (14) (15) (16) (17)
(20)
(21)
(22) (23)
(ref 12). Addition of radicals to olefins also seems to be too slow to compete effectively (ref 18). Even if C4 radicals were removed, in the cyclopentene experiments only sequence (2) would be affected. However, butadiene, the most likely product of (2) is least affected of ali the C4's by the phase change. J. A. Kerr and A. R . Trotman-Dickenson, Progr. React. Kinef., 1, 100 (1961); J. M. Tedderand J, C. Walton, ibid., 4, 37 (1967). We have not discussed the possible consequences of the fact that our system contains both singlet and triplet atoms. For the purpose of this paper we need only note that the existence of a unique route to acetylene formation cannot be simply explained as involving energy equilibrated intermediates, whether singlet or triplet, since both types of intermediates should give C4 and Cz radioactive products in the way discussed above. We do not rule out the very reasonable possibility that the rapid hot formation of acetylene which we note involves a hot triplet atom as an abstracting reagent. while the path which involves insertion followed by unimolecular decay involves oniy a singlet. Since this comparison involves results from two different laboratories, the question of the effect of a systematic difference in the two sets of results is of interest. The most drastic effect would occur i f the condensed phase yields were systematically lower than those quoted here. As one exampie, if the condensed phase yieids were reduced by a factor of 1.5, the direct yield in the gas phase would be 47 f 7%. This is an extreme assumption. so it seems unlikely that such systematic differences would alter our general conclusion that the direct reaction accounts for more than half of the acetylene yield in the unmoderated system. For discussions of ethylene production by insertion of a CH radical see the following: A. P. Wolf and G . Stocklin, Abstracts 146th National Meeting of the American Chemical Society. Denver, Coio., Jan 1964, p 326: D . E. Clark and A. F. Voigt. J. Amer. Chem. SOC., 87, 5558 (1965): G. F. Palino and A. F, Voigt. ibid.. 91, 242 (1969): and ref 15. Hot abstraction of monovalent atoms by C atoms is known (ref 4) Note Added in Proof. Professor A. F. Voigt, private communication, has recently informed us that later work in his laboratory showed that the yields reported in ref 15 should be divided by 1.23. With the corrected yields the direct yield for cyclopentane should be 12.5 f 1.2, and from cyclopentene 8.0 f 3.3. The direct yield for CH1 group from cyclopentane is 2.5 and estimated direct yield from cyclopentene using the method of Rose16 is 8 . 9 % The main conclusion of the paper, that a hot reaction leading to nonequilibrated intermediates accounts for the bulk of the acetylene yield, is unchanged.
Heterogeneous Loss Reaction of Carbon Monosulfide' R. J. Richardson,* H. T. Powell, and J. D. Kelley McDonnell Douglas Research Laboratories. McDonnell Douglas Corporation. St. Louis, Missouri 63766
(Received May 3. 7973)
Publication costs assisted by McDonnell Douglas Corporation
A mass spectrometer study of the loss of gas-phase carbon monosulfide is presented; the principal loss mechanism is found to be a heterogeneous wall reaction producing carbon disulfide and a carbon-rich wall deposit. Carbon monosulfide lifetimes are found to be of the order of minutes in a clean vessel but are reduced to seconds once a surface layer of the deposit is formed. The loss of gas-phase CS a t room temperature does not appear to be accompanied by formation of a solid CS polymer as has been widely assumed.
Introduction Carbon monosulfide (CS) can be produced in the gas phase from the electrical discharge2a or thermal dissociation2b of carbon disulfide (CS2) vapor. The CS molecule is not a reactive free radical in that it has a singlet ground state3 and has been observed to persist a t low pressures for times ranging from a few seconds to many minutes.*
The mechanism for CS disappearance involves reaction a t the vessel surface; hence, the lifetime depends on surface to volume ratio. Dewar and Jones2a postulated that the principal surface reaction responsible for CS loss is a simple polymerization a t the wall. In a later investigation, Hogg and Spice5 presented a qualitative argument that the formation of a (CS), polymer from CS would be exothermic. Although The Journal of Physfcal Chemistry, Vol. 77. No 22, 1973
