2412
C . B. KISTIAKOWSKY AND BRUCEH. MAHAN
1701.
79
power dependency of ra appearing in equation S. The value v = 1OIs molecules is undoubtedly high. From a consideration of the geometry of the closest packing of molecules on a surface Brunauer'" gives for the area covered by one molecule
surface coverage case, using different catalyst systems. The agreement of the theoretical and experimenta! temperature dependencies both for the low surface coverage cases such as that investigated by Harrison and McDowell and for the higher surface coverage case investigated here, together with the pressure dependencies investigated for these two cases streiigtheiis the model assumed in the whcre JZ is the molecular weight of the gas ad- theory for the conversion. This agreement is all sorbed, 61 is the density of the liquefied gas, and N A the more striking in view of the fact that the theory is Avogadro's number. Taking the density of predicts a negative temperature dependency for the liquid hydrogen as 0.070 g. ~ m . - ~the , value v = low surface coverage case and a positive tempera7 X lo1*molecules cm.-2 can be calculated. ture dependency for the high surface coverage case. The significance of the calculations concerning I t is a pleasure t o acknowledge the assistance of the absolute rate is that the same choice of values Dr. Clarence M. Cunningham in the preparation for these variables gave agreement with experiment of the catalysts and of Mr. L. E. Cox, foreman of for both the low surface coverage case and the high the Cryogenic Laboratory Shop, in the construction of the catalyst chamber. (14) S. Bruiiauer, "The Adsorption of Gases and Vapors," Princet o n University Press, Princeton N. J., 1945, p. 287.
COLUIIILIUS, OHIO
[ C O N T R I B U T I O N FROM THE
GIBBSCHEMICAL LABORATORY, HARVARD UNIVERSITY]
The Photolysis of Methyl Ketene BY G. B. KISTIAKOWSKY AND BRUCEH. MAHAN' RECEIVED DECEMBER 19, 1956 The gas phase photolysis of methyl ketene was investigated using light of 3650, 3130 and 2400 A. wave length. The principal photolytic products were found to be CO, CnH4 and C4Hs. The experimentally determined effects of temperature, wave length of light, methyl ketene and inert gas concentrations are explained by a mechansim involving vibrationally excited ethylidene radicals rearranging a t measurable rates into ethylene. The alternative mechanism involving only photoexcited methyl ketene molecules is shown to be inconsistent with observations.
Introduction Extensive investigations of the photolysis of ketene2-' have demonstrated the formation of the rnethylene radical and have provided some information on its kinetic behavior. The photolysis of the next homolog, methyl ketene, has not been described in the literature. By analogy, i t should result in the formation of the ethylidene radical, CH3CH. Previous attempts to study the chemistry of this radical have involved the pyrolysis and photolysis of diazoethane, CHsCHN2, and acetaldazine, CH3CHNNCHCHs. The decomposition of the latter was found to be quite complicateds and apparently does not involve the ethylidene radical to any important extent. Investigations of the properties of the ethylidene radical produced from diazoethane have been hampered by the hazards involved in handling this material. Rice and Glasebrooks observed no reaction of the ethylidene radical with antimony mirrors when pyrolyzing diazoethane a t 650" and higher. The ( 1 ) General Electric Fellow, 1955-1956. ( 9 ) (a) R. G. W. Norrish and G. Porter. Disc. Faraday SOC.,a, 97 (1947): (b) G. B. Kistiakowsky and N. W. Rosenherg, T ~ I JOURS NAL, 73. 321 (1950). (3) M . Vanpee and F. Grard, Bull. soc. cirim. bclg., 60, 208 (1951). ( 4 ) G. 13. Kistiakowsky and W. L. Marshall, THISJOURNAL, 74, 88 (1952). ( 5 ) A . N.Strachan and W . A. Noyes. Jr., i b i d . , 76, 3258 (1954). (6) €? Gcsser a n d E. W.R Steacie, C a n . J . Chcm., 34, 113 (15nG). (7) J. Chanmugam and M. Burton, THISJOURNAL, 78, 509 (1956). ( 8 ) K .K. Brintun, ibid., 7 7 , 842 (1955). (9) F 0. Rice a n d A. L. Glasebrook, i b i d . , 6 6 , 741 (1934).
only decomposition products detected were ethylenc and nitrogen. The authors concluded that if ethylidene was formed a t all under their conditions, it isomerized to ethylene so fast as to exclude all other reactions. Barrow, Pearson and Purcell10 observed the rcmoval of selenium and tellurium mirrors when pyrolyzing diazoethane a t 400". As the furnace temperature approached 600" the rate of mirror removal progressively decreased, thus confirming the speculation of Rice and Glasebrook on the thermal instability of the ethylidene radical. The products resulting from the reaction with selenium mirrors were shown by Barrow, et al., to have the properties of selenoacetaldehyde. The same product was obtained when the authors decomposed diazoethane photochemically in the presence of selenium mirrors. The ultraviolet photolysis of diazoethane was also studied by Brinton and Volmanll who detected small amounts of butene-2, hydrogen and acetylene among the products, in addition to ethylene and nitrogen. The authors suggested that butene-2 originated from the reaction CHsCH $- CHBCH +CHaCHCHCHz
Repeated explosions of diazoethane led to the discontinuation of this work. As the following will ( I O ) R F Barrow,
r
(,
Pearson and K. IT Purcell, C r o n s Faraday
Soc , 31, 880 (1535)
(11) R . K B r i n t o n and 1) (1951).
H '.'olmdn
I
C h r m r h y s , 19, 1391
PHOTOLYSIS OF METHYLKETENE
May 20, 1957
show, the photolysis of methyl ketene provides a inore satisfactory source of ethylidene radicals. Experimental Details Methyl ketene was prepared by the method of Jenkins.la The vapor of propionic anhydride was pyrolyzed a t low pressure in a quartz tube heated externally to 600". The products passed through a trap cooled t o -78" where unreacted anhydride and propionic acid were condensed. Methyl ketene was collected m a subsequent trap which was cooled to -161 ". more volatile gases being removed continuously by a diffusion pump. The contents of the second trap were analyzed as 99% methyl ketene and were further purified by two slow bulb-to-bulb distillations The delivoery bulb was kept a t -78" and the receiver bulb a t -111 , continuous pumping by a diffusion pump assisting in the removal of non-condensables. The first and last quarters of each distillation were discarded. The freshly distilled product was 99.9% absorbed by Ascarite and no ethylene or other lower boiling gases were ever detected as impurities. Although the purified methyl ketene was stored in a trap cooled with liquid nitrogen, the amount of impurity nonabsorbable in Ascarite slowly increased with the age of the preparation and cokld not be removed by a repetition of the purification procedure described above. The impurity was not identified chemically but the mass spectrum of older preparations suggested the presence of some CI and CS hydrocarbons. An attempt to remove these impurities by a low pressure column fractionation failed, since methyl ketene fapidly polymerized in the still pot and on the column packing: The presence of the impurities in methyl ketene, which may have amounted t o 0.5% in some kinetic experiments, made it advisable to express the results of photochemical experiments mainly in terms of carbon monoxide and ethylene yields, since analyses for these products were not complicated by the above-mentioned impurities. The ketene which was used as an internal actinometer for the quantum yield meawrements was prepared by the pyrolysis of acetic anhydride and was purified and stored in an identical manner to methyl ketene. Hydrogen used in the analytical procedure was Matheson Extra Dry grade (99.8%), which was passed over heated platiiiized silica gel and Mg(C104)a to remove oxygen and water vapor. Oxygen from an Airco tank was treated similarly before use in the combustion analyses. The n-butane was Phillips Petroleum Company Research Grade material (99.9%) and was used after removing noncondensables a t 161 Sulfur hexafluoride, Matheson C.P. Reagent grade, was distilled twice from -111 to --161°, the fvst and last quarters of each distillation being discarded. Carbon dioxide, Matheson Bone Dry grade (99.8%), was used after the removal of non-condensables a t - 161O . Diethyl ketone, used for actinic measurements, was Eastman White Label grade, subjected to two bulb-to-bulb vacuum distillations before use. The photochemical experiments were carried out in a static system. This consisted of the usual pumping arrangement, cold traps and bulbs for the storage of the required chemicals, a gas mixing manifold, a reaction cell, a quartz spiral manometer and a semi-micro gas analytical system. The light source for almost all the experiments was a low pressure mercury arc of the type described by Bates and Taylor.ls Its high intensity made it especially suited for this research, since enough products could be obtained for analysis after only five minutes irradiation time. Methyl ketene dimerizes at a measurable rate in the vapor phase. By keeping the pressure of methyl ketene a t 30 mm. and below and Iimiting the runs t o five minutes, dimerization was kept t o negligible proportions (considerably less than 0.5Y0 in the reaction celi). The lamp consisted essentially of three concentric quartz tubes. The discharge was maintained in the outermost annular space by a 20 A. direct current, a t a 47-volt potential drop. The light intensity was kept reasonably constant by a servo-mechanism controlled by an RCA 935 photoelectric cell.
-
.
(12) A. D.Jenkins, J . Chem. SOC.,2563 (1052). (13) J. R. Bates and H.S. Taylor, THISJOUXNAL, 49, 2438 (1927).
2413
A Pyrex reaction cell was axially located in the innermost space of the lamp. To maintain a constant temperature, either water or a filter solutlon was circulated through the annular space immediately surrounding the reaction cell. The other annular space of the lamp could be filled with a second filter solution, the thickness of each layer being 7 mm. To isolate the 3130 A. group of the mercury spectrum the outer filter space was filled with an aqueous solution containing 300 g. of NiSO46H20 and 75 g. of CoSOd.7H20 per liter. A solution containing 0.5 g. of KlCrO4 per liter of water was circulated through the inner compartment. T o isolate the 3650 A. spectral region the circulated medium was an aqueous solution containing 0.02 g. of 2,7-dimethyl-3,6diazacyclohepta-I,6diene hydrochloride per liter while the second solution contained 500 g. of CuSO4.5H20 and 75 g. of CoS04.7H20 per liter. A few experiments were made using the strongly reversed resonance line of a British Thomson-Houston high pressure mercury lamp. The radiation from this almost point source was collimated by a quartz lens, passed through a 3 cm. filter of chlorine gas (1 atm. pressure) and through another filter cell containing nitrogen saturated with mercury vapor a t room temperature. Following the filters were a cylindrical quartz reaction cell of 12 cm. length and a front surface aluminized mirror to reflect the radiation back into the reaction cell. The asymmetrically broadened resonance line of this lamp extended from appro~imately~2550 to 2300 A . , the average wave length being about 2400 A. The reaction mixtures were prepared by admitting the components successively into a one-liter flask and measuring their pressures with the quartz spiral manometer. A glass stirrer, driven magnetically, ensured rapid mixing of the components prior to admission to the reaction cell. Since the rate of thermal dimerization of methyl ketene was affected by inert gases and was subject to wall effects, it was impossible to prepare and keep inert gas-methyl ketene mixtures of accurately predetermined composition. This mixture was therefore made up to the approximate composition deslred and a sample of it was analyzed while the irradiation was taking place. The progress of the (dark) dimerization of methyl ketene and of the photochemical reaction were followed manometrically, using the quartz spiral manometer, but reliance was placed on the chemical analysis of the reaction mixtures. This was carried out in a system similar to that described by Nash.14 The product gases were passed through a trap cooled by liquid nitrogen and the non-condensables separated, measured and analyzed by combustion over CuO. Methyl ketene was separated from the condensables by passing them through either Ascarite or anhydrous magnesium perchlorate. The latter agent proved to be an excellent absorber of methyl ketene and thus provided means for analyzing mixtures of methyl ketene and carbon dioxide. The condensable reaction products wer; then separated into two fractions, one volatile a t -161 , and both fractions were analyzed by combustion with added oxygen on platinized silica gel, or catalytic hydrogenation. Samples could be withdrawn for infrared or mass spectrometric analysis. The performance of the entire analytical system was checked repeatedly with known gas mixtures.
Experimental Results 1. The Absorption Spectrum and the Nature of the Products.-The absorption spectrum of methyl ketene was compared with that of ketene using a Cary recording spectrophotometer. As Fig. 1 shows, the two spectra are very similar, except that the absorption of methyl ketene is slightly shifted to lower frequencies. The low resolution tracings showed no evidence of vibrational structure in either spectrum. Figure 1 shows that with the radiation of a mercury arc filtered only by the Pyrex walls of the reaction cell, the effective radiation must consist of the 3130, 3440 and 3650 A. line groups. Experiments designed to determine the nature of the reaction products were carried out with this polychromatic (14)
L. K.Nash. Ind. Eng. Chcm., 18, 505
(1946).
G. B. KISTIAKOWSKY AND BRUCEH.PILLHAK
2114
Vol. 79
light into a mixture of carbon monoxide, ethylene and butene 2CH3CHCO
2
ol
4000
I 3000
1
3500
4-nhv --+
I
I1
1
2500
A.
Fig. 1.-Absorption spectra of methyl ketene ( I ) and of ketene (11), both in a 10 cm. layer a t 86 mm. pressure.
radiation. Followed manometrically, the reaction was observed to be accompanied by a pressure increase, the rate decreasing with irradiation time in a manner quite similar to that observed with ketene. This fact suggests that the intermediates formed in the photochemical decomposition of methyl ketene react with the stable products of the reaction just as methylene reacts with the ethylene iormed in the decomposition of ketene. Subsequent experiments on the methyl ketene system showed that ethylene was one of the reaction products, and that the rate of pressure increase was lowered by adding ethylene to the reaction mixture. Gaseous products which were not condensed in a trap a t -196" were found analytically to be pure carbon monoxide. A mass spectrometric examination of this fraction indicated the presence of traces of methane. The fraction non-condensable a t - 161" was shown by catalytic hydrogenation to be a pure mono-olefin or an equimolar mixture of acetylene and ethane. X mass spectrometric analysis showed it to be ethylene with a t most O.Syoethane and no higher hydrocarbons. The infrared spectrum of the sample showed the 10.52 p band of ethylene and no other bands, because of the small size of the sample available. The hydrogenation of the fraction non-volatile a t -161" showed i t to contain on the average one double bond per mole. Combustion with oxygen, the results of which are shown in Table I, identified
L'CO
T
(2 - x)CnHr f
CJIa
(1)
Table I1 shows that this is quantitatively correct and also demonstrates that the composition of the hydrocarbon fraction depends on the pressure of methyl ketene in the reaction cell. The higher the pressure the less ethylene and the inore butene is obtained, but the sum of ethylene plus twice the amount of butene remains equal to carbon monoxide, as indicated by eq. 1. 'rABLE
11
THE COMPOSITION O F PRODUCTS CHROMATIC RADIATIOS AT 24' -CHaCHCOInitial pressure mm. pmoles
2.6 4.6 4.8 6.9 8.5 9.0 10.7 11.6 17.7 19.3 19.i 25.6
3.8 10.2 10.7 15.3 18.9 20.0 23.8 25.9 39.4 42 9 43.8 56.9
co
FORMED IN
300
POLY-
BY
SECONDS
produced, pmoles
(CO)/ (CzHaj
(CIHI)/ (CzHd
1.01
1.22 1.39 1.14 1.61 1.61 1.60 3.10 2.32 2.25 2.41 2.74
0.13
1.12 l.:ii L34 1.45 1.63 2.43 2.51 3.98 .'3.97 5.92 .'3.40
1.98
.19 .10 32 .:37 .43 ,53 .63 .68 .G7 .GO
--
,I;>
Total hydrocarbons a s CP, rmoles
1.04 1.10 1.42 1.39 l.L% 1. i 8 2.38 2.45 4.17 3.85 4.76 3 66
2. Quantum Yields.-Primary quantum yields of ketene and met45-1 ketene decompositions under exposure to 3130 A. radiation a t 23' were determined using diethyl ketone a s the internal actinometer. The absorption coefficients of all three were tletermined with the Carp spectrophotonleter. Only the ratios, of course, were required for the calculations. The quantum yield of diethyl ketone was found to be approximately unitylj a t room temperature and low radiation intensities. At higher intensities, comparable to those used here, the yield may be as low as O.G.I6 The latter experiments were made with diethyl ketone concentrations one-half to oneTABLE I third as great as those of Table 111, thus favoring TV.0 COMBUSTION -4NALYSES O F THE P R O D U C T FRACTIOSradical recombination processes. The value to be used in the calculations thus lies between 1.0 and CONDENSABLE AT - 161' 0.6 and Table 111 shows therefore that the primary 411 quantities are in micromoles quantum yield of ketene decomposition is between I I1 1.12 and 0.67. Experiments were made with dif0.636 0.586 Condensable a t - 161 ferent pressures of ketene a t this wave length and - 1.70 Mole no. change on combustion - 1.82 the ratio of carbon monoxide production to the -1.91 -1.75 Calcd. for CdHs pressure of ketene was found to be independent of 2.41 2.64 COZin products pressure. Since the fraction of absorbed radiation 2.54 2.34 Calcd. for CIHs was small, this shows the independence of the quantum yield of pressure and so suggests, but does the material as a Cd hydrocarbon. A mass spectro- not prove, that the quantum yield is unity. In metric analysis was consistent with the identifi- any case, the uncertainty does not affect the validcation as a C4mono-olefin, but the size of the sam- ity of the conclusions to be made below. ple precluded the identification of the butene isoTo determine the primary quantum yield of mer present. methyl ketene decomposition some assumption The results of these analyses were taken to (15) W Davis, THIS J O U R N 4 L . 70, 1868 (1948) mean that, within the accuracy of the measure(16) K 0 Kutschke, M H J Wi]nen and E W R Steacie r b i d , ments, methyl ketene is decomposed by ultraviolet 74, 714 (1852). O
May 20, 1957
PHOTOLYSIS OF METHYL KETENE
TABLEI11 QUAXTGM YIELDOF KETENEDECOMPOSITION THEPRIMARY AT 3130 A,"
co, labs, moles/ quanta/sec. sec. X 10-11 X lO-'S
P, mm.
Compound
37
(C2Hs)ZCO (C2Hj)zCO
32
1.43 1.28
2415
'""I
Io, quanta/
sec. X lo-"
3.3 3 4
1.43 1.2s
ilV. 3 35 x 10'6 17 CH2CO 3.6 1.6 Ratio of primary quantum yield of ketene to quantum yield of diethyl ketone; 1.12. For these experiments the lamp was run at reduced intensity.
must be made regarding secondary reactions. Consideration of the data presented in Table I1 and analogy with the ketene mechanism suggest the set of reactions CHiCHCO hv +CO f CHICH
+
CH,CH CH2CH2 CHiCH f CHiCHCO --+ C4H6
+ CO
(2)
The amount of carbon monoxide' directly resulting from the primary process is therefore COprimary = C2H4 f 1/2(coobsd
- C2H4)
(3)
Use of this equation in the quantum yield calculations avoids the least accurately determined quantity, the yield of butene. On this basis, the primary quantum yield of carbon monoxide, using 3130 A. wave length radiation, is found to depend slightly on the pressure of methyl ketene and to tend to the same value as that of ketene (unity?) a t low pressures, as shown in Fig. 2 .
0
10
20
Pmm. H g
Fig. 2.--Priniary quantum yields of meth>.l ketene decomposition by radiation of 3130 A. wave length, calculated assuming that of ketene is unity.
The primary quantum yigld of methyl ketene decomposition in the 3650 A. wave length region was determined a t 24' using ketene as the internal actinometer. The quantum yields for ketene a t this wave length were interpolated from the graph given by Strachan and K ~ y e s . ~ The results are shown in Fig. 3 . The total light intensity entering the reaction cell a t this wave length was 2 X quanta/second. The decrease of the quantum yield with increasing pressure of methyl ketene is seen to be more exaggerated than the correspond-
Fig. 3.-Primary quantum yields o(I) of methyl ketene decomposition in radiation of 3650 A . wave length. Line I1 shows data of Strachan and Noyes (ref. 5 ) obtained with ketene at this wave length.
ing behavior of ketene reported by Strachan and Noyes. 3. Effects of Temperature.-The provisional reaction mechanism specified above suggests that if the unimolecular rearrangement of the ethylidene radical into ethylene and the competing step (attack on a methyl ketene molecule) have different activation energies, the carbon monoxide-ethylene ratio should change with temperature a t constant methyl ketene concentration. Experiments were therefore carried out a t 5 and 49" using polychromatic radiation. The results, together with those of Table I1 obtained a t 24") are plotted in Fig. 4. The carbon monoxide-ethylene ratio is seen to be a linear function of the concentration of methyl ketene. Temperature has no observable effects. For purposes of comparison the butene-ethylene ratio is also plotted in this figure. The data are consistent with a straight line having zero intercept. As already noted in a preliminary publication,17 the forms of the relations between the methyl ketene concentration and the carbon monoxide-ethylene and butene-ethylene ratios are consistent with the provisional mechanism proposed above. 4. The Effects of Radiation Frequency.Separate measurements of the carbon monoxideethylene ratio in the initial stage of the reaction were made using spectrally isolated radiation of 3650, 3130 and 2400 A. wave length. The results are presented graphically in Fig. 5 . I t is seen that while the behavior remains qualitatively the same over this entire spectral region, the probability of forming ethylene, rather than butene, a t a (17) G. B. Kistiakowsky and B. H. Mahan, J Chem Phys , 14, 922 ( 19.38)
I
G.B. KISTIAKOWSKY A N D BRUCE H. MAHAX
2416
r
3.0 __--
VOl. 79
, i
04'
i 5/
2
03c
o/
o,/'
,'
:>
\
I //
,/01?3
0 It
O
L
k
,
--A
io
~
15
.L.~2C
PtOtOl ___.
Pmm. Hg
' 2 H 3 C ti C 0
.
Fig 4.-Line I shows the carbon mo1ioxide:ethylene ratios obtained at diflerent pressures of methyl ketene and different temperatures in polychromatic radiation: dots, at 5'; circles, a t 21'; triangle., at 49". Line I1 shows the hutene:ethjleiie ratios a t 21". I
Fig. 6.-Effect of inert gases on a function ( J f the carbon rno~ioxide:ethyleneratia:solid triangle, I I O inert gas; dots, carbon dioxide; circles, n-butane; open triaiip!(.s, sulfiir hexafluoride; 3130 .I) radiation.
of the total pressure to the pressure of methyl
ketene. While it seemed highly probable that sulfur hexafluoride and carbon dioxide would not participate chemically in the reaction, butane might be suspected of reacting with ethylidene radicals. 1 1 sample of the products non-condensable a t -161" was therefore subjected t o a mass spectrometric examination, which showed only a negligible amount of ethane to be present. This does not prove the absence of an addition reaction'* but i t does make more legitimate the inclusion of butane in Fig. 6. 'rABLE
I\T
EFFECTS OF INERT GASESWITH 3130 A. RADIATION Column 3 gives the rate of priiiiary CO production in molecules prr second CHICHCO, mm.
31302\.
a
/
/ e
23.5 5.4 15.9
Cog, mini.
31.9 24.4 l l i ) .4 S1'8, mm.
24.6
10.3 6.0
125.4 117.1 78.0 n-CJLlo, mm.
15.5
24.2 4.2 P mm. Hg.
Fig. 5.-Tlie carbon morioxide:ethylene ratios in monochromatic radiation of three wave lengths as functions of methyl ketene pressure.
given pressure of methyl ketene, increases rapidly with increasing energy of the light quanta used in the photolysis. eFects of 5. The Effects of Inert Gases.-The some inert gases were studied using 3130 A. radiation a t 24". The results in Table IV are shown g r a p h i d l y in Fig. 6, where the quantity ((CO)/ (cLIIi) 1) 'F'(;~I~cII~o is p l o t t d against the ratio
30.2
22.3 7.3 5.8
98.5 125 1 81.6 40.2 102.2 66.7 67.1-
0.120 .110 ,443
,070 .I19 ,245 ,292
An attempt n a s made to determine the effect of inert gas when the mixture was irradiated with 3650 light. Preliininar:,i runs indicated that the total yield of CO vr-as decreased and the ratio of carbon monoxide to ethylene was increased by the addition of an incrt gas (carbon dioxide and '12-
a.
(18) W E Doering K 0 l
