1464
P. G. BLAKE AND K. J. HOLE
The Thermal Decomposition of Methyl Ketene
by P. G. Blake and I(. J. Hole Chemistry Department, University College, Cardiff,Wales (Received October l g , 1966)
The thermal decomposition of methyl ketene has been studied in a static system over the ranges 360-540" and 20-200 mm pressure. I n carbon-coated vessels the primary decompositions are to carbon dioxide and pentadiene-2,3 and to carbon monoxide and butene-2, respectively. The decompositions are similar in rate, that to CO having the higher activation energy. Subsequent reactions of the olefins produce many other products, especially at higher temperatures. Both decompositions are of three-halves order and are inhibited by isobutene. A chain mechanism is therefore suggested and the low Arrhenius parameters are obtained, and the independence of the rate on surface/volume ratio indicates that the chains begin and end on the carbon surface.
Introduction Kinetic work on the lower ketenes has been largely confined to ketene itself; in particular, the photolysis of ketene has been extensively studied, since this is a good source of methylene radicals. l s 2 The pyrolysis of ketene produces mainly carbon dioxide and allene, but carbon monoxide becomes a major product at higher temperatures.8~~It was proposed that carbon dioxide and allene were formed by a bimolecular process, and that methylene radicals, produced by the subsequent decomposition of allene, attacked ketene ultimately giving carbon monoxide. Only the photolysis of methyl ketene has so far been investigated. Kistiakowsky and his co-workers6ss have shown the carbon monoxide, ethylene, butene-2, and acetylene are the main products and proposed a mechanism involving the ethylidene radical. The thermal decomposition of methyl ketene is reported here. It was thought to be of interest to compare this with the photolytic study and also with the pyrolysis of ketene. Biradicals have been postulated in the latter and in the thermal decomposition of diazomethane and diazoethane, and the possibility exists that they might be formed in this case. Experimental Section Materials. Methyl ketene was prepared by the pyrolysis of propionic anhydride at 390-420'. The product was purified by redistillation in vacuo at -80 to -90" to prevent polymerization, only the middle fraction being retained and stored at -196". The literaThe Journal of Physical Chemistry
ture boiling point' of -56" is incorrect and seems to correspond to the temperature at which rapid exothermic dimerization of the liquid begins. An approximate boiling point of -23" was obtained by extrapolating vapor pressure measurements in the range -105 to -70' to 760 mm on a log p l / T plot. The methyl ketene was completely absorbed by anhydrous magnesium perchlorate and was shown by gas chromatography not to contain more than 1% of ketene. The ultraviolet spectrum was closely similar to that obtained by Kistiakowsky and Chong. It seems advisable to prepare methyl ketene at as low a temperature as possible to reduce ketene formation to a minimum, and 400" gives a purer product than 550". Nitric oxide was prepared by treating saturated aqueous sodium nitrite solution with 10% sulfuric acid in the presence of mercury, and purified by passing through soda lime, silica gel, and a trap at -78". The infrared spectrum showed that all NO2 had been removed. Isobutene, obtained from the Distillers Co. Ltd., was (1) J. Chanmugam and M. Burton, J . Am. Chem. SOC.,78, 509 (1956). (2) H.M. Frey, Prow. Reaction Kinetics, 2 , 137 (1964). (3) J. R. Young, J . Chem. SOC.,2909 (1958). (4) W.B. Guenther and W. D. Walters, J . Am. Chem. SOC.,81, 1310 (1959). (5) G, B. Kistiakowsky and B. H. Mahan, ibid., 7 9 , 2412 (1957). (6) D.P. Chong and G. B. Kistiakowsky, J . Phys. Chem., 68, 1793 (1964). (7) A. D.Jenkins, J . Chem. SOC.,2563 (1952).
THERMAL DECOMPOSITION OF METHYL KETENE
pursed by distillation. Gas chromatography indicated the presence of traces of other olefins. Apparatus and Technique. The high-vacuum apparatus was of a conventional static type. The reaction vessel was situated in an electrically heated aluminumbronze block thermostat, the temperature of which was controlled by a platinum resistance thermometer and Sunvic proportional controller RT2. Vessel temperatures were measured by a standardized chromel-alumel thermocouple and Doran potentiometer. Temperature fluctuations during a run did not exceed h0.25" at 540". An unpacked silica vessel and one packed with silica tubes were used, both of ca. 150 cc volume and having surface/volume ratios of 1.5 and 10.1 cm.-', respectively. A mean dead space of 2.1% was corrected to the appropriate temperature and allowance was made for it. A rapid surface reaction took place on silica and both vessels were coated with relatively thick carbon films by the repeated and prolonged pyrolysis of isobutene in them until the rate of decomposition of methyl ketene fell to a constant and reproducible value, which was the same in both vessels. Air must be excluded between as well as during runs if reproducible results are to obtained. Greaseless taps with P.T.F.E. diaphragms were used near the reaction vessel ; otherwise, glass high-vacuum taps were employed. Pressures were read to h O . 1 mm using a glass spiral manometer with mirror attachment. The usual procedure was to admit methyl ketene to the vessel, make frequent readings of the pressure, and pump out the whole of the gas for analysis after the appropriate reaction time (30 sec to 10 min). Most attention was paid to the early stages of the decomposition and rate constants were calculated from initial rates. Analysis. Samples were transferred to the analytical section of the apparatus, which consisted of two WardLe Roy stills, a mercury diffusion pump, a McLeod gauge, a Toepler pumpgas buret, and a gas chromatograph, and had take-off points for external analysis. Products were identified by a combination of infrared, mass spectrometric, and gas chromatographic techniques. Hydrogen was occasionally measured by analyzing the permanent gas fraction on an A.E.I. MS3 mass spectrometer. CO, Con, CH,, CZH+CZH6, and methyl ketene were measured on the unfrozen sample to reduce the risk of dimerization. Methyl ketene was estimated by the infrared absorption of the >C=C=O group at 2150 cm.-', using a Perkin-Elmer Infracord 137. The absorption is strong and no dimerization occurred in the
1465
infrared cell at the low pressures used; for the same reason, absorption in this region by CO was quite undetectable. CO, C02, CH,, GH4, and C2H6 were measured by gas chromatography using a 1.2-m activated silica gel column at 25", a katharometer detector, and hydrogen as a carrier gas. Ca to Cg hydrocarbons were analyzed on a duplicate run by freezing down in the Ward stills to room temperature and pumping into the gas buret. Methyl ketene and COZwere removed by absorption on soda lime and the hydrocarbons separated on a 6.1-m column of dinonyl phthalate on kieselguhr at 25". An attempt was made to estimate methyl ketene dimer in the system using the vapor phase infrared absorption of the carbonyl group in the lactone ring at 1750 cm-l, but, owing to the low volatility of the liquid dimer at room temperatures, the results are very approximate.
Results The decomposition was studied between 360 and 540" and from 20 to 200 mm pressure. Some semiquantitative runs were also made at lower temperatures. Produck Some representative results are given in Table I. The major products were COZ,CO, pentadiene-2,3, butene-2, ethylene, butadiene-1,2, and methane; smaller amounts of methyl ketene dimer, propylene, ethylene, isobutene, hydrogen, pentadiene-1,3, and propane were formed. The principal products were produced at substantially the same rate in packed and unpacked vessels, but there were variations of up to 40% in minor products, and, in particular, no pentadiene-1,3 was found in the packed vessel. At lower temperatures COZ predominates, and the initial rate of formation of pentadiene-2,3 is approximately that of COZ and is equal to it at 361" (Figure 1). However, polymerization and decomposition of pentadiene-2,3 cause its concentration to pass through a maximum, and it is reduced to a minor product at high temperatures. The ratio C0/C02 increases with temperature, eventually exceeding unity at high temperatures and large percentage reaction (Figure 2) , and butene-2 pressure increases with CO, although it again passes through a maximum. Butadiene-1,2 is the only other major product below 400", but CH4 and C2H4increase with temperature and become the major hydrocarbon products above 500". There is no relation between pressure change and methyl ketene reacted, and there is an initial pressure decrease at all but the highest temperatures. Kinetics. The rates of decomposition of methyl ketene and of formation of CO and COz are the same in Volume 70, Number 6 May 1066
1466
P. G. BLAKEAND K. J. HOLE
Table I : Principal Products of Methyl Ketene Decomposition (All Pressures Are in Millimeters) Time, min
ACHICHCO
AP
1 2 5
48.2 60.7 79.0
-5.9 -14.4 -24.0
1 2 5
51.6 65.6 84.7
-5.3 -12.1 -16.3
406.2", 4.3 6.0 7.7
2
32.6
-2.5
457.3', 5.0
1 2
30.8 36.7
-1.6 -0.9
487.2', 4.5 7.3
co
coz
C6Ha
C4Ha
C4HO
ClH4
CH4
406.2', ~ C H , C H C O 100 mm, unpacked vessel 4.5 12.4 8.2 2.9 5.7 17.4 5.8 2.5 7.0 21.4 5.1 4.5
1.4 1.3 2.4
0.3 0.9 1.2
0 0.5 0.8
packed vessel 5.9 1.0 7.8 3.3 4.9 4.0
0.6 1.8 2.1
0.2 0.2 0.3
0.2 0.2 0.3
mm, unpacked vessel 9.1 4.1 1.4
0.6
0.9
0.8
1.1 1.4
0.3 1.8
0.6 1.7
~ C ~ C H C100 O mm,
11.7 17.4 18.6 ~ C H # ~ H C 50 O
~ C H , C H C O 50
6.5 8.1
mm, packed vessel 2.4 2.3 1.6 3.4
b o z = 103a4exp( - 14,80O/RT) I.'" mole-'/' sec-l ~CR,CHCO =
-
103s9exp( 14,90O/RT) l.'/' mole-'/2 sec-l
in the unpacked vessel, being averages of the (similar) values at 20,50, and 100 mm pressure, and by kco =
106.2
e x p ( - 2 5 , 1 0 0 / ~ ~ L'/~ )
sec-1
b o z = lo3.' exp( - 13,7W/RT) l.'Iz mole-'/' sec-I ~ H , C H C O=
104e4exp( - 16,10O/RT) 1.'Ia mole-'/' sec-'
in the packed vessel. The values from k c H c H C O are of little significance since dimerization was occurring and making a larger contribution at lower temperatures.
1 0
I 6
I 10
Time, min.
Figuro 1. Product-time curves for the decomposition of 50 mm of methyl ketene a t 361.0".
both vessels within about lo%, despite the sevenfold difference in surface/volume ratio. The kinetic order was 1.5 in each case and rate constants obtained from initial rates are given in Table 11. Below about 500" an order of 1.5 was also obeyed during the course of each run, since plots of [CHaCHCO]-'/' us. time were linear. At higher temperatures there waa some curvature, but the decomposition of methyl ketene is then so rapid that rate constants will not be very accurate. The Arrhenius parameters are given by = 105.9 exp(-24,200/~~)l.'I1 mole-'/' sec-1
The J O U Tof~Physical Chembtru
AP
5
CH4 C4Hs
C4Hs CrHs
0 0
5
10
Time, min.
Figure 2. Product-time curves for the decomposition of 50 mm of methyl ketene at 487.2'.
1467
THERMAL DECOMPOSITION OF METHYL KETENE
Table I1 : Some Three-Halves-Order Rate Constants for Methyl Ketene I)ecomposition and CO and COS Formation sec-1) Has Been Multiplied by 101 [k (1.'I2 PCHICHCO,
r,
mm
CC
Vessel
kCHaCHCO
kcog
kco
50 50 100 50 50
361.7 406.2 406.2 457.3 457.3 487.2 487.2 487.2 502.3 54" 3
Unpacked Packed Unpacked Unpacked Packed Unpacked Packed Unpacked Packed Packed
0.55 1.26 1.18 2.39 2.68 3.65 3.80 3.27 5.24 8.50
0.21 0.46 0.40 0.83 0.91 1.62 1.34 1.38 1.76 2.51
0.03 0.12 0.12 0.32 0.30 0.98 0.95 0.87 1.24 2.51
20
50 100 50
50
Methyl Ketene Dimer. The pressure drop at 360 and 406" and the condensation observed on product removal suggcsted that dimer was present. An attempt was made to study the equilibrium 2CH&H=C=O
CH3-CH-C=CH-CH3
1
o=c-0
1
at various low temperatures, in order to measure K and AH for the dimerization, and thus estimate equilibrium pressures of dimer in the reacting system, but it became clear that other equilibria were involved and it was not possible to measure the equilibrium constant. Appreciable pressures of dimer were found in the range 275--406', the amounts decreasing with increasing temperature. Product element balances struck at 360 and 406" without allowing for dimer are deficient in C, H, and 0 by 40 and 25%, respectively. If the dimer is assumed to contain the missing oxygen, then C and H as well as 0 now balance, and observed pressures agree closely with those calculated from the products and methyl ketene gone. Although it was not possible to measure dimer with any accuracy, estimates made are compatible with the above procedure. The oxygen deficiency falls with increasing temperature and vanishes at 542'. This, coupled with the fact that A H is negative for dimerization (inferred from the liquid phase) implies that dimer pressure decreases to very small values a t the top of the temperature range. Carbon and hydrogen deficiencies above 500" were about 40%, reflecting the complex polymerizations of the olefinic products. Inhibition of the Decomposition. Isobutene and nitric oxide were used and the effect on the rates of formation of CO and COz and of disappearance of methyl ketene is shown in Table 111. Rate constants in the presence of isobutene are only quoted to two figures since the
analytical points were more scattered. Both reactions are inhibited, and the average values are 32% for CO and 38% for COz. Table 111: Effect of Isobutene on Rates of Decomposition of Methyl Ketene and Formation of CO and CO? [k (1.'I2 sec-1) Has Been Multiplied by IO] T,
PCHaCHCOv
OC
mm
406.2 406.2 487.2 487.2 502.3 502.3
50 50 50 50 50 50
Pi-C&Hs, mm kCHaCHC0
0 200 0 200 0 200
1.3 1.1 3.5 3.0 5.6 4.8
kco
kCOz
0.14 0.10 0.92 0.59 1.2 0.7
0.42 0.22 1.5 1.1
1.7 1.2
Results with nitric oxide were not reproducible, slight inhibition resulting at first, but later runs being accelerated. Nitric oxide is known to attack carbon surfaces at these temperaturess and this is the probable cause of the effect observed.
Discussion The results show trhatboth carbon dioxide and carbon monoxide are produced by processes of three-halves order which are inhibited by isobutene, and that approximately stoichiometric amounts of pentadiene-2,3 and butene-2, respectively, are formed a t the same time. Reaction rates are independent of surface/volume ratio in carbon-coated vessels but the low Arrhenius constants suggest that the surface is involved.gJO It thus seems probable that the decomposition of methyl ketene is a surface-initiated radical chain reaction, and that heterogeneous termination of chains must predominate, since the rate remains independent of surface/volume ratio. It will be simpler to discuss the mechanisms of COZ and CO formation separately, and then to consider the decomposition as a whole. The Formation of Carbon Dioxide. The fact that pentadiene-2,3 is formed in amounts equal to COz suggests an intermediate state resembling the lactone dimer above. The dimer could be formed first and then decompose, or the transition state could resemble the dimer. However, a simple molecular process of the above type would obey second-order kinetics and be unaffected by inhibitors. The ketene decomposition (8) H. Watts, Trans. Faraday Soc., 54, 93 (1958). (9) 9. Glasstone, K. J. Laidler, and H. Eyring, "The Theory of Rate Processes," McGraw-Hill Book Co., Inc., New York, N. Y.,1941, p 389. (10) P. G. Ashmore, "Catalysis and Inhibition of Chemical Reactions," Butterworth and Co. Ltd., London, 1963,pp 19, 284.
Volume 70,Number 6 M a y 1966
1468
P. G. BLAKEAND K. J. HOLE
was said to be second order, but the results of Young fit an order of 1.5 rather better. I n methyl ketene, a t least, a simple process must be ruled out, but the nature of the products makes it most probable that a cyclic intermediate must play some part in the decomposition. The following mechanism is proposed CH3CH:C0
+ S (surface)
--f
+ S-H
(1)
CH-CH~
(2)
CH&H :CO CH2CH:CO
+ CH8CH:CO
--+
CH~-CH-~
I I
0-c=o CH2CH=C-CH-CHa
+ h~ +CHSCH + CO CHaCH + CH3CHCO +C4H8 + CO CHaCHCO
--+
I I
0-c=o
+ CH2-CH=C=CH--CH, CH2--CH=C=CH-CH3 + CH&H :CO CHzCH :CO + CHaCH=C=CHCH3 2CHzCH:CO + S + (CH2CH: C0)2 + S COz
(3) (4) (5)
If the steady-state approximation is made for the radicals involved, the rate of decomposition is given by -d[CH3CHC01 - - - klS[CH3CHCO] dt
surface are often less than half of the corresponding homogeneous value. The very low value of A (103.4 1,"' mole-"z sec-l) reflects the surface initiation (A1), or the low value of 81/& and also the low probability of reaction 2, the addition of a radical to a carbon-carbon double bond with the formation of a four-membered ring. The Formation of Carbon Monoxide. Both stoichiometry and the measurement of products associate the formation of butene-2 with that-of CO, and the results at 406" conform to the 1 to 2 ratio expected. The formation of butene in the photolysis of methyl ketene by the steps
+
If the chains are long enough for the first term to be neglected
and the evidence for methylene and ethylidene in the pyrolysis of diazomethane and diazoethane11s12suggest that butene may be formed by a similar process in this case. However, the activation energy of the pyrolysis of diazomethane is 35 kcal mole-', which is equal to the heat of the reactionla CH2N2+T H 2
+ Nz
and the corresponding heat for methyl ketene is estimated to be +68 f 5 kcal. This seems to rule out the direct formation of ethylidene, since the activation energy for CO formation is only 24 kcal. The CO pressuretime curve was sigmoid in ketene pyrolysis and it was suggested that CO resulted from the attack of radicals formed by the decomposition of the product allene on ketene. In methyl ketene the maximum rate of formation of CO is at the beginning of the reaction and the above explanation is not tenable. The following chain mechanism is advanced to account for the formation of CO
The mechanism predicts the observed products, order, CH3CH:C0 S (surface) + inhibition, and independence of surface. The surface site8 in initiation need not necessarily be the same as in CH2CH:CO S-H (6) termination. In this event the rate equation above CH2CH:CO --f CH-CH CO (7) must be multiplied by (S1/SS)'/',where SI and Sg are the "concentrations" of the respective sites, and the CHFCH CH3CH:CO + independence of rate on surface/volume is maintained. CH,CH=CHCH2 CO (8) '/2(E1 - Es), The over-all activation energy E = E2 and the A factor equals 2A2(A1/&)1/Por ~ A ~ ( A I / A s ) ~ / ' -CH,CH=CHCH2 CH3CH :CO + (S1/Ss)'" if initiation and termination sites are not the CHzCH: CO CHaCH=CHCHa (9) same. E5 will be near zero and E2, for the radical addition, probably between 3 and 7 kcal/mole, which makes El 20 kcal since E 15 kcal. The initiating (11) B. S. Rabinovitch and D. W. Setzer, J. Am. Chem. Soe., 83, 750 (1961). step suggested is essentially one of hydrogen abstrac(12) R. F. Barrow, T. G. Pearson, and R. H. Purcell, Trans. Faraday tion by a solid free radical and a low value for E1 is SOC.,35, 880 (1939). understar da' le. I n any event the large (negative) (13) G.von Bunau, P. Potzinger, and G. 0. Schenk, Tetrahedron, 21, heats of adsorption mean that activation energies a t the 1293 (1965).
+
+
+
+
+
-
The Journal of Physieal Chemistry
-
+
+
+
THERMAL DECOMPOSITION OF METHYLKETENE
2CH-CH
+S+ CHFCH-CH=CH~
+S
1469
(10)
Steady-state treatment leads to -d [CH&HCO] = 2$( dt
i:)
[CH3CHC0]I/'
if the chains are long, and predicts an over-all activation energy equal to Es l/z(Ee- Elo), and an A factor of 2As(A6/AI0)"'. These values will be considered below. The Decomposition as a Whole. Chain mechanisms have been proposed to account for the COz and CO formation. The decomposition must, however, be regarded as a whole and the effects of cross interactions must be considered. Only one chain-initiating step has been postulated; the termination processes are of the same type and, although cross-termination between the vinyl and CHzClHCO radicals must be equally possible, this should not seriously affect the mechanism. Reactions 4 and 9, the abstraction of hydrogen from methyl ketene by the large olefinic radical, are the same in both cases, and the essential differences between the COZ and CO forming reactions are in steps 2 and 7, and 3 and 8. In (2), the CH2CHCO radical adds to methyl ketene, in (7) it decomposes giving CO. I n (3), the cyclic radical suffers unimolecular decomposit.ion to C02 and in (8) the vinyl radical displaces CO from methyl ketene. The observation that the activation energy for CO is 9.4 kcal higher and the A factor 1 0 2 a 4 higher than for C02
+
must be explained. An exact comparison of the individual expressions for the activation energy cannot now be made since, for example, the addition of CHZCHCO radicals to methyl ketene, which eventually leads to C02, competes with their unimolecular decomposition to CO. However, it seems likely on general grounds that the addition will have the lowest activation energy and A factor of reactions 2, 3, 7, and 8, and that this is responsible for the difference in the Arrhenius equations for COPand CO. Since the kinetics of the steps postulated above are largely unknown and must be inferred from known cases, the suggested mechanisms are tentative in character, but appear to provide the most satisfactory explanation of the experimental results. One general difficulty is that it is not easy to see why homogeneous termination of the chains does not occur, since the radicals seem to be too large to need third body stabilization. The nondependence of rate on surface/volunie ratio and the need for surface initiation suggest that this process cannot be important a t the relatively low pressures studied. Minor Products. The minor products are compatible with the above mechanism, but the complexity of the reactions undergone by olefins around 500" makes a detailed consideration of them unprofitable. Acknowledgment. K. J. H. wishes to thank the Department of Scientific and Industrial Research for a maintenance grant.
Volume 70,Number 6 M a y 1966
