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THERMAL ISOMERIZATION O F C Y C L O P R O P A N E AT
In calculating fT,$ as a function of time, it was necessary to correct for the fact that the dilatometric experiment was made a t 40’ after keeping the sample a t 25’ for 48 days, while the dynamic shear measurements were made a t 38.9’ after 72 days a t room temperature. The srnall differences in the magnitude of the free volume and in the time scale of the isothermal expanslion process arising from these discrepancies were estimated from the thermal expansion coefficients and the data of Fig. 3, and the time scale of the isothermal expansion process a t 40 was corrected using the log aT function to the corresponding time scale at 38.9’ where the shear experiments were performed. After about 50 min., when thermal equilibrium has been reached in the torsion pendulum, the agreement and those calbetween the empirical values of log culated from eq. 7 is excellent, providing quantitative evidence that the shear relaxation times during the isothermal expansion are controlled by the average free volume. At earlier times, where the temperature is changing, thc empirical values are larger than those calculated. The difference shows again an additional temperature effect, which appears immediately upon change of temperature before the time-dependent free volume changes become effective. The magnitude of this instantaneous effect is again about 20% of tlhe total. Reduction Anomaly at Low Temperatures and Long Elapsed Times.-The drop in the magnitude of G’’ a t voluminal equilibrium a t temperatures below 33”, seen in Fig. 10, which reflects a depression in the short-time plateau of H as in Fig. 11,is seen also in Fig. 19 a t long elapsed times. The similarity in behavior suggests that this reduction anomaly is associated with frlee volume alone, just as the relaxation time shift factors are. As a crude test of this hypothesis, the vertical separations of the various curves below the high-ternperature equilibrium composite curve in Fig. 19 have been measured and the total vertical displacement from the latter curve is plotted in Fig. 21 against the fractional free volume. The points for voluminal equilibrium a t various temperatures and for systems in the course of isothermal contraction a t various times a,ll fall on the same curve, so that whatever the nature of the anomaly it appears to be related to free volume alone. The same conclusion can be drawn from the close agreement of the second and third columns in Table I, since a t the lower temperatures the method of reduced variables does not apply and tan 6 is affected
I
161
L O W PRESSURES I
I
I
2 .o
21
22
1
c3
s Q
0.10 -
0.05-
0
1
1.9
1
2.3
f x 102. Fig. 21.-Vertical displacement of log G” in Fig. 19, measured from curve a t voluminal equilibrium above 33.8’, plotted against fractional free volume. Key for temperatures and elapsed tirnes same as in Fig. 18. Crossed points refer to data a t voluminal equilibrium.
by both relaxation time shifts and change in the sha,pe of the relaxation spectrum. The different curves in Fig. 19 correspond to a progressive depression of the plateau in H (Fig. 11) with decrease in free volume. If the retardation spectrum L is calculated, it is found that this also undergoes a progressive drop in magnitude while remaining fairly flat. The symptoms are not those of a 0 mechanism31v32; it is difficult to interpret them without more understanding of the molecular origin of viscoelasticity in the glassy state. Acknowledgments.-This work was supported in part by the National Science Foundation and in part by the Army Research Office. MTeare much indebted to Dr. T. P. Yin and Mr. R. Schmelzer for work on the apparatus, to Mr. P. Mangin for help with some of the experiments, and to Mr. L. R . Shultis for making many calculations. (31) J. D. Ferry, W. C. Child, Jr., R. Zand, D. M. Stern, M. L. Williams, and R. F. Landel, J . Colloid Sci., 12, 327 (1957). (32) J. D. Ferry and K. Ninomiya, In J. T. Bergen, “VisooelasticltyPhenomenological Aspects,” Academic Press, New York, N. Y., 1960, p. 55.
THE THERMAL ISOMERIZATlON OF CYCLOPROPANE AT LOW PRESSURES BY A. D. KENXEIIY A N D H. 0. PRITCHARD Chemistry Department, Universitv of Manchester, Manchester 18, England Received July I S , 1961 The isomerization of cyclopropane has been studied in a 1-1. vessel a t 490” down t o a pressure of 6 x 10-4 mm. At these low pressures, the rate constant becomes h-st order again in cyclopropane, and dependent on surface area, because the molecules are predominantly energized by collisions with the walls instead of collisions with other gas molecules. The activation energy was measured in this low pressure first-order region.
Some years ago, one of us1 collaborated in an attempt to study the isomerization of cyclopropane a t 5 X mm. in a flow Elystem; the experiment failed, apparently (1) H. 0. Pritchard, R. G. Sowden, and A. F. Trotman-Dickenson Disrusszons Faradav SOC.,17, 90 (1954).
because of some kind of streaming phenomenon, such as has recently been observed a t higher pressures by Batten.2 However, with the development of gas chromatography using very high sensitivity ionization (2) J. J. Batten, Australian J . A p p l . Sci., 12, 11 (1961).
-4. D. KENSEDY AND H. 0. PRITCHARD
162 0
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I
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8
42
3 -1.0 3 3
- 1.5
- 2.0 I
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-1.0 -0.0 -t log p (mm.). points taken from ref. 3; 0 , experiments in unpacked vessel; 0 , experiments in packed vessel. -3.0
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- 4.0
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1.30 1.32 1.34 103/~. Fig. 2.-Arrhenius plot for thermal isomerization of Cyclopropane in a packed reaction vessel a t 8 X 10-3 mm. pressure.
detectors, it is now possible to analyze the products of a reaction carried out in a static system a t less than mm. pressure. Our aim in these experiments was to observe the change in kinetics which must occur .\1-hen the mean free path exceeds the size of the reaction vessel and energization becomes principally a wall process instead of a gas phase one. Experimental The apparatus and procedure were similar in principle to experiments performed previously in this Laboratory3 except that a 1-1. spherical Pyrex vessel was used instead of the original 2-1. cylindrical one. The required pressure of cyclopropane was introduced into the reaction vessel by performing a series of expansions on a known quantity of gas: the gas buret in which the cyclopropane was originally measured and the whole series of expansion volumes were fitted with polythene diaphragm valves to reduce any loss of gas by absorption. The reaction vessel itself was isolated from the pumping system by a wide-bore mercury cut-off and a trap cooled in solid COZ to prevent mercury vapor from entering the reaction zone. The sampling section likewise consisted of a set of expansion volumes isolated by polythene diaphragm valves, leading to a 4-ft. chromatographic column which was built into the vacuum line: the column was packed with 207, of molar AgNOt/glycol on firebrick and was operated a t -30". The detector used was the Pye Argon system which is accurate to approximately & l O ~ o for hydrocarbon analyses on the small quantities that were available.
Experiments were carried out in the region of 490' from 7 inm. pressure down to 6 X 10-4 mm. At the higher pressures, the rate constants coincide with those obtained previously3; as the pressure is reduced, the rate constant falls toward the secondorder limit, but before this is reached, the rate constant levels off and the reaction becomes first order again in cyclopropane, as shown by the plot of log k / k , us. log p in Fig. 1. The reaction is essentially first order a t 10-3 mm., when the mean free path is about 7 cm., compared with the diameter of the vessel, which is 13 cm. The measurements then were repeated with the reaction vessel packed. The vessel was completely filled with 169 4-cm. lengths of 10 mm. 0.d. Pyrex tubes (8 mm. i.d.) so that the total surface area was increased from 530 to 4390 cm.2 and the reaction volume fell from 1060 to 870 ml. The reaction now became first order slightly above 10-2 mm. pressure, when the mean free path was about 0.7 cm. compared with the mean free travel of the order of 1 cm. The rate constant for the formation of propylene in the low-pressure first-order region increased by a factor of about 2 . 5 on packing the vessel. The activation energy in the low pressure first order region was measured a t 8 X lO-3mm. in the packed vessel over the temperature range 477417". The use of the packed vessel meant that we could operate at a higher pressure, thus allowing us to make more than one analysis for each run; also, since the rate constant is higher, the length of each run is shortened considerably. The activation energy was found to be 57.2 + 2 kcal. with a frequency factor of 10l1.6sec.-'; the freyuency factor as of course dependent on the geometry of the packing. The Arrhenius plot is shoan in Fig. 2.
Discussion We conclude, for reasons advanced below, that in the lorn pressure first-order region, the nature of the isomerization of cyclopropane is unchanged, the only difference being that energization takes place predominantly on the walls instead of in the gas phase. First, it is necessary to discuss the possibility of some systematic experimental error in these observations. We do not think that there is any significant error in our estimate of the cyclopropane pressure: this was obtained from the original quantity of gas measured out, and the appropriate expansion ratio; me did not find any discrepancy between the amount of gas we assumed went into the reaction vessel, and the amount that was available for analysis afterward. Nor is it likely that the total pressure in the reaction vessel differed much from the cyclopropane pressure. The lowest pressure a t which we could operate was 6 X mm., which is much in excess of the attainable vacuum in the reaction vessel (better than 10-j mm.) ; also the pressure of mercury vapor in the vessel should be well below mm. Nor is there likely to be much error in our estimate of the reaction time. At the low pressures, where the pumping speed is likely to be slow, runs usually lasted about 20 or about 40 hr. so that errors due to slow pumping would be negligible. Even a t the highest temperature of 517", the shortest run was 3 hr., so that even here, no significant error is likely. Two alternative interpretations of the observations are possible, one that we have a mall energized reaction, and the other that we have a true heterogeneous reaction involving an adsorbed species; the third possibility envisaged by Butler4 in a recent paper is ruled out because we never observed any product other than propylene in any of our experiments. We believe that in this reaction, the second postulate is unnecessary because the observations fit the mall energization scheme quite well. In the unpacked vessel, the number of molecule-m7all collisions equals the number of mole-
(3) H. 0. Pritchard, R. G. Sowden, and A. I?. Trotman-Dickenson, Proc. Roy. SOC.(London), 8217, 563 (1952).
VoI. 67
(4) J
N . Butler, J . A m . Chem. Soc., 84, 1393 (1962).
Jan., 1963
FOElnLkTION O F E T H Y L E N E I N
cule-molecule collisions a t about 5 X mm.; the corresponding figure for the packed vessel is about 5 X 10+ mm, These are approximately the pressures where the low pressure first-order regions begin in each case and this is consistent with the interpretation that collisions with the walls are some 3-5 times as efficient as collisions with other cyclopropane molecules. Furthermore, if we regard the walls as acting instead of gas molecules as far as energizatioii is concerned, the temperature coefficient of the reaction shows the expected behavior. The variation in temperature
PHOTOLYSIS OF ETHYL ACETATE
163
coefficient with pressure has been observed for azomethane5 and l,l-dimethylcyclopropane.6 In both cases, as the pressure is reduced, the activation energy falls by about 5 to 7 kcal. and the frequency factor Falls by a factor of lo2 to 104. Our activation energy is 8 f 2 kcal. below the high pressure value, and the frequency factor, although it is to some extent dependent on the surface area, is down by a factor of about l o a to 104. (6) C. Steel and A. F. Trotman-Dickenson, J . Chem. Soc., 976 (1959:l. (6) M. C. Flowers and H. It. Frey, ibid., 1167 (1962).
IXTRAMOLECULAR REARRAKGEMENTS. V. FORMATION OF ETHYLENE I N THE PHOTOLYSIS OF ETHYL ACETATE FROM 4 TO 500°K.1 BY P. AUSLOOSASD RICHARD E. REBBERT Sational 3ureau of Standards, Washington, D. C. Received J u l y 16, 1962 The formation of ethylene in the gas, liquid, and solid phase photolysis of CH,COOCZH, CHsCOOCDzCD211, and CHJCOOC2D5has been investigated a t various wave lengths and temperatures. The intramolecular isotope effect increases with a decrease in temperature in both the gas and liquid phase photolysis. In the liquid phase, an activation energy difference of 1.0 lrcal./mole was obtained for D and H-atom transfer in the photolysis of CHBCOOCD2CD2H. A variation in wave length influences the formation of ethylene in the gas and solid phases but does not affect the intra- and intermolecular ieiotope effects in the liquid phase. A comparison of the product yields in the mercury sensitized decomposition with those obtained in the direct photolysis, as well a8 a study of the effect of biacetyl on the ethylene yield, indicates that the formation of ethylene by an intramolecular rearrangement of the ester molecule excited to an upper triplet state cannot be ruled out.
Introduction It is now well established2that esters containing one or more p-hydrogen atoms in the alkyl group may decompose photochemically by an intra,molecular rearrangement into an olefin and the corresponding acid. In the present work, the gas, liquid, and solid phase photochemical decompositions of CH3COOC2H6, CII3COOCD2CD2H,and CH3COOC2D6have been investigated in order to obta,in a better understanding of the intra- and intermolecular isotope effects on the primary process. The comparison of these results with those obtained on the photolysis of CH3COCH2CRDCD2R,3 and on the pyrolysis of isotopically labeled ethyl a ~ e t a t ewas , ~ expected to lead to interesting correlations. Experimental Apparatus.-The gas phase was photolyzed in a quartz cell (10 cm. in length, 5 cm. in diameter) enclosed in a heavy alunninum furnace provided with double quartz windows. The temperature was automatically controlled within half a degree. The liquid phase was irradiated in a quartz cell (0.05 cm. in depth, 2.5 cm. in diameter) provided with two outlets, one of which was sealed a f k r filling and the other closed by a break seal. The liquid cell was immersed in a Pyrex dewar flask with double quartz windows. Cold ethyl alcohol was the refrigerant for experiments (carriedout from 193 t o 273°K. The solid phase experiments were carried out in a stainless steel low-temperature dewar The ethyl acetate was deposited on a gold plated brass plate, in order t o ensure good thermal conduction. The flow rate for deposition of the sample was
~-
( I ) This research was supported by a grant from the U. S. Public Health Service, Department of Health, Education, a n d Welfare. (2) (a) I'. Ausloos, Can. J . Chem., 36, 383 (1968); (b) P. Ausloos, J . -4% Chem. Soc., 80, 1310 (1958); (0) M. H. J. Wijnen, ibid., Sa, 3034 (1960); (d) R. Borkowski and P. Ausloos, ibid., 83, 1053 (1961). (3) R. P. Borkoivski and P. Ausloos, J . Phys. Chem., 65, 2267 (1961). (4) (a) A. T. Blades and P. W. Gilderson, Can. J . Chem., 38, 1401 (1960); (b) A. T. Blades and P. W. Gilderson, i b i d . , 38, 1407 (1960). ( 5 ) Hofrnan Free Radical Research Dewar, D-1288.
about 2 cc. (STP)/min. I n most experiments about 30 cc. (STP) of gas was depoLitedon the brass plate. In the mercury photosensitized experiments a flat spiral lowpressure mercury arc waa used in conjunction with a Corning filter 7-54. A Hanovia SH-100 lamp was used in all other experiments. The incident wave length range was varied by ulsing Corning filters 9-54 and 7-54, transmitting radiation a t Rave lengths greater than 2150 and 2250 A,, respectively. Because ethyl acetate absorbs mostly below 2350 d., the effectiye wave length regions were roughly 2150-2350 A. and 2250-2350 A. Materials.-Ethyl-d5 acetate and ethyl-& acetate were obtained from Merck, Sharp & Dohme of Canada, Ltd. Mass spectrometric analysis showed that the three batches of ethyl-& acetate used in this work contained about 5% ethyl-& acetate, the remainder consisted of CHaCOOCDzCDzH. It was not gossible t o tell what fraction of the ethyl-& acetate was CH,COOCDzCDHz or CH~COOCHDCDZR;consequently, the results have not been corrected for the presence of this impurity. At any rate, this correction would only be a minor one and certainly would not affect any of the conclusions drawn in this paper. The ethyl-d5 acetate batch contained 2.5% ethyl-d., acetate. Chromatographic analysis indicated that after a trap-to-trap distillation a t --80°, the total chemical impurities amounted to not more than lye, ethanol being the major impurity. CH8COOC2H6was obtained from Eastman Kodak Co. After distillation the impurities did not exceed 0.1%. Analysis.-The analytical system consisted of a solid nitrogen trap, a modified Ward still, an automatic Toepler pump, antd a Toepler gas buret. The carbon monoxide-methane-hydrogen fraction was removed a t -210'. Ethylene and ethane subsequently were distilled off a t -180'. Propane and carbon A11 fractions were analyzed dioxide were removed a t -150'. mass spectrometrically using a Consolidated mass spectrometer Model 21-101. Standard samples of CzD4 and C2D3H obtained from Merck, Sharp & Dohme were run on the same mass spectrometer in order to calculate the isotopic distribution of the ethylenes.
Results In order to study the relative importance of a H atom t o a D atom transfer in the intramolecular rearrangements of the labeled ethyl acetates, ethylene
