PHOTOLYSIS AND PYROLYSIS O F ACETALDEHYDE
949
observed activation energy. These observations could easily be tested by using high concentrations of rare gas in the photolysis and seeing if the activation energies remain unchanged. I t is also quite conceivable that a part of the small amount of methane which is produced at 60°C. and lower is produced by the reaction
+
+
+
CHaCO CHaCOCHa --t CHd CO CHzCOCHs (i.e., some of the acetyl acting as “active” methyl radicals). This might then explain the ‘‘anomaly” observed by Noyes in the activation energies for methane production a t low and high temperatures. This would not necessarily do damage to the relation which he has found for oc,,/Qi’&a, since a t the lower temperatures where the acetyl radical concentration is highest, its ratio to the concentration of methyl radicals might well be such as to permit a small contribution to the production of methane and yet maintain the form of the ratio constant.
M. SZWARC (University of Manchester, Manchester, England) added the following comment in hhe proofs: I t seems that Dr. Laidler misunderstood my expression “normal.” The term has therefore been explained fully in footnote 9 of my paper. FREE RADICALS I N THE PHOTOLYSIS AND PYROLYSIS OF ACETALDEHYDE’,’ PAUL D . ZEMANY AND MILTON BURTONa
Research Laboratory of the General Electric Company, Schenectady, New York, and Department o j Chemistry, New York University, University Heights, New York City Receiued August 10, 1960 INTRODUCTION
The Rice-Herzfeld theory of free-radical chain reaction (14) accounted both for the observed order and for the activation energy of the pyrolysis of acetaldehyde (11) by the folloming mechanism:
+ HCO + CHIC0 CH3 + CO
CHICHO --t CH3
R
+ CHSCHO
-+ R H
CH3CO -+
R
* Presented
+ R + chain end
(1)
(2) (3) (4)
before the Symposium on Anomalies in Reaction Kinetics which was held under the auspices of the Division of Physical and Inorganic Chemistry and the Minneapolis Section of the American Chemical Society a t the University of Minnesota, June 19-21,1950. ’ This paper is ahstracted from a thesis submitted by Paul D. Zemany to the Faculty of the Graduate School of Kew York University in partial fulfillment of the requirements for the degree of Doctor of Philosophy;June, 1950. a Present address: Department of Chemistry, University of Notre Dame, Notre Dame, Indiana.
950
PAUL D. ZEMANY AND MILTON BURTON
F. 0. Rice and coworkers had used the Paneth mirror technique (12) to show the production of free radicals in the pyrolysis at 743°C. Others who found such mirror evidence included Pearson and Purcell (10) and Letort (3). Allen and Sickman tested the assumption that methyl radicals propagated the chains by a demonstration that such radicals resultant from the pyrolysis of aaomethane induced the decomposition of acetaldehyde at 300°C. ( l ) , Le., a t a temperature 150°C. lower than the minimum temperature a t which the pyrolysis of acetaldehyde had hitherto been observed. An objection offered to the major significance of free radicals in the pyrolysis of acetaldehyde was that actual observations of free-radical production related to temperatures 200-300"C. above those of the kinetic studies. Patat and Sachsse ueed the para-ortho-hydrogen conversion (9) in an effort to establish the degree of contribution of the free-radical chain mechanism to the overall reaction a t 550'C. They found clear evidence for the production of free radicals but concluded that it was only about & of that required to account for the observed rate. In consequence of these various results, considerable doubt existed that methyl radicals were present a t all a t the lowest temperatures a t which the pyrolysis of acetaldehyde had been studied. To test this point Burton, Ricci, and Davis (2) extended the lower limit of observation of methyl radicals to 475°C. by experiments with radiolead mirrors. .4t practically the same time, Staveley and Hinshelwood (17) concluded from studies of the pyrolysis of acetaldehyde involving nitric oxide that no significant portion of the pyrolysis went by a free-radical mechanism at the lower temperatures of study. They suggested that the so-called 3/2 order of the reaction generally assumed was in reality a first order, changing to second order, and that the kinetics were adequately explained in terms of a single rearrangement process CHsCHO -+ CHI f CO
(1')
involving about eighteen square-terms. I t was found that propylene did inhibit the decomposition of acetaldehyde; later Smith and Hinshelwood (16) reinvestigated the effect of nitric oxide and found that the inhibition had been masked by its catalytic effect. The earlier view of Hinshelwood shortly received substantial support from some work of Morris (7), Tvho studied the pyrolysis of mixtures of carefully pwified acetaldehyde and acetaldehyde-dc (stored over hydroquinone) and wits unable to find any significant quantity of methane-d or of methane-&, both of which would have been expected if the mechanism were according to the RiceHersfeld scheme of reactions 1 to 4. Morris suggested as one conclusion from his result that Burton, Ricci, and Davis had found radicals which resulted perhaps from a side reaction involving a trace of oxygen in their reactant (cf. Letort (4)). Since the lead-mirror technique, very difficult at best, is insensitive in the presence of oxygen, Taylor and Burton (19) suggested the porsibility of a
PHOTOLYSIS AND PYROLYSIS OF ACETALDEHYDE
951
free-radical chain involving an induced internal conversion in one elementary SkP
R
+ CHJCHO
+
R
+ C K + CO
which might apply also in the photolysis. The work here reported was intended to establish, by studies both of the photolysis and of the pyrolysis of acetaldehyde, the degree of free-radical production, the fractional contribution of free-radical chains to the overall rate, and the nature of the free-radical chain mechanism. The procedure involved continuous mass-spectrometric analysis of samples of mixtures of acetaldehyde and acetaldehyde-d4 under pyrolytic and photolytic conditions and continuous determination of relative concentrations of the various possible methanes as well as other products. A t this point we may state that the results unequivocally show kinetically significant production of free radicals in the pyrolysis of acetaldehyde. EXPERIlldENTAL Matffials
Acetaldehyde (City Chemical Company) was distilled under nitrogen, transferred to a vacuum system, degassed, and then distilled several times from frozen carbon tetrachloride (-23OC.) to carbon dioxide-acetone (-78"C.), and from there to frozen acetone (-95'C.), using middle fractions in each case. It was not cooled below its freezing point (- 123.5"C.) because it tends to polymerize on freezing ( 5 ) . For many of the runs it was prepared by the depolymerization of paraldehyde by sulfuric acid, and purified as described above. Acetaldehyde-d4 was synthesized from deuteroacetylene, itself made according to the method of Lind, Jungers, and Schifflett (6). A good sample of baked calcium carbide was treated with deuterium oxide (99.87 per cent purity, obtained from the Stewart Oxygen Company), procured through the Atomic Energy Commission. Mass-spectrometric analysis of the resultant deuteroacetylene showed an H:D ratio of 0.0035 as determined by peak heights. After purification the deuteroacetylene was converted to acetaldehyde-d4 by the method of Zanetti and Sickman (22). The acetaldehyde finally produced had an H:D ratio of 0.0031. The latter was purified in the same way as acetaldehyde. Both were stored in evacuated bulbs. Two different samples of Eastman Kodak Company White Label hydroquinone were used without further purification. The gases used for obtaining reference mass spectra, e.g., methane, ethane, and carbon monoxide, were obtained from the Matheson Company. Apparatus All analyses were done on a mass spectrometer. The spectrometer was a precursor t o the General Electric Analytical Mass Spectrometer modified to permit continuous magnetic scanning and recording of the various masses.
952
PAUL D. ZEMANY AND MILTON BURTON
The reaction cell itself (figure 1) contained the leak to the mass spectrometer for most of the runs. The temperature was maintained to &3"C. by a single setting of ri variac connected to a nichrome heating element. At about 400°C. thermocouples 10 and 12 (figure 1) measured a temperature about 3' lower than did thermocouple 11. Two reaction cells were used. The second, however, had a glass capillary in place of the Fernico cup. The rate at which the sample was withdrawn through the faster of the two leaks was less than 0.2 per cent per hour. Most runs were completed within a few hours, so that this loss did not affect interpretation of
I!
SPECTROMETER
SAMPLE SYSTEM
FIG.1. Reaction cell-leak. 1, quartz window; 2, Fernico cup leak made by punching a small hole through the thin metal, peening shut, then etching t o the desired size (this was devised by Dr. F. J. Norton of this Laboratory); 3, Pyrex body of leak; 4 , aluminum foil wrapping; 5, asbestos wrapping; 6, nichrome heating element; 7, asbestos wrapping; 8 , thick layer of thermal insulation; 9, graded seal t o Pyrex; 10, 11, a n d 12, thermocouples between layers 4 and 5.
results. The volume of the cells was about 50 ml. In a typical run the reaction cell was heated to the desired temperature before introduction of the reaction mixture. After a few minutes the pressure was noted, the stopcock closed, and the strip chart recorder on the mass spectrometer immediately started. This time was considered the start of the run. The recorder ran continuously at constant speed, and thus provided a time base for the various data recorded. Generally the range of masses 12 to 50 or higher was swept at 10- to 15-min. intervals, a frequency sufficient to insure detection, identification, and measurement (23) of any compound present a t any time in quantities above the minimum detectable quantity for that particular compound (Le., about 0.1 per cent for a typical substance in an uncomplicated case). Quantities of each component used
PHOTOLYSIS A S D PYROLYSIS OF ACETALDEHYDE
953
were estimated by comparison of peak heights after calibration runs on pure substances. Following the suggestion of J. Turkevich and coworkers (20) the assumption \vas made that the parent peaks of isotopic isomers had nearly equal sensitivities. I t was necessary to compare peak heights ( 2 4 , rather than to calculate directly the partial pressure, because of change of sensitivity with temperature as well as for other reasons. The initial pressure of reactant was noted. Since the total amount of material in the reaction cell did not change appreciably during the run, comparison of peak height ratios sufficed for analysis. As a check on the possibility of important side reactions involving oxygen as an impurity, after the reaction cell and its contents were closed off from the rest of the system, the gas in the sample system was condensed by liquid nitrogen and any residual gas noted and analyzed mass-spectrometrically through a leak in parallel with the reaction cell. When a small amount of oxygen mas deliberately added to the acetaldehyde, it could be separated by this procedure. In a typical case with the reaction cell filled a t 15 mm. the noncondensable gases amounted to 3 microns and consisted of carbon monoxide and methane. An micron of oxygen calibration showed that under these conditions 0.67 X oxygen would have been detected; i.e., the oxygen present did not exceed 4.5 X 10P mole fraction. In no case was any oxygen detected in the materials employed in the runs. A complication in the estimation of the composition was the adsorption of acetaldehyde in the mass spectrometer. This adsorption caused the response of the spectrometer to lag behind a change in the partial pressure of acetaldehyde on the leak; heating of the lines and the spectrometer, which would presumably minimize this effect, was not convenient. Holyever, it was possible in most cases to estimate the approsimate value of the partial pressures. The principal aim of this technique was to determine whether mixed methanes were found in the decompositions. An accuracy of 5-10 per cent ~ v n sconsidered adequate and was readily attained in the measurements. The light source, a General Electric H-4 lamp with jacket removed, was placed so as to shine through the quartz window of the reaction cell. The H-4 lamp is a self-reirersed mercury arc which gives most of its energy at longer wave lengths. A fen runs were conducted in a more conventional manner, by filling a vessel with the reactants and then sampling the products. Much information can be obtained from a single run. In the following discussion of results several typical runs will be described in detail. RESULTS AiYD DISCCSSIOX
1. Photolysis at 140°C. Three photolyses were carried out with the reaction cell at 140°C. under comparable conditions of illumination and other factors. The intensity of illumination was varied by change of position of the H-4 lamp. Satisfactory constancy in intensity of the lamp was verified by monitoring with a photocell.
954
PAUL D. ZEMANY AND MILTON BURTON
( a ) Acetaldehyde: The cell was filled to a pressure of 6.20 mm. with acetaldehyde. The cell was closed off from the sample system after about 2 or 3 min., and the illumination and recorder started simultaneously. The mass range 12 to 50 was scanned repeatedly a t intervals of 8-10 min., with an occasional scan to higher masses, to check the presence of materials of higher molecular weight. From the chart one could, at the conclusion of the run, measure peak heights and times. Some of the data are plotted in figure 2. Figure 3 shows the corresponding partial pressures of the various compounds. The initial point of the acetaldehyde peak height in figure 2 is an illustration of the adsorption effect mentioned previously. I t lies below the peak height curve extrapolated back to
2509
2000
f
1500
x
I IO00
500
I
I
20
40
I
60' Minulrs
, 80
I
0
FIG.2. Peak heights observed on maas spectrometer in 140'C. photolysis of 6.20 mm. of acetaldehyde. zero time. The other substances involved do not show this effect. Figure 3 is corrected for known deviations of this type. It shows expected increase in carbon monoxide and also the methane:carbon monoxide ratio less than unity found by other investigators. This method of analysis now shows that this low ratio is the result of simultaneous production of ethane, which accounts for the difference between carbon monoxide and methane. The hydrogen yield was not directly measured but is presumably very much like that of ethane (cf. the next section). The ratio CH4:CO:Htis then about the same as that reported by other workers, who used more conventional methods of gas analysis. (b) Aceta2dehyde-d.: In similar fashion 7.13 mm. of acetaldehyde44 was photolyzed. Figure 4 (analogous to figure 3) summarizes the variation of partial
PHOTOLYSIS A S D PYROLYSIS O F ACETALDEHYDE
955
pressure values, calculated from the corresponding peak height data. We might mention incidentally the production of peak heights at masses 50 and 5 2 , corresponding t o less than 0.1 per cent of the total pressure at the end of the run. The 5 2 peak could be the parent one of C2DsO or C8Ds.An attempt a t identification by fractionation and concentration vas unsuccessful. The parent peak
Minutes
FIG.3. Pressures of various components in the 140°C. photolysis of acetaldehyde calculated from mass spectra.
Minufei
FIQ.4 . Pressures of acetaldehyde-& and products resulting from its photolysis a t 140°C.
for acetone-ds was also noted, along with its contribution to the 46 peak. The amount produced was roughly 0.1 per cent of carbon monoxide. This experiment shows a lower rate of photolysis under comparable conditions than was found for the acetaldehyde and also a greater relative yield of ethaneds compared to ethane in the acetaldehyde case. This result is consistent with a very naive suggestion of a higher activation energy of the chain-continuing step (reaction 2 ) resulting from the lower zero-point energy of the acetaldehyde-& molecule, and with the shorter chain length that would also occur.
956
PAUL D. ZEMANY AXD MILTON BURTON
(c) A third run at 140°C.was on a mixture of equal parts of acetaldehyde and acetaldehyde-& a t a total pressure of 5.80 mm. Figure 5 gives pressure as a function of time for reactants and products. 4 t about the midpoint of the run the ratio of peak heights (roughly proportional to partial pressure) for Hz, HD, and DI was 73:18:56. Even without calibration this result demonstrates a substantial excess of HZand Dz over HD, resultant undoubtedly from intramolecular rearrangement and decomposition into ultimate molecules in a single elementary act. This result merely demonstrates that not all the hydrogen comes from free formyl but that some of it may come from a rearrangement process which does not contribute significantly to
x I -
I
20
30
40
50
I
I
I
I
60
70
80
90
Minutes
FIG.5. Pressures obtained for some of the coniponents in the photolysis of a mixture of acetaldehyde-dd and acetaldehyde at 140°C.
the overall effect. Even under these extreme conditions (lorn pressure and high light intensity) the yield of hydrogen was less than 5 per cent that of the acetaldehyde decomposed. Since this experiment was not especially pertinent to this work, it was not repeated. I t does indicate that hydrogen and ethane yields may not be exactly the same. The following points are worthy of particular note. The rate of photolysis of acetaldehyde-& in the mixture is increased and that of photolysis of acetaldehyde decreased compared with that observed for the separate pure reactants under roughly comparable conditions, including total energy absorption. The ratio of methane-d3 to methane-& for the photolysis under these conditions was about 1.6. This result is consonant with the interpretation that the reaction between CDI and CHBCHO has a lower activation energy than the reaction betveen CDI amd CD3CDO.
PHOTOLYSIS AND PYROLYSIS O F ACETALDEHYDE
957
Although ethane-de, ethane, and ethane-da were found, negligible amounts of ethane-& and ethane-d4 were produced. I t is presumed that the other mixed ethanes were missing, but the spectrum in their region could not be interpreted because the spectrum of ethane-ds was not known. 2. Photolysis ut 290°C.
Only one experiment on photolysis at higher temperature is here recorded. It was conducted on acetaldehyde-& at 290OC. according to the technique al-
2c 2 e
.-.-E
I 0)
2n IO t
a
0 40
80
120
I60
Minutes
FIG.6. Photolysis of acetaldehyde-& at 2oo°C., showing the effect of a 17-hr. interruption in the illumination. The discontinuity in the lines showing the amounts of product is due largely t o the fact that cracking patterns in the m a s spectrometer were measured before the instrument had been allowed to warm up sufficiently after the interruption in illumination.
ready set forth. I t is included primarily not because it shows anything new or unexpected but because it shows substantial absence of effect of interruption of the photolysis (see figure 6) and because in this case the ethane-& fraction, was very carefully identified by accurate measurement of the mass of the CzD4 fragment. After the run, ethane-ds was separated by isolation of the first fraction of gas evolved from material condensed in liquid nitrogen. This fraction was mixed with air and the mass spectrum obtained. The distance, on the chart, between C2DSand C2D4fragments (mass difference = 2) was 21.8 cm. The 32 peak (which includes oxygen) was nicely resolved into a doublet (8) with a
958
PAUL D. ZEM.4NY AND MILTON BURTON
separation of 7.2 mm., corresponding to a mass difference of 0.065. The peak which thus had a mass of 32.065 was ascribed to C2Dd. Using mas3 packing fractions, CzDd has a weight of 32.06647 mass units. 3. Pyrolysis ut 510OC. Pyrolyses were conducted like the photolyses in the rezction vessel shown in figure 1. The technique was exactly the same except that no illumination was used. In a typical run an equimolar mixture (&0.5 per cent) of acetaldehyde and acetaldehyde-d4 \vas introduced at a pressure of 15.2 mm. into the reaction vessel preheated at 510°C. and the spectrophotometric analysis was imme-
0
40
60
Minutes
FIG. 7. Pyrolysis of a mixture of acetaldehyde and acetaldehyde-& a t 520°C. The amounts of total CDO and CHO compounds are given rather than CHICHO and CDaCDO because of the isotopic exchange which occurs. The dotted line represents an estimate of the acetaldehyde-& pressure, which decreases because of both isotopic change and pyrolysis.
diately begun. Figure 7 shows the results of one such run. The principal features are as follows. The acetaldehyde pyrolyzes about 1.G times as rapidly as acetaldehyde-& in a mixture of the two gases. This result is in agreement with that of J. R. E. Smith (15), who found the ratio to be 2.5 for the pure separate reactants.' Since the pyrolyses of both components in our case involve chains in which both CHJ and CDa participate, no closer agreement can be expected. The relative rates are, in a naive sense, related to the lower zero-point energy of the deutero compound and the consequent higher activation energy for removal of the deuterium atom. Complexity of the mass spectrum and lack of knowledge of the mass spectra of the mixed isotope aldehydes in the pertinent region forbade direct observation of yield of methane. Information regarding that compound can be inferred Note that Morris (7) later found the value to be 1.3-1.4
PHOTOLYSIS .4ND PYROLYSIS OF ACETALDEHYDE
959
only from other data. On the other hand, figure 7 clearly shows that m:thsned, and methane-d4 are formed in the ratio 1.2: 1. This result is in agreement with
the hypothesis that most methane is formed via radicals, Le., by reactions analogous to reactions 1 and 2. The difference of the ratio from unity is to be attributed to a slightly lower activation energy for rzaction 2a as compared with reaction 2b
+ CHJCHO -+ CDSH + CHaCO CD3 + CDBCDO-+ CD4 + CDSCO CD3
(24 (2b)
as our uncritical notions of the higher zero-point energy of acetaldehyde would indicate. However, the fact that the ratio of methane-d, to methane-& is not as high as in the photolysis may indicate some reaction by a direct split to ultimate molecules. In this case it would amount to about 15 per cent of total reaction, if it is assumed that the ratio of methane-& and methane-d4 produced by the free-radical split is constant (see also Section 5 , below). Isotopic isomerization occurs simultaneously with pyrolysis and at a very similar rate. Since formyl radicals disappear by reaction 2 the most probable source of this isomerization is the hitherto undiscussed reaction
CD,
+ CHsCHO -+
+ CHa
(5a)
CH3
+ CD,CDO+CHZCDO + CD3
(5b)
CDaCHO
and its analog which probably occurs by a simple inversion process of relatively low activation energy. The conclusion as to low activation energy is obtained by comparison of the initial yields of the mixed aldehydes with those of the mixed methanes. The latter are formed at a comparable rate. Since reaction 2 is a lowactivation-energy process, reaction 5 must likewise be. Both CD3CH0 and CH3CDO pyrolyze (and may rearrange) at characteristic rates which confuse any attempt at establishment of rate constants from the data of this run. We should note explicitly that the loir initial concentration of mixed-isotope aldehydes forbids the conclusion that the mixed methanes are initially Formed from such compounds. They can be formed only by the chain mechanism.
4. Pyrolysis of pure acetaldehyde The objection can be offered to the interpretation of the previous section that the results are true but attributable to the presence of an impurity, such as oxygen. In order to avoid such a possibility, Morris incorporated hydroquinone in his aldehydes and in that way reduced the specific rate of the pyrolysis to a value approaching that of the propylene-inhibited rate. To test that possibility in this work several runs were made with pure acetaldehyde. At 663 mm. pressure, the half-time was 220 min. at -185°C. These results are comparable to those reported by Smith and Hinshelwood (16) and by Letort f o r t h e uninhibited reaction.
960
PAUL D. ZEMhNY AND MILTON BURTON
We may note that we do not attempt a direct comparison with the r e d t i of Morris. The latter held that the only function of the hydroquinone was to remove oxygen and suggested by implication that it had no other inhibiting effect. An examination of the hydroquinone me employed in a test of Morris' work showed that it contained certain low-boiling impurities. When these were removed, the hydroquinone exhibited a residual vapor pressure of 10 microns a t 8°C. measured by an effusion method (21). This is the order of magnitude that one would expect by extrapolation of higher temperature data (18). We found that acetaldehyde which had been treated with hydroquinone and then distilled twice before pyrolysis did not behave differently from the untreated material in pyrolytic experiments, as determined by production of equal amounts of noncondeneable products (carbon monoxide plus methane) with time. However, deliberate addition of hydroquinone to the reacting acetaldehyde did reduce its rate of decomposition. In duplicate runs at 5OO"C., 13 mm. of acetrldehyde decomposed at about twice the rate of acetaldehyde and hydroquinone. The hydroquinone was introduced at its \rapor pressure at room temperature (30°C.). Hydroquinone did not show any appreciable effect in the photolysis a t the lower temperature. Because the chain length in the photolysis may be several orders of magnitude shorter than in the pyrolysis and so many more radicals are produced in the photolysis, it is quite likely that the meager supply of hydroquinone was depleted by the radicals initially produced, after which time the reaction proceeded at its normal rate. Hydroquinone is tln excellent chain stopper and it is very probable that it or some impurity in it mas acting as an inhibitor in the pyrolysis. 5. Pyrolysis of acetaldehyde-d, and acetaldehyde at 465'C.
In another pyrolysis a t 465OC. (figure 8) the ratio of methane-da to methaned, mas initially about 1.0. Again using the hypothesis that the ratio observed in the photolysis holds for the chain reaction, 25 per cent of the decomposition was by a direct molecular split, i.e., somewhat more than at the higher temperature. It would seem that as the temperature of pyrolysis is increased the fraction of decomposition via a free-radical chain also increases. These calculations are based on the assumption that the difference in activation energies of the controlling reactions is sufficiently small for the ratio of methaneda and methanedl formed by the free-radical mechanism to be substantially insensitive to temperature. However, it must be emphasized that our conclusion regarding the reality of the rearrangement decomposition is not affected by this assumption. Substantially, we find that the ratios of methane-d to methane-& at various temperatures are roughly as follows: -
TEYPEPATUPL
Photolysis at 140°C.. ....................................... Pyrolysis at 465°C.. ........................................ Pyrolysis at 51O0C.. ........................................
CHS:CDd
1.6 1 .o 1.2
~-
96 1
PHOTOLYSIS AtiD PYROLYSIS O F ACETALDEHYDE
The minimum at 4G5OC. requires the conclusion that at least part of the decomposition at 465°C. is uiu an ultimate molecule mechanism. The particular value of 25 per cent may be too high. Furthermore, this conclusion does not suggest that the mechanism is by rearrangement in the gas phase. I t may occur on the walls of the vessel. On this point we have no evidence. S o t shown on the figure is the ratio of acetaldehyde-d4 pyrolyzed to ethane-& formed. That ratio \\-as found to be of the order of 2000. Analyses were not conducted sufficiently accurately to permit a more exact statement. The ethaneI .o
0.9 0.8
0.? 0.6
0.5 $4 l Lo a
;3-
-
/ '0
FIG.S. 1'1-rolysis of rz mixture of acetaldehyde-& and acetaldehyde a t 465°C. The ordinate a t the right applies to curves I and 11. Curve I shows the fraction of depletion of acetaldehyde-dr resulting from forniation of CDsCHO. Curve I1 gives the total fractional depletion of acetaldehyde-dr by all isotope exchange processes. The difference between I and I1 is largely attributable to formation of C2D2H20.
Cis can come only from CDa radicals. In the limit two CD, radicals are required
for each C2D6. Thus, since the total yield of methane-& and methane-& formed approximates the amount of acetaldehyde-d4 decomposed, it follows that the chain length of processes including steps analogous to reactions 2 and 3 is of the order of lo00 at a minimum. The yield of ethane decreased as temperature or pressure increased in both the photolysis and pyrolysis. In the pyrolysis at 600 mm. it was not detected. The rate of formation of CD3CHO and C2DIHZO from CDZCDO is also shown in figure 8. I t is evident that C?D2H20is not produced by a primary reaction.
962
PAUL D. ZEMANY AND MILTON BURTON
I t piobably is formed by a second inversion (reaction 5a) after internal rearrangement of CDSCHO to CDzHCDO. Several assumptions had to be made to calculate the rate of formation of C2DzH20. First, it was assumed that the parent peak of isotopic isomers of acetaldehyde had the same sensitivity. Second, since the ratio of the 47 peak (parent for CDsCHO) to the residual 48 peak after subtracting the contribution of CDaCDO was initially constant, it was assumed that no CD~HZO was initially formed, and that the ratio observed was the ratio for 46 to 47 from CzDsHO. Later in the reaction the 48 peak increased above that required for CD&DO and CD3CH0,and the excess was ascribed to C1D2H20. Because ,it can also be formed by a similar series of reactions from CHaCHO, only half of the amount observed originates from CD3CD0 and is so depicted. This is also in accord with the observation that CD2H2is not initially observed, but does occur after the reaction has progressed somewhat. CONCLUSION
The results of this study are consistent with the hypothesis that the pyrolysis of acetaldehyde proceeds largely by a free-radical chain of the Rice-Herzfeld type. Apparently, in addition to the Rice-Herzfeld chain-propagation step there also is a reaction involving exchange of free alkyl radicals and the alkyl radical of the acetaldehyde molecule. This process is of activation energy nearly as low as that of the chain-propagation step and has no effect on the steady-state concentration of alkyl radicals. There is also a direct split into ultimate molecules by a reaction which may make an appreciable contribution a t the lowest temperature, but becomes of minor importance as the temperature is increased. SUMMARY
1. The photolysis (with the fully reversed mercury lamp) and pyrolysis of acetaldehyde have been studied continuously by a mass-spectrometric technique involving the use of isotopic hydrogen. 2. The photolysis of acetaldehyde at 140°C. yields methane and carbon monoxide in a ratio slightly less than unity. The difference is ascribed to ethane simultaneously produced. It is inferred that an amount of hydrogen roughly equivalent to ethane is also formed. 3. The photolysis of acetaldehyde-& under similar conditions gives similar results but a t a lower rate explicable on the basis of the difference in aero-point energies of the two aldehydes. 4. The photolysis of mixed acetaldehyde and acetaldehyde4 yields the expected mixture of products. However, the ratio of H2, HD, and DZ produced (in relatively small yield) indicates that a small fraction of the reaction proceeds concurrently by rearrangement in a single elementary act, which does not add significantly to the total effect. 5. In the pyrolysis of mixtures of acetaldehyde and acetaldehyde4 primarily produced free radicals apparently play an important part. Production of methane-dr and methane-& was directly shown by mass-spectrometric techniques and the formation of methane and methane-d may be inferred. By comparison of the yield of methanes with the yield of ethane-de it is estimated that the
4
PHOTOLYSIS AND PYROLYSIS OF ACETALDEHYDE
963
chain length for the Rice-Herzfeld pyrolysis of acetaldehyde is of the order of IO00 a t 465°C. under the experimental conditions. 6. The free-radical chain reaction in the pyrolysis of acetaldehyde includes a low-activationenergy, inversion-type isotopic isomerization, which does not affect the steady-state concentration of radicals. 7. The initial rate of pyrolysis of acetaldehyde under the conditions of these experiments is comparable t o the results of previous workers on the uninhibited reaction. It was shown that the oxygen present was below a level which could make a significant contribution. 8. Hydroquinone vapor can act as an inhibitor to the pyrolysis, but not in the photolysis, because of the relatively shorter chain lengths in the photolysis. Storage over hydroquinone does not affect the rate of pyrolysis. 9. There is a concurrent pyrolytic split of acetaldehyde into ultimste molecules. This reaction makes a measurable contribution at the lower temperatures of pyrolysis, but becomes of minor importance as the temperature is raised. The authors wish to express their appreciation to the General Electric Company for u6e of the facilities of their Research Laboratories in the completion of this investigation. They also acknowledge with thanks the advice and assistance of Dr. F. J. Norton, particularly with problems involving employment of the mass spectrometer. REFERENCES (1) ALLEN,A. O., AND SICKMAN, D. V.: J. Am. Chem. SOC.66, 1251, 2031 (1934). (2) BURTON,M., RICCI,J. E., AND DAVIS,T. W.: J. Am. Chem. SOC.62, 265 (1940). (3) LETORT,M.: Thesis, Paris, 1937. Thia reference is given by E. W. R. Steacie in Atomic andFreeRadicaZ Reactions, p. 129, Reinhold Publishing Corporation, NewYork (1946). (4) LETORT,M.: J. Chem. Phys. 34, 428 (1937). (5) LETORT,M., AND DUVAL,X.: Compt. rend. 216, 608 (1943). (6) LIND,5. C., JUNOERS, J. C., AND SCHIFFLETT, C. H.: J. Am. Chem. SOC.67,1032 (1935). (7) MORRIS,J. C.: J. Am. Chem. SOC.63, 2535 (1941); 66,584 (1944). (8) NORTON,F. J.: Phys. Rev. 76, 1957 (1949). F., AND SACHSSE, H.: Naturwissenschaften 23, 247 (1935). (9) PATAT, (10) PEARSON, T. G.,AND PURCELL,R. H.: J. Chem. SOC. 1936, 1151. K.F.: J. Am. Chem. SOC.66, 284 (1934). (11) RICE, F. O., AND HERZFELD, W. R., AND EVERINO, B. L.: J. Am. Chem. SOC.64,3529 (1932). (12) RICE,F. O., JOHNSTOX, (13) RICE,F. O., AND POLLY,0. L.: J. Chem. Phys. 6, 273 (1938). (14) RICE, F. O., AND RICE,0. K.: The Atiphatic Free Radicals. The Johns Hopkine University Press, Baltimore (1935). (15) SMITH,J. R. E.: Trans. Faraday SOC.36, 1328 (1939). (16) SMITH,J. R. E., AND HINSHELWOOD, C. N.: Proc. Roy. SOC.(London) AlBO, 237 (1942). L. A. K., A N D HINSHELWOOD, C. K.:J. Chem. SOC.1036, 812; Nature 137, (17) STAVELEY, 29 (1936). (18) STULL,D. R . : Ind. Eng. Chem. 39, 517 (1947). H. A., AND BURTON, M.: J. Chem. Phys. 7, 414 (1939). (19) TAYLOR, (20) TURPEVICH, J., FRIEDMAN, L., SOLOMON, E., A N D WRIOATSON, F . M.: J. Am. Chem SOC.70, 2638 (1948). (21) VERHOCK, F. H., AND MARSHALL, A. L.: J. Am. Chem. SOC.61, 2737 (1939). D . V.: J. Am. Chem. SOC.68, 2034 (1936). (22) ZANETTI,J. E., AND SICKMAN, P. D.: Anal. Chem., in press. (23) ZEMANY, (24) ZEMANY,P. D., AND PRICE,F. P.: J. Am. Chem. SOC.70, 4222 (1948).
964
DISCUSSION
DISCUSSION
E. R. BELL(Shell Development Company): In support of Zemany and Burton’s proposed inversion CHSCHO
+ CD3 e CD3CHO + CH3
(1)
there might be cited the work of Rust, Seubold, and Vaughan (J. Am. Chem. SOC.70, 4253 (19$8)), who demonstrated that secondary alcohols are formed in the free-radical decomposition of butyraldehyde and heptaldehyde in the liquid phase a t 115-13OoC. Presumably this involves the step H
R
+ R’CHO + RCR’
(2)
0
the oxy radical subsequently abstracting a hydrogen atom from the aldehyde. The reverse of reaction 2 has been demonstrated by these same authors (J. Am. Chem. SOC.72, 338 (1950)): namely, that secondary alkyloxy radicals in the vapor phase at 195%. decompose to alkyl radicals and aldehydes. It seems improbable, however, that the simple inversion should be a generally occurring process. For example, whereas reaction 1 might proceed without complications, a reaction such as the one given below might tend to give another set of products after forming a similar intermediate : CH3 H H -CH3
+ (CHa)*CHCHO e-
\I
I
C-C-CH,
’A
CH3
CHaCHO
+
I
CH3CCHa
