THE THERMAL DECOMPOSITION O F TETRAMETHYLLEAD* BY J. H. SIMONS, R. W. McNAMEE, AND CHARLES D. HURD
Several noteworthy experiments on the pyrolysis of metal alkyls have been announced in recent, years. That of Paneth and Hofeditz' is best known since these investigators found that some active body, capable of removing lead mirrors, was produced as tetramethyllead was heated. Paneth assumed that the active body was free methyl radical. The identit,y of the decomposition products was not studied. Such information would be of importance, especially in view of the assumption of free radicals made by Rice,* by Taylor and Jones,3 and others in postulating the mechanism of certain simple organic reactions. Experiments on the thermal decomposition of ethylsodium in the solid phase have been made by Schorigin4and by Carothers and C ~ f f m a n .Geddes ~ and Mack6 determined the products formed from tetraethylgermanium and found its rate of decomposition. Taylor and Jones decomposed diethylmercury, tetraet,hyllead, and dimethylmercury ; but did not analyze the products completely, as their interests were on the effects of the presence of these substances upon the reactions of ethylene. Jones and Werner? decomposed various alkyl derivatives of mercury and lead in acetic acid solution. The purpose of the present work was to decompose the vapors of tetramethyllead by a flow method under conditions similar to those used by Paneth and Hofeditz and to compare the product's so formed with those obtained by a static method in which the experimental conditions of heating were greatly different. I n the flow method the vapor of tetramethyllead was drawn through a short heated region a t a very high velocity after which it was rapidly cooled. The pressure was kept below 2 mm. of mercury. It was hoped that by this method the initial active substances could be removed to a cold part of the tube before their readjustment into stable compounds. The conditions used duplicated those of Paneth and Hofedita with the exception that no carrier gas was used. When hydrogen was used for this purpose, a lead mirror was readily removed by the active substance. In the static method the material was heated for a considerable period of time in sealed pyrex tubes. Each bulb contained sufficient of it to give a pressure of an atmosphere or two a t the temperature used. The temperatures varied bet,ween 2 6 5 ° - 6 2 0 0 , as contrasted with 5 j o o - 8 2 0 0 in the flow method. Furthermore, aside from tem-
* Contribution from the Chemical
Laboratory of Northwestern University. Pnneth and Hofeditz: Ber., 62, 1335 (1929). *Rice: J. Am. Chem. Soc., 53, 19jg (1931). Taylor and Jones: J. Am. Chem. Soc., 52, I I I I (1930). Schorigin: Ber., 43, 1931 (1910). Carothers and Coffman: J. Am. Chem. SOC.,51, 588 (1929). Geddes and Mack: J. Am. Chem. SOC.,52, 4372 (1930). ' Jones and Werner: J. Am. Chem. SOC.,40, 1257 (1918).
940
J. H. SIMONS, R . W. McNAMEE, AND CHARLES D. HURD
perature differences the pressure was some five hundred times greater, the time of contact was thousands of times greater, and the formation of the final products took place in the same region where the active substances were created so that the readjustment must have occurred in the presence of considerable quantities of both the original substance and the reaction products. Great care was exercised to insure the absence of impurities, especially oxygen and organic iodides, which Paneth indicated prevented the removal of the mirrors. This caution was also based upon the work of Geddes and Mack, who found that 0.1% of oxygen greatly increased the rate of the decomposition tGtraethylgermanium.
Experimental Procedure The tetramethyllead was prepared from methvlmaenesium iodide and lead chloride in the usual way. It was purified by distillation, the distillate coming over between 48-50' a t roo mm. To remove any residual iodides, the distillate was left for a week in contact with freshly precipitated silver oxide. Then it was redistilled, and the distillate dried with anhydrous calcium chloride. It was distilled into the apparatus and the retained gases boiled out. The entire apparatus was pumped out to a pressure of I x IO-^ mm. of mercury or less before the flow method was used, and the bulbs were pumped out to a similar vacuum before the tetramethyllead was distilled into them. I
FIG.I
Y
Flow Method. The apparatus used consisted of a high vacuum bench with the necessary pumps, McLeod gauge, etc. For the floy method of decomposition the tetramethyllead was contained in a small bulb, which could be immersed in cooling baths. This was connected first through a U-tube (made of 6 mm. pyrox tubing) which was heated by immersion in a molten salt bath, then through a second U-trap which was cooled by a solid carbon dioxide and acetone bath t o remove any undecomposed material carried over, and finally to both the high vacuum pumping system and the apparatus shown diagrammatically in Fig. I . In this piece of apparatus there was sufficient mercury to fill both the bulbs C and B and at the same time not to expose the lower outlet of bulb A. One outlet of the threeway stopcock was connected to a vacuum pump and the other to a source of compressed air. Connection to the rest of the apparatus was made a t H. A had a volume of 1000cc; B, 500 cc; and C, roo CC. The height of the tube between G and H was I O O cm. to hold the mercury in place.
THERMAL DECOMPOSITION OF TETRAMETHYLLEAD
941
After the apparatus had been pumped out and the residual gas boiled out of the t,etramethyllead, the appropriate temperat'ure baths were put into place. The container for the tetramethyllead was cooled to about - z o o with ice and salt. The temperature of the hot bath was controlled by a Bunsen burner and was measured with a chromel-alumel thermocouple. The U-tube was immersed in the heated liquid to a depth of 5 cm. or less. The stopcock, which separat,ed the container from the rest of the apparatus, was not opened. By pumping the air out from above the mercury in A, the bulbs C and B were emptied; and when G was exposed, the gas from the decomposition filled them. This gas was trapped in C by allowing air to enter A and so forcing the mercury up into B after which stopcock F was closed. By pulling the mercury down again another quantity of gas was trapped in B, which was in turn forced into C. By repeating this operation C could be filled to any desired pressure. I t took 2 0 0 to 300 of these operations t'o fill the bulb (100cc.) with gas at, atmospheric pressure. The gas so obtained was forced through E into a gas pipet and was analyzed. The opening of the large volume of empty space in B to the rest of the apparatus drew the gas through the heated region at very high velocity. The time of contact was estimated by varying the time that the opening a t G was made and determining the shortest, time necessary t,o fill B. From this and the relative volumes of B and the heated region an approximate value of the contact time could be calculated. In these experiments it was judged to be less than 0.001second. For temperatures above 600' the first U-tube was replaced by a horizontal piece of quartz tubing. This was heated by a small electric furnace built around it. The thermocouple junction was placed in the center of the furnace next t,o the outside of the quartz tube. Temperature measurements made in this manner or those made by having the thermocouple junct,ion in the molten salt bath give the temperature of the outside of the tube. This is somewhat higher than that of the gas which is rapidly streaming through the inside of the tube. The values so obtained can, however, be used for comparisons. Static Method. The bulbs used in the static method were of about I O O cc. capacity and had capillary tips on both ends. One of these was sealed shut and the other sealed to a manifold, which connected through a stopcock to the container of tetramethyllead. After the evacuation a visually determined quantity of the liquid was distilled into the bulbs. They were then sealed off and weighed. After being heated in a furnace for a given length of time, the gases contained in them could be forced into the analysis apparatus by the following means. The capillaries were scratched with a file. Over one of them was placed a rubber tube filled with mercury and connected to a mercury reservoir. The capillary was then broken while it was inside the rubber connection. As the tube contained approximately enough of the original substance to produce one atmosphere pressure at room temperature, there was very little rush of the gas out of the bulb or of mercury into it. A
J . H. SIMONS, R. W. McNAMEE, AND CHARLES D. HURD
942
short rubber tube on the upper capillary connected it to the analysis apparatus. Breaking this capillary enabled the gas to be forced out by means of the mercury with very little air introduced. The weight of the cleaned bulb and the capillary tips subtracted from the weight of the bulb before it was opened gave the weight of the tetramethyllead decomposed. The analysis were made by a modified Orsat apparatus with the method described by Hurd and Spence.I
Results The results of the flow method are given in Table I and those of the static method in Table 11. I n runs 7 to 1 1 of Table 11, both tarring and carbonization were observed. Runs 1 1 and 14 showed diminishing carbonization and no tarring. The deposits in runs 15 to 17 was a sputtered film of lead, loosely held, and easily dissolved in mercury. I n these three runs very little carbonization was observed. As the initial temperature a t which decomposition took place in the bulbs was 265", as contrasted to the initial temperature of 425" which was required to obtain a deposit in the flow method, it is evident that corresponding temperatures in the latter are about 150"higher than in the former. There are two possible explanations for this difference. One is that the rapidly moving gas has not come into temperature equilibrium with the walls of the tube, and the other is that the temperature a t which the corresponding reactions take place is higher due to the much shorter time of contact and lower pressure. A bulb held a t 26.5" for twenty hours still contained tetramethyllead, as was shown by the formation of liquid on cooling to room temperature. This showed the reaction a t this temperature to be very slow. At 550' the reaction was complete in less than ten minutes.
TABLE I Decomposition of Tetramethyllead by Flow Method Run
I
550 Temperature of tube "C Average Pressure, mm. of Hg. o 5
4 700
4
5
55 0
700
820
1.5
2.0
1.1
2 0
2
Percentage by Volume of Gaseous Products Acetylene Isobutylene Propylene Ethylene Hydrogen Methane Ethane
0 .I
0.5
0.0
0.0
0.0
1.8 0.8 1.8
5.1
0.8
0.3 0.4 18.4 9.2 47.8 23.9
2.4
10.4 82.8
1.5
0.3 0.3
2.5
2.8
4.9 24,,9
0.1
31.0
58.7
6j.j
0.4 2.4
14.6' 33.2 49.8
Hydrogen was added to the tetramethyllead before decomposition, thereby increasing the percentage of hydrogen in this case. a
Hurd and Spence: J. Am. Chem. SOC.,51, 3353 (1929).
THERMAL DECOMPOSITION OF TETRAMETHYLLEAD
:%
9 W
I
N
I
I I
I
.
N
-00 ,
0 0
.
N
.
N Q N
N ,
943
. .
P - N
000 N
N
944
J . H. SIMONS, R . W. McNAMEE, A N D CHARLES D. HURD
Probably the most outstanding difference in the course of the low temperature reaction from that of the high temperature reaction is carbon formation. The appearance of the firmly attached black mirror of carbon and lead which resulted in the former case was quite different from the loosely-held, sputtered, gray surface of lead (without carbon) which was deposited a t higher temperatures. The former would not distill away when heated in z)ucuo, nor would the mirror disappear when it was subjected to the action of methy 1 radicals in the manner of Paneth and Hofeditz. Furthermore, the mirror was not soluble in mercury. I n contrast, the sputtered lead mirror did all three of these things. The unusual production of carbon at the lower temperatures but not at the higher and also the change in the reaction products with change in temperature point to the conclusion that the decomposition takes place by two separate sets of reactions. One of these is comparatively slow and takes place a t the lower temperatures. From the closely adhering character of the deposit, this is judged to be a wall reaction. The other is a fast reaction which takes place a t the higher temperatures and forms the loosely held sputtered lead deposit. From the similarity of this deposit to that reported by Geddes and Mack’ in the decomposition of tetraethylgermanium, it is assumed that this is a homogeneous reaction. They concluded that their reaction was at least 98% homogeneous. Carbon formation was prominent in runs 7 to 1 1 where the temperature ranged between 265-365’. I n these runs methane and ethane comprised 87-94% of the gaseous products and the olefines ethylene, propylene and isobutylene 1o-5% of the products. All five of these hydrocarbons are stable a t temperatures* 265-365O in quartz or pyrex tubes. At least, under these conditions there is no appreciable carbon formation. The carbon which was noticed from tetramethyllead has its origin from some source other than the paraffins and olefins in the gaseous products.
Discussion Previous workers have postulated that the tetramethyllead molecule in its decomposition formed a lead atom and hydrocarbon fragments. Paneth and Hofeditz have considered these fragments to be free methyl radicals. Whether such is the case or not is irrelevant to the present discussion. I t is convenient, however, to retain the term radical in this connection, despite a possible error in so doing. In order to explain the products formed, it will be assumed that a t the lower temperatures a slow reaction takes place in the condensed phase on the walls of the vessel, and a t the higher temperatures the reaction is rapid and homogeneous in the gas phase. Because the latter is rapid, it consumes the tetramethyllead before any considerable quantity has reacted according to LOC.cit. Hurd: “The Pyrolysis of Carbon Compounds,” 5-79
(1929).
THERMAL DECOMPOSITION O F TETRAMETHYLLEAD
945
the former reaction. Only the products methane, ethane, ethylene, and hydrogen will be considered, assuming the other products formed in relatively small quantities to be due to side reactions. When the decomposition occurs on the walls of the vessel in a condensed phase, the molecules of the reacting substance are packed tightly together. Under these conditions the methyl radicals, as they are disrupted fiom the lead atom, are in close contact with other tetramethyllead molecules and so can extract hydrogen from them to form methane in quantity. By coming in contact with other methyl radicals on the walls, they can form ethane. As the temperature is raised, the reaction proceeds more rapidly, and the activity of the methyl radicals increases to form even higher proportions of methane. This trend is seen in runs 7 , 8, 9, and I O . Lower temperatures in a condensed phase would also tend to favor side reactions. The deposit left on the wall would consist of lead, carbon, and tarry hydrocarbons. As the temperature is further raised, the concentration of methyl radicals increases; and this gives greater opportunity for their combination to form ethane. This trend is seen in runs 1 1 , 12, and 13. At jso", runs 15 and 16,the high temperature reaction begins to set in. The methyl radicals are now released in the gas phase and have the opportunity of a variety of reactions. I n a two-body collision between two methyl radicals we would expect ethylene and hydrogen to be formed, one or both of which would be highly activated. As a matter of fact, the production of ethylene and hydrogen was found to be strikingly increased, and both were present in approximately equivalent amounts. A three-body collision in the considerable gas pressure in the bulb is also highly probable. Two methyl radicals meeting in a three-body collision could form ethane by releasing the energy of combination to the third body. Even at the higher temperatures there is probably some of the low temperature wall reaction taking place to produce some of the methane. Methane could also be formed in the gas phase by a reaction of the methyl radicals with the hydrocarbons present or with molecular hydrogen. In the flow method of decomposition, there is an entirely different set of conditions. The active methyl radicals are preserved in a comparatively stable condition, since they are rapidly removed from the heated region. As Paneth and Hofeditz were able to detect the active substance in their experiments as long as 0.026 second after its formation a t a gas pressure of z mm. of mercury, it must be relatively stable. I n these experiments it remained in the heated region less than 0.001 second. Not having sufficient energy to form ethylene and hydrogen by a two-body collision, it reacts to form ethane by a three-body collision or wall reaction. Actually, a high proportion of ethane was found in these experiments. At jsoo a carbonaceous deposit indicated a wall reaction which was similar to that formed in the tubes at lower temperatures and accounts for the methane content. Methane could also be formed by reaction between the methyl radicals and hydrogen or hydrocarbons present. That the methyl radicals require more energy to form ethylene and hydrogen than to form ethane is seen in the increasing proportion of ethylene with increased temperature.
946
J. H. SIYONS, R.
W.
McNAMEE, AND CHARLES D. HURD
A comparison of runs I and 2 which were at the same temperature, but at different average pressures shows that at the lower pressure considerably higher percentage of ethane is formed and lower percentages of methane, ethylene, and hydrogen. At the lower pressure the methyl radicals have a greater mean free path and so leave the heated region and the carbonaceous deposit on the wall more readily. Furthermore, they have less opportunity to meet tetramethyllead molecules in the heated region. This is in agreement with our suggestion that they would combine in the cold portions of the tube to form ethane. Run 4 was at the same temperature as run 3, but in the former a small amount of hydrogen was allowed to leak into the tetramethyllead container. I t is seen that this reduced the percentage of ethane with a corresponding gain in the amount of methane. This indicates a reaction between the methyl radical and hydrogen. Lead Mzrrors. These experiments suggest the reason for the difficulties encountered in the removal of lead mirrors by methyl radicals, when the mirrors are prepared by pyrolyzing tetramethyllead. Unless sufficiently elevated temperatures are taken, carbon, which is not removed by methyl radical, is formed in the mirror. If satisfactory temperatures are used, quartz tubing is essential since pyrex glass would collapse under the reduced pressure in the apparatus. To be sure metallic mirrors which are not contaminated with carbon can be readily formed by vaporizing the metal and condensing it on the glass surface, but this does not concern us here. The presence of traces of oxygen may accelerate the decomposition on the wall, and this explains why its complete removal is necessary. Correlatzon of Exzstzng Data. Paneth and Hofeditz report that the decay of the methyl radical is linear with time after its formation, suggesting a reaction which is first order with respect to the radicals. I n line with the above work this can be explained in the following manner. In their experiments the concentration of hydrogen was very much greater than that of the methyl radicals. This would give but little opportunity for collisions between methyl radicals to form either ethane or ethylene and hydrogen, reactions which would be second order with respect t o the methyl radicals. A reaction between the methyl radicals and molecular hydrogen to form methane and active hydrogen atoms would, however, be first order with respect to the radicals, thereby satisfying the conditions. When a hydrocarbon, instead of hydrogen, serves as atmosphere for the methyl radicals, Rice' has found the decay to be second order with respect to the radicals. This suggests that the methyl radicals do not react with hydrocarbon molecules in the cold, an assumption which is in agreement with the results and hypotheses presented in this paper. I n the decomposition of ethylsodium, Carothers and Coffmanl found that the chief products were ethylene, hydrogen, and ethane in the ratio of 8, 4, and I respectively. Ethylsodium is a salt-like compound3 with a very low 1 F. 0. Rice: Paper presented before the Physical Chemical Division of the American Chemical Society a t the Buffalo meeting, September, 1931. * Loc. cit. 8 Hein and Schramm: 2. physik. Chem., 151 A, 234 (1930).
THERMAL DECOMPOSITION O F TETRAMETHYLLEAD
947
vapor pressure and, therefore, the decomposition takes place in the solid phase, If the compound has an ionic crystal lattice, which it,s properties indicate, the sodium ion is as close to the beta carbon in any one of the adjacent ethyl ions as to the alpha carbon. This physical condition would facilitate the disruption into ethylene and sodium hydride in the manner which these authors postulate as the niajor reaction. They obtained indications of sodium carbide as a product of the pyrolysis. I n explaining this and also the formation of ethane, they have postulated a mechanism which assumed as intermediate products NaCHzCHzNa and NazCHCHNaZ. I n the ionic crystal lattice these should be written z Na+ CHZCHZ--and 4 Naf CHCH----. I t seems highly improbable that doubly and quadruply negatively charged hydrocarbon ions would be formed. This difficulty can be overcome by assuming that t,he ethyl ion or radical, as it is disrupted from the crystal lattice, reacts in this condensed phase in a manner similar to the low temperature condensed phase reaction for tetramethyllead. It should give ethane and leave a carbonaceous residue, which would probably contain Eodium carbide analogous to t,he formation of methane and carbon in our low temperature experiments. The small amount of ot'her products formed by side reactions would not be detected by the method of analysis these authors used. A striking contrast is shown between the thermal decomposition of tetraethylgermanium and that of ethylsodium. The former is a nonpolar substance that is easily vaporized. The metal atom is probably centrally located between the hydrocarbon groups and adjacent to the alpha carbon atoms. There is no possibility of a reaction similar to the major one which takes place in the decomposition of et'hylsodium. The reaction is homogeneous in the gas phase. The chief products which Geddes and Mack' found were ethane, ethylene, hydrogen, methane, and higher olefins in amounts respectively of about 4, 2, I , I , and I . These are not so greatly different from the products found in our high temperature reaction especially when it is taken into consideration that the ethyl radicals have the opportunity for a reaction which is denied the methyl radicals. That is, they can undergo disproportionation into the two stable products, ethane and ethylene. Meinert2has recently studied the pyrolysis of tetraethyllead. I n common with our results on the lower homolog, his results also point, to two different reactions. One of these is a homogeneous reaction in the vapor phase. The other is a reaction in the condensed phase. His experiments on the decomposition of the liquid gave rise to the products which are analogous to the products we have postulated in our condensed phase reaction. The Production of Isobutylene. Isobutylene is one of the lesser products formed in the thermal decomposition of tetramethyllead. Its amount, about 5 7 0 , is greatest a t the lowest temperature used, where the wall reaction predominates, and decreases with a rise in temperat'ure. Its presence might be
+
+
LOC.cit. R. N.Meinert: Paper presented before the Physical Chemical Division of the American Chemical Society a t the Buffalo meeting, September, 1931.
948
J. H. SIMONS, R. W. McNAMEE, AND CHARLES D. HURD
overlooked were it not for the fact that in two independent cases isobutylene has been reported. Hurd and Spence' reported small yields of it from the pyrolysis of n-butane and recently2 considerably higher yields have been realized. I n the electrolysis of methylmagnesium bromide in ether solutions, Evans and Lee3 found that isobutylene made up 10-17% of the anode gases when the concentration of the electrolyte varied from 1.45 to 1.09 molar. When the experiments with tetramethyllead, n-butane, and methylmagnesium bromide are compared, the only apparent point in common is h a t all may give a free methyl radical as a transient product. Evidently, therefore, by some obscure mechanism, isobutylene is formed from it. summary
The thermal decomposition of tetramethyllead has been studied by the use of two different methods. I n one the substance was heated for a considerable time a t normal pressures in sealed bulbs, whereas in the other it was heated at reduced pressures, using a flow method which gave an extremely short contact time. The products of the reaction were identified. The results indicate two sets of reactions, one taking place on the walls of the vessels and the other being a homogeneous gas phase reaction. The low temperature reaction produced carbon and tarry substances and, curiously, also some isobutylene. An attempt has been made to explain the mechanism of the formation of the products and also to correlate other experiments of a similar nature with this information. Certain essential factors have been pointed out regarding the Paneth and Hofeditz experiment of removal of lead mirrors by free methyl radicals. Evanston, Illinois.
LOC.cit. Hurd, Eilers and Pilgrim: unpublished data. Evans and Lee: Paper presented before the Physical Chemical Division of the American Chemical Society at the Buffalo meeting, September, 1931; F. H. Lee, Ph.D. dissertation, Northwestern University, 1931. 2
