THE SIGNIFICANCE OF PYROLYSIS TEMPERATURES*
Attempts have been made in the pest to interpret the behauior of molecules of organic compounds when decomposed by pyrolysis, with the aid of the heats of formation of the several bonds involved in the decomposition. I t appeared that the intensity factor of the energy employed must be of importance and experiments were carried out to determine the effect of temperature on the breaking of the bonds between atoms. Although the carbon-to-carbon linkages in parafin hydrocarbons are assumed to have the same heat of formation, i t was found that the temperatures at which the several bonds broke were different. B y regulating these temperatures i t was found possible to break the bonds successively and thus determine from the products of pyrolysis the order in which the bonds yielded to the infEuence of heat. The results emphasize the importance of the study of pyrolysis from this point of v i m i n order to learn more of the effect of heat energy on atomic linkings and the effect of the configuration of molecules on these linkings.
. . . . . .
The behavior of molecules of organic compounds when subjected to the action of heat at such temperatures that decomposition takes place has been studied for many years. The interest which led to the many researches in the field of pyrolysis has been largely in the products formed, and, until recently, little attention has been paid to the mechanism of the changes and the energy factors invo1;ed. For a number of years investigations hsve been in progress in the Research Laboratory of Organic Chemistry of the Massachusetts Institute of Technology which had as their aim a study of the effect of the structure of molecules on the bonds between the atoms. A definite bond in a typical compound was first investigated; derivatives of this compound formed by the replacement of a hydrogen atom by certain atoms or groups were next prepared, and the analogous bond in these studied in the same way and under the same conditions as those employed in the case of the type compound. The methods used in studying the bonds were such that quantitative measurements could be made, and the results expressed as numbers could be compared with one another. Two methods have been used. In one, the bond investigated was broken by causing the compound to react with a second substance under such conditions that the rate a t which the reaction took place could be accurately measured; in the second, the temperature was determined to which the compound had to be
* Contribution from the Research Laboratory of Organic Chemistry. Massachusetts Institute of Technology. No. 83. This paper coven substantially the material presented in a lecture sponsored by the A. R. L. Dohme Foundation, delivered at The Johns Hopkins University. March 18, 1932. 1890
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PYROLYSIS TEMPERATURES
1891
heated in order to bring about the breaking of the bond a t such a rate that decomposition was evident. It has been shown in the case of two types of compounds that there is a striking relationship between the relative rates a t which the members of a series of analogous compounds react with a fixed reagent and the relative temperatures a t which compounds containing the same bond undergo pyrolysis. I shall limit myself in this address to the consideration of temperatures of pyrolysis and the use of these temperatures in interpreting the behavior of the several bonds in certain types of molecules when these bonds are broken as the result of the action of heat energy. Thermochemical data give much information in regard to the quantity of heat energy involved in the formation of the bonds between atoms in organic compounds. Attempts have been made to interpret the behavior of molecules when subjected to pyrolysis by utilizing this quantity factor. I t seemed probable that the intensity factor of the energy was of importance as well as the quantity factor when a bond is broken by heat. When bonds are broken by electrical energy the intensity factor is of paramount importance. No decomposition takes place until the required voltage is applied. These considerations led to the study of the significance of temperature in pyrolysis. Experiments were undertaken to determine whether, in the case of a particular compound, a temperature could be found below which the compound did not decompose and a t or above which decomposition took place. In seeking for such a temperature 'the compound, butane for example, was heated a t a definite temperature aild observations made to determine whether expansion took place continuously. This would occur if the molecule decomposed into two or more molecnles as the result of pyrolysis. If there was no evidence of decomposition after heating for one hour the temperature was raised to a fixed point and new ohservations were made. Finally, temperatures were reached a t which decomposition occurred a t definite and measurable rates. By plotting the rates a t different temperatures i t was possible to find a point a t which a measurable change occurred when the compound was heated for one hour and below which no such change occurred. Near this so-called "cracking temperature" observations were made a t intervals of about 5 degrees. It is possible that cracking might take place when a compound is held for a long time a few degrees below its cracking temperature so determined. The values obtained may not have any absolute significance. Many experiments have been undertaken to test this point. For example, a compound that decomposed a t a measurable rate a t a certain temperature showed no signs of decomposition when heated for 9 hours a t 4 degrees below the cracking temperature or for 16 hours 9 degrees below this point.
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Further experiments are now in progress to study the effect of the time of heating on cracking temperatures. In the work to be described the members of a series of compounds were heated in the same way in order to obtain comparable results. Experiments showed that the cracking temperature of a compound is influenced by contact substances. This phase of the subject is being studied on account of its theoretical and practical interest. In the investigation of the effect of the structure of similar molecules, e. g., the paraffin hydrocarbons, on cracking temperatures the compounds were studied under the same conditions. The results are, therefore, comparable; and as the differencesbetween the crackmg temperatures of different molecules are large compared with any possible error in these temperatures the conclusions are significant. For example, the cracking temperature determined for normal pentane was 391' and for normal hexane 343", a difference of 48 degrees. In these hydrocarbons the bond broken was that between the second and third carbon atoms. The cracking temperatures bring out a difference in the behavior of the bonds severed by heat. No such diierence is associated with the heat of formation of bonds as calculated from thermochemical data; all the carhonto-carbon linkages in pentane and hexane are assumed to have the same heat of formation. Our present knowledge of the quantity factor of the energy involved does not serve to tell us which bond in a particular hydrocarbon yields first to the action of heat or to indicate that different hydrocarbons vary markedly in &ability toward heat. The intensity factor of the energy, which is associates with temperature, gives us valuable information and brings out differences that are significant. The compounds investigated were selected because preliminary work showed that when pyrolyzed they decomposed in each case in a simple way into two substances that could be readily identified. The pyrolysis could he associated with the rupture of a particular bond. Malonic acid and its substitution products are decomposed by heat a t relatively low temperatures into carbon dioxide and the corresponding aliphatic monobasic acid. A series of such derivatives was made' and the temperatures a t which decomposition begins were determined in order to find out the effect of the change in the radical present in the acid on the temperature required to break the carbon-to-carbon bond. COOH KHC
Heat RH.C.COOH
+ COX
In the second series, ethers of the structure (CsH&C-&R were studied, the radicals (R) being the same as those introduced into malonic NORRISAND YOUNG, I.Am. Chem. Soc., 52,5066 (1930)
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PYROLYSIS TEMPERATURES
1893
acid.% A typical decomposition in this series is represented by the following equation: Heat
(CsH&C4-CHaCHa
---t (C6Hl)CH
H + O=C.CH,
In these two series the compounds were heated a t the rate of approximately one degree per minute and the temperature noted a t which a measurable rate of decomposition occurred. The results of the experiments are shown in Table I.
">
Methyl Ethyl 11
20
12
I t can be seen from Table I that the ra$icals fall in the same order in the two series when measured by their effect on pyrolysis temperatures. The results indicate that these temperatures %re significant, and that the intensity of the heat energy employed is an important factor in the pyrolysis. These encouraging results in this new method of studying atomic linkages led to the investigation of the pyrolysis of simple aliphatic hydroc a r b o n ~ . ~It seemed of particular interest to determine whether in these compounds the several bonds were broken successively by heat a t different temperatures, and if this were the case to find out a t what temperature each bond was severed. Such results 'would add much to our knowledge of the effectof radicals and their positions on the carbon-to-carbon and the carbon-to-hydrogen bonds. Certain saturated and unsaturated hydrocarbons were heated in a vessel of pyrex glass to which was attached a manometer to measure the changes in pressure which occurred when decomposition took place. The compounds were heated a t a number of temperatures and the rates a t which pyrolysis took place were measured a t these temperatures. When the NORRIS ~ r YOUNG. n I.Am. C h m . Soc., 52,753 (1930)
' NORRISAND THOMPSON. &did., 53, 3108 (1931).
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rates were plotted against temperature, intersecting straight lines were obtained. A sample plot is reproduced in Figure 1. Experiments were next undertaken to determine whether the temperatures indicated by the crossing of the lines were significant or whether the results should be represented by a smooth curve through the points, which would indicate a g+adually increased rate of decomposition with increasing temperature. The results of the study of n-pentane will illustrate how this question was answered.* The hydrocarbon was passed slowly through a tube of pyrex glass held a t a temperature between the cracking temperature (400') and the temperature a t which the rate of decomposition changed as indicated on the plot (423"). The products of pyrolysis were examined and found to consist only of compounds that would be formed as the result of breaking of a single carbon-to-carbon bond-the one between the second and third carbon atomsnamely, ethane and propylene in equivalent amounts and ethylene and propane, also in equivalent amounts. When the pyrolysis was carried out within the second temperature range indicated in the plot (423451") there were obtained the compounds produced in the first temperature range and in addition the products which resulted from the breaking of the bond between the first and second carbon atoms, namely, meth400 500 ane and butylene. No hydrogen was found in the products of pyrolysis until Tem~erature.'C. the temperature was above that of the RATECURVE: n-PENTANECRACKIN~ upper limit of the second range (451'). sCALE DIv, = 0,09CC. AT 40These results showed that when npentane is decomposed by heat the bonds break one after the other as the temperature is increased. Similar results were obtained with the other hydrocarbons studied. The results of the experiments are given in Table 11. An examination of the results leads to important conclusions in regard to the effectof the structure of the hydrocarbons on their relative stabilities toward heat. This property varies with the length of the chain and the
* The detailed description of these experiments will be published later.
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PYROLYSIS TEMPERATURES
TABLE II Products of the F'yrolysis of Hydrocarbons First Temoerature Ranee Crocking S a c a d Aonoge Temp., Break, Tamp. of 'C. 'C. Pylolgsis
400 391 343
450 426 391
440 419 362
+C - k c
C
++ C-C=C C-C-C
(a) C--C
(b) c==C (a) C-C
(a)
P*odudr
+ C-C-C=Ct c=c + c-c-c-c
(4)' (1)
(lo)* (1)
c-c-C
I
C c-C-c-C
I
C
C-C-C-C-C
I
C c-c-kc-C
339 4W
419 421
416 416
+
C-C-C C-C=C C (?)**
+
+
433 448 441 C (?)** Second Temperature Range Second Braoh. OC.
Third Brrok, OC.
426
450
446
C
423
450
437
(a)
b
+ C-C-C=Ct
C-kC C-C-C
+ C-C + C=C
(2)
(1)
I
C
450- 492 H2 *The numbers in parentheses indicate the relative extent to which the simultaneously occurring reactions took place. t The product was a butene; the position of the double band was not established. *' Methane was the only product identified.
presence or absence of side chains and unsaturated linkages. For example, the replacement of a single bond in isopentane by a double bond, as the result of which trimethylethylene is formed, raises the cracking temperature 50 degrees.
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This new information about the particular compounds studied is of interest, but the results of the investigations have a broader significance in that they throw new light on the behavior of molecules when subjected to the action of heat energy. They bring out the important facts that different bonds break at diere& temperatures, that the structure of the molecule has a marked effect on these temperatures, that the rates at which honds break after decomposition begins varies with the nature of the bond broken. The results also indicate that the composition of the mixture of products obtained when a compound undergoes pyrolysis is determined by the rates of the several independent reactions that take place and the relation between the temperature used and the cracking temperatures of the several bonds involved in these reactions. I t seems improbable that the use of heats of formation of the several honds could serve as a means of interpreting pyrolytic decomposi
