Abstract
Full Text
PHYSICAL CHEMISTRY
L. V. SHEVELKOVA, A. M. BRODSKII, R. A. KALINENKO,
Corresponding Member of the USSR Academy of Sciences K. P. LAVROVSKII
THE MECHANISM OF FORMATION OF SECONDARY PRODUCTS IN HIGH-TEMPERATURE CRACKING OF ETHANE
The present paper is devoted to an investigation of the mechanism of formation of secondary products and, in particular, methane under the conditions of high-temperature cracking of ethane. The experiments were carried out by the previously developed procedure in a flow system with a turbulent reactor \((^{1})\). Small amounts of acetylene labeled with radiocarbon \(C^{14}\) were added to the initial ethane. The work is a continuation of previous studies, in which the cracking of ethane was investigated by us under analogous conditions with additions, respectively, of radioactive methane \((^{2})\) and ethylene \((^{3})\).* Two series of experiments were carried out at temperatures of \(800—920^\circ\) and a pressure of 90 mm Hg, using corundum and quartz, respectively, as heat carriers.
The procedure for carrying out the experiments and analyzing the products has been described in detail earlier \((^{2,3})\).
The first series of experiments (corundum heat carrier) was carried out on a mixture of composition \(99.99 + 0.01\%\ C_2H_2\). The specific activity of \(C_2H_2\) was \(1.75 \times 10^6\) imp/cm\(^3\)·sec, and that of the entire initial mixture was 173 imp/cm\(^3\)·sec. The second series of experiments (quartz heat carrier) was carried out on a mixture of composition: \(99.97\%\ C_2H_6 + 0.03\%\ C_2H_2\). The specific activity of \(C_2H_2\) in this series of experiments was \(3.0 \times 10^7\) imp/cm\(^3\)·sec, and that of the entire mixture was 9100 imp/cm\(^3\)·sec. Detailed compositions of the products are given in \((^{3})\).
Tables 1 and 2 give the specific and partial activities of the products obtained. The partial activities \(A_i\) represent the fraction of radioactivity falling on a given component in 1 cm\(^3\) of reaction products. The value \(A_i\) is equal to the corresponding specific activity multiplied by the fraction of the \(i\)-th component in the reaction products. From the data given in the tables it is evident that the specific and partial activities of methane and coke-1 depend substantially on the material of the heat carrier, in contrast to the compositions of the products and the total cracking constant \(k_1\), on which the heat-carrier material has no noticeable effect. Of greatest interest is the circumstance that, when corundum is used as the heat carrier, the greater part of the radioactivity of \(C_2H_2\) passes into methane (up to 80–90% of the initial amount), coke-1 (10–30%), and, to a lesser degree, into divinyl (2–3%) (Table 2). The ratio of the specific activity of \(CH_4\) \((a_{CH_4})\) to the specific activity of \(C_2H_2\) \((a_{C_2H_2})\) in deep experiments approached \(1/2\), while the ratio \(a_{\text{coke}}/a_{C_2H_2}\) was equal to \(1 \div 2\) (Table 1). This indicates that the source of formation of a significant part of \(CH_4\) and all or almost all of coke-1 under the conditions studied was \(C_2H_2\). The specific and partial radioactivities of coke-2, as well as of the remaining products, including \(C_4H_6\), were considerably lower than the specific radioactivity of \(C_2H_2\).
In the experiments using quartz as the heat carrier, the specific activities of coke-1 and methane were, respectively, 2–10 and 20–200 times lower than \(a_{C_2H_2}\) (Table 4). The conversion of \(C_2H_2\) into \(CH_4\) and coke-1 in this case
* These investigations made it possible to show the predominantly molecular character of the formation of ethylene from ethane under the conditions considered and to establish the radical character of the reactions forming side and secondary products \(C_3H_8\), \(C_4H_{10}\), \(C_3H_6\), and \(C_4H_8\). The mechanisms of formation of \(CH_4\), \(C_4H_6\), \(C_3H_4\) (allene and methylacetylene), and coke remained not fully clarified. The point is that, in investigations \((^{3})\), the specific activities of \(C_2H_4\) and \(C_2H_2\) in the reaction system were identical, as a result of which it was difficult to draw an unambiguous conclusion as to whether, for example, the source of \(C_4H_6\) is two \(C_2H_4\) molecules or \(C_2H_2\) and \(C_2H_4\) molecules, and also to estimate the role of reactions involving \(C_2H_4\) and \(C_2H_2\) in the formation of \(CH_4\) and coke.
amounted, respectively, to approximately 4.5 and 7% of the initial amount of \(C_2H_2\) at 918 and 873°, and to approximately 15 and 18% at 830° (Table 2). The total fraction of \(CH_4\) formed through \(C_2H_2\) was small and, in the range 830—
Table 1
Specific activities \(a_i\) of products of ethane pyrolysis (in imp/cm\(^3\)·min)*
| Heat-carrier temperature, °C | \(h\), sec\(^{-1}\) | \(t\), sec | \(a_{CH_4}\cdot 10^{-3}\) | \(a_{C_2H_6}\) | \(a_{C_2H_4}\cdot 10^{-3}\) | \(a_{C_2H_2}\cdot 10^{-3}\) | \(a_{C_3H_8}\cdot 10^{-3}\) | \(a_{C_3H_6}\cdot 10^{-3}\) | \(a_{\text{acetylene}}\cdot 10^{-3}\) | \(a_{M.\,al}\cdot 10^{-3}\) | \(a_{C_4H_{10}}\cdot 10^{-3}\) | \(a_{C_4H_8}\cdot 10^{-3}\) | \(a_{C_4H_6}\cdot 10^{-3}\) | \(a_{\text{coke}}\cdot 10^{-3}\), 1 | \(a_{\text{coke}}\cdot 10^{-3}\), 2 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Heat carrier: corundum** | |||||||||||||||
| 841 | 5 | 0,034 | 13,30 | 30,30 | 3,16 | 34,50 | 1,45 | ||||||||
| 847 | 6,5 | 0,053 | 6,95 | 5,77 | 1,75 | 18,33 | 0,93 | ||||||||
| 886 | 19 | 0,0117 | 4,30 | 2,06 | 0,023 | 82,00 | 0,094 | 0,284 | 0,063 | 1,05 | 5,60 | ||||
| 882 | 24 | 0,024 | 2,87 | 1,23 | 0,030 | 21,80 | 3,94 | 1,96 | 9,52 | ||||||
| 893 | 30 | 0,024 | 2,87 | 9,40 | 1,09 | 11,50 | 2,00 | ||||||||
| 896 | 34 | 0,0211 | 2,87 | 7,36 | 0,79 | 11,82 | |||||||||
| Heat carrier: quartz*** | |||||||||||||||
| 806 | 2,5 | 0,0718 | 177,7 | 616 | |||||||||||
| 830 | 6 | 0,0385 | 113 | 1,42 | 2903 | 418 | 682 | ||||||||
| 828 | 7 | 0,0418 | 82,8 | 1,87 | 3090 | 2,74 | 26,00 | 344 | 778 | ||||||
| 834 | 9 | 0,0678 | 48,2 | 1,97 | 1194 | 135 | 863 | ||||||||
| 877 | 22,5 | 0,0194 | 17,00 | 15,80 | 1,64 | 2800 | 1,92 | 11,10 | 204 | 357 | 1,52 | 23,7 | 145 | 364 | |
| 877 | 24 | 0,0230 | 10,00 | 12,62 | 1,77 | 1628 | 8,67 | 125 | 279 | 0,96 | 29,7 | 112 | 292 | ||
| 885 | 31,5 | 0,0219 | 13,90 | 1,31 | 1610 | 9,45 | 88,5 | 191 | 1,71 | 17,9 | 97,0 | 369 | |||
| 918 | 80 | 0,0147 | 6,12 | 33,15 | 1,42 | 502 | 1,67 | 9,85 | 81,0 | 117,5 | 2,48 | 14,1 | 40,2 | 129 | |
| 918 | 88,3 | 0,0150 | 7,26 | 23,65 | 1,24 | 595 | 1,59 | 9,00 | 90,0 | 148,3 | 10,2 | 46,0 | 200 |
* The accuracy of determination of the specific activities was 3% (relative).
** The specific activity of \(C_2H_6\) in the initial mixture was \(1{,}75\cdot 10^6\) imp/cm\(^3\)·sec.
*** The specific activity of \(C_2H_6\) in the initial mixture was \(3\cdot 10^7\) imp/cm\(^3\)·sec.
**** Coke-1 represented carbonaceous deposits on the heat carrier. Coke-2—polymeric products deposited in the upper cold part of the reactor.
Table 2
Partial activities \(A_i\) of products of ethane pyrolysis (imp/min·cm\(^3\) of gas obtained)
| Heat-carrier temperature, °C | \(t\), sec | \(A_{CH_4}\) | \(A_{C_2H_6}\) | \(A_{C_2H_4}\) | \(A_{C_2H_2}\) | \(A_{C_3H_8}\) | \(A_{C_3H_6}\) | \(A_{\text{acetylene}}\) | \(A_{M.\,al}\) | \(A_{C_4H_{10}}\) | \(A_{C_4H_8}\) | \(A_{C_4H_6}\) | \(A_{\text{coke-1}}\) (as \(CO_2\)) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Heat carrier: corundum | |||||||||||||
| 841 | 0,034 | 119 | 5,3 | 0,9 | 10,6 | ||||||||
| 847 | 0,053 | 103 | 1,2 | 1,2 | 13,4 | ||||||||
| 886 | 0,0117 | 75,3 | 1,3 | 3,5 | 46,8 | 0,07 | 0,11 | 0,05 | 0,13 | 3,9 | |||
| 882 | 0,024 | 80 | 0,4 | 7,5 | 28,4 | 0,13 | 2,2 | 20,0 | |||||
| 893 | 0,024 | 72,0 | 13,0 | 1,0 | 23,5 | ||||||||
| 896 | 0,0211 | 72,3 | 25,0 | 2,4 | 28,0 | ||||||||
| Heat carrier: quartz | |||||||||||||
| 806 | 0,0718 | 1500 | 470 | 5380 | 126 | 1393 | |||||||
| 830 | 0,0385 | 1210 | 222 | 6000 | 109 | 1225 | |||||||
| 828 | 0,0418 | 1380 | 250 | 5600 | 2,7 | 38 | 376 | 1075 | |||||
| 834 | 0,0678 | 800 | 525 | 5500 | 231 | 1530 | |||||||
| 877 | 0,0194 | 290 | 8,1 | 360 | 5560 | 1,85 | 14,0 | 15,3 | 1,6 | 4,75 | 283,5 | 530 | |
| 877 | 0,0230 | 195 | 5,8 | 430 | 6420 | 17,4 | 13,7 | 1,2 | 7,4 | 396 | 670 | ||
| 885 | 0,0219 | 290 | 370 | 3480 | 16,2 | 12,2 | 20,0 | 1,2 | 2,3 | 194 | 1220 | ||
| 918 | 0,0147 | 250 | 9,0 | 430 | 3660 | 1,93 | 28,5 | 12,9 | 7 | 1,2 | 2,5 | 220 | 334 |
| 918 | 0,0150 | 224 | 6,3 | 400 | 3410 | 2,23 | 25,1 | 22,4 | 17,8 | 6,3 | 344 | 240 |
918° amounted to 1–7% of the total amount of \(CH_4\). The participation of \(C_2H_2\) in coke formation was considerably greater: about 20–50% of the coke was obtained from \(C_2H_2\). A comparison of the specific activities of \(CH_4\) and of the radical \(CH_3\) shows that formation of that part of \(CH_4\) which is obtained from \(C_2H_2\) does not proceed through the radical \(CH_3\). The specific activity of \(CH_3\) in the volume can be estimated as follows. Since in (3) it was shown that \(C_4H_{10}\) is formed upon recombination of two \(C_2H_5\) radicals, while \(C_3H_8\) is formed from \(CH_3\) and \(C_2H_5\), \(a_{C_2H_5} = \frac{1}{2}a_{C_4H_{10}}\) and \(a_{CH_3} = a_{C_3H_8} - a_{C_2H_5}\). From the data of Table 1
it follows that in experiments using quartz as the heat-transfer medium, \(a_{\mathrm{CH_4}}/a_{\mathrm{CH_3}} = 10—20\), i.e., there is a route for obtaining radioactive \(\mathrm{CH_4}\) in addition to \(\mathrm{CH_3}\).
From the data on the formation of \(\mathrm{CH_4}\) obtained in the present work and in \((^3)\), it may be concluded that the amount of \(\mathrm{CH_4}\) formed from \(\mathrm{C_2H_4}\) and \(\mathrm{C_2H_2}\) during cracking of \(\mathrm{C_2H_6}\) in the temperature range \(800—920^\circ\) and at a pressure of 100 mm Hg, when quartz is used as the heat-transfer medium, does not exceed 10% of the total amount of \(\mathrm{CH_4}\) in the reaction products*.
It should be noted that an increase in methane formation upon adding small amounts of \(\mathrm{C_2H_2}\) \((^4)\) and \(\mathrm{C_2H_4}\) \((^5)\) to \(\mathrm{C_2H_6}\) was previously observed during cracking of \(\mathrm{C_2H_6}\) at lower temperatures.
The first-order rate constant for the overall reaction of consumption of \(\mathrm{C_2H_2}\) to form various products \((k_2)\), calculated for experiments using quartz as the heat-transfer medium, was \((18 \mp 2)\ \mathrm{s^{-1}}\) and practically did not change with increasing temperature. This may be explained by the large contribution of \(\mathrm{C_2H_2}\) consumption in a heterogeneous process limited by transport. In experiments using corundum as the heat-transfer medium, the constant \(k_2\) was determined with considerably less accuracy and was \(200—800\ \mathrm{s^{-1}}\).
Using the value found for \(k_2\), the rate constant for the formation of \(\mathrm{C_2H_2}\) from \(\mathrm{C_2H_4}\) \((k_3)\) was calculated:
\[ k_3=\frac{[\mathrm{C_2H_2}](1+k_2t)}{[\mathrm{C_2H_4}]\,t}, \tag{1} \]
where \(t\) is the ratio of the volume of the reaction zone to the volumetric flow rate of the products under the experimental conditions. In experiments with quartz as the heat-transfer medium, the values of \(k_3\) at 830, 885, and \(920^\circ\) were, respectively, 0.1, 0.5, and \(1.8\ \mathrm{s^{-1}}\). The variation of this constant with temperature is described by the formula:
\[ k_3=4.3\cdot 10^{15}\exp(84.5\pm 4\ \mathrm{kcal})/RT\ \left(\mathrm{s^{-1}}\right). \tag{2} \]
When considering the pathways for formation of other products of \(\mathrm{C_2H_6}\) cracking, it should be taken into account that the specific activities of all products, including \(\mathrm{C_4H_6}\), allene, and methylacetylene, were tens of times lower than that of \(\mathrm{C_2H_2}\). This indicates that only an insignificant fraction of these compounds was formed with participation of \(\mathrm{C_2H_2}\). Since for all products, except methane and coke, the ratios \(a_i/a_{\mathrm{C_2H_2}}\) and \(A_i/A_{\mathrm{C_2H_2}}\) (\(a_i\) and \(A_i\) are, respectively, the specific and partial activities of the \(i\)-th components) did not depend on the material of the heat-transfer medium, it may be concluded that the formation of these products is homogeneous in character.
The data obtained in the present work agree with the conclusion, made in \((^3)\), that under the conditions of \(\mathrm{C_2H_6}\) cracking the formation of \(\mathrm{C_3H_8}\) and \(\mathrm{C_4H_{10}}\) occurs by recombination of the radicals \(\mathrm{CH_3}\) and \(\mathrm{C_2H_5}\), \(\mathrm{C_3H_6}\)—by decomposition of the \(\mathrm{C_4H_9}\) radical, and \(\mathrm{C_4H_8}\)—by decomposition of the \(\mathrm{C_4H_9}\) radical and recombination of \(\mathrm{C_2H_3}\) and \(\mathrm{C_2H_5}\). On the basis of \((^3)\) and the present work it should be considered established that the formation of the main amounts of \(\mathrm{C_4H_6}\) under the conditions studied occurs with participation of two molecules of \(\mathrm{C_2H_4}\) (or radicals formed mainly from \(\mathrm{C_2H_4}\)). It may be proposed that the formation of \(\mathrm{C_4H_6}\) and \(\mathrm{C_3H_4}\) (allene and methylacetylene) proceeds according to the scheme:
\[ \begin{array}{rlrl} \mathrm{C_2H_4}+\mathrm{R} &\to \mathrm{C_2H_3}+\mathrm{RH}, & \mathrm{C_2H_5}+\mathrm{C_2^{*}H_2} &\to \mathrm{C_4^{*}H_7}, \qquad (b^{*})\\ \mathrm{C_2H_3}+\mathrm{C_2H_4} &\to \mathrm{C_4H_7}, \qquad (a) & \mathrm{C_4H_7}\,(\mathrm{C_4H_7^{*}}) &\to \mathrm{C_4H_6}\,(\mathrm{C_4H_6^{*}})+\mathrm{H}, \qquad (c,c^{*}) \end{array} \]
* Here the formation of \(\mathrm{CH_4}\) actually from \(\mathrm{C_2H_4}\) and \(\mathrm{C_2H_2}\) is meant. The influence of \(\mathrm{C_2H_4}\) and \(\mathrm{C_2H_2}\) on the rate of \(\mathrm{CH_4}\) formation by reactions of the type \(\mathrm{C_2H_4}+\mathrm{C_2H_5}\to \mathrm{C_4H_9}\to \mathrm{CH_3}+\mathrm{C_3H_6}\) has not been taken into account here. The possible influence of reactions of this type in methane formation cannot be evaluated from the ratio of the radioactivities of \(\mathrm{C_2H_4}\), \(\mathrm{C_2H_2}\), and \(\mathrm{CH_4}\):
\[ \begin{array}{c} \mathrm{RH}\downarrow\\ \mathrm{CH_4} \end{array} \]
\[ 2\mathrm{C}_2\mathrm{H}_3 \to \mathrm{C}_4\mathrm{H}_6, \tag{d} \]
\[ \mathrm{CH}_3 + \mathrm{C}_2^{*}\mathrm{H}_2 \to \mathrm{C}_3\mathrm{H}_5^{*}, \tag{e} \]
\[ \mathrm{C}_3^{*}\mathrm{H}_5 \to \mathrm{C}_3^{*}\mathrm{H}_4 + \mathrm{H}, \tag{f*} \]
\[ \mathrm{C}_4\mathrm{H}_7 \to \mathrm{C}_3\mathrm{H}_4 + \mathrm{CH}_3. \tag{g} \]
Molecules containing \( \mathrm{C}^{14} \), and the pathways of formation of the principal amounts of labeled products, are marked with an asterisk. In this case, since \(a_{\mathrm{C}_4\mathrm{H}_6} < a_{\mathrm{C}_3\mathrm{H}_4} \ll a_{\mathrm{C}_2\mathrm{H}_2}\), it should be assumed that only an insignificant fraction of the corresponding molecules was formed with the participation of \(\mathrm{C}_2\mathrm{H}_2\) (reactions \((b^*)\) and \((f^*)\)).
In order to decide whether the principal amounts of \(\mathrm{C}_4\mathrm{H}_6\) are formed by reaction (c) or (d), it is necessary to estimate the concentration of \(\mathrm{C}_2\mathrm{H}_3\). If one assumes approximate equality of the rate constants for recombination of two \(\mathrm{C}_2\mathrm{H}_3\), two \(\mathrm{C}_2\mathrm{H}_5\), and \(\mathrm{C}_2\mathrm{H}_5\) with \(\mathrm{C}_2\mathrm{H}_3\), then, assuming that \(\mathrm{C}_4\mathrm{H}_8\) is formed mainly by recombination of \(\mathrm{C}_2\mathrm{H}_3\) and \(\mathrm{C}_2\mathrm{H}_5\), we have:
\[ \frac{[\mathrm{C}_2\mathrm{H}_3][\mathrm{C}_2\mathrm{H}_5](1+k_{\mathrm{C}_4\mathrm{H}_{10}}t)} {[\mathrm{C}_2\mathrm{H}_5][\mathrm{C}_2\mathrm{H}_5](1+k_{\mathrm{C}_4\mathrm{H}_8}t)} \ll \frac{[\mathrm{C}_4\mathrm{H}_8]}{[\mathrm{C}_4\mathrm{H}_{10}]} \simeq 0.2 \div 0.5, \]
where \(k_{\mathrm{C}_4\mathrm{H}_8}\) and \(k_{\mathrm{C}_4\mathrm{H}_{10}}\) are the rate constants for the disappearance of \(\mathrm{C}_4\mathrm{H}_8\) and \(\mathrm{C}_4\mathrm{H}_{10}\). With
\[ (1+k_{\mathrm{C}_4\mathrm{H}_{10}}t) \simeq (1+k_{\mathrm{C}_4\mathrm{H}_8}t),\quad [\mathrm{C}_2\mathrm{H}_3]/[\mathrm{C}_2\mathrm{H}_5] \simeq 0.2 \div 0.5. \tag{3} \]
Assuming that all \(\mathrm{C}_4\mathrm{H}_6\) is formed by recombination of two \(\mathrm{C}_2\mathrm{H}_3\), we obtain:
\[ [\mathrm{C}_2\mathrm{H}_3](1+k_{\mathrm{C}_4\mathrm{H}_8}t)/ [\mathrm{C}_2\mathrm{H}_5](1+k_{\mathrm{C}_4\mathrm{H}_6}t) \simeq [\mathrm{C}_4\mathrm{H}_6]/[\mathrm{C}_4\mathrm{H}_8] = 5 \div 15. \tag{4} \]
Substituting relation (3) into (4), we have:
\[ (1+k_{\mathrm{C}_4\mathrm{H}_8}t)/(1+k_{\mathrm{C}_4\mathrm{H}_6}t) \simeq 30. \tag{5} \]
Since the contact time in the experiments is \(0.015\)—\(0.05\) sec, relation (5) can be fulfilled only at very large values of \(k_{\mathrm{C}_4\mathrm{H}_8}\) (of the order of \(200\)—\(1000\ \mathrm{sec}^{-1}\)) and small values of \(k_{\mathrm{C}_4\mathrm{H}_6}\), which appears unlikely. Thus, the assumption of the predominant formation of \(\mathrm{C}_4\mathrm{H}_6\) by reaction (d) leads to a contradiction. It is possible, however, that the assumption made concerning equality of the rate constants for radical recombination is not fulfilled, and the recombination of two \(\mathrm{C}_2\mathrm{H}_3\) for some reason proceeds more than an order of magnitude faster than the recombination of \(\mathrm{C}_2\mathrm{H}_3\) with \(\mathrm{C}_2\mathrm{H}_5\).*
In conclusion, it should be noted that the influence of the surface on the cracking rate has been studied many times. However, the question of which particular processes the surface can influence and what the specific chemical mechanism of this influence is still cannot be considered unambiguously clarified. The use of labeled atoms in the present work made it possible to trace, under \(\mathrm{C}_2\mathrm{H}_6\) cracking conditions, a clearly expressed dependence of the rate of decomposition of \(\mathrm{C}_2\mathrm{H}_2\) on the nature of the surface. This rate changes by a factor of 10–20 for different surfaces, and this change is not reflected in the composition of the main products of \(\mathrm{C}_2\mathrm{H}_6\) cracking or in the rate of the overall conversion of ethane.
Institute of Petrochemical Synthesis
named after A. V. Topchiev
Academy of Sciences of the USSR
Received
7 VIII 1964
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* The assumption that the principal amounts of \(\mathrm{C}_4\mathrm{H}_6\) are formed by decomposition of \(\mathrm{C}_4\mathrm{H}_7\), from which, by reaction (g), allene and methylacetylene are also obtained, likewise leads to difficulty. Under such an assumption, the rate of decomposition of \(\mathrm{C}_4\mathrm{H}_7\) at the C—C bond must be lower than the rate of its decomposition at the C—H bond, since the content of \(\mathrm{C}_3\mathrm{H}_4\) in the cracking products is approximately 30 times lower than that of \(\mathrm{C}_4\mathrm{H}_6\). It is not excluded, however, that decomposition of \(\mathrm{C}_4\mathrm{H}_7\) at the C—H bond is facilitated, since it leads to formation of the energetically favorable structure of divinyl.