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What sort of products will be formed when we subject a higher alkane (say decane) to pyrolysis by heating at 773-973 K in the presence of $\ce{H2/Ni}$? I know that lower aliphatic alkanes and alkenes will be products that will be formed but will other products such as aliphatic alkynes and saturated and unsaturated cyclic compounds be formed (say cyclopropane and cyclopropene)?

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  • $\begingroup$ Pyrolysis of alcanes is often carried out with heavy hydrocarbons from kerosene or higher fraction of petroleum and mineral oils. It produces a mixture of lighter alkenes and alkanes. Alkynes are not produced significantly. But with $\ce{H2/Ni}$, aliphatic alkenes are reduced to alkanes. The proportion of aromatic compounds is about $10$% in petroleum. No cyclopropane ou cyclobutane are obtained by pyrolysis of petroleum. $\endgroup$
    – Maurice
    Commented Dec 11, 2023 at 16:33
  • $\begingroup$ @Maurice Oh yeah I completely ignored the fact that we were using H2/Ni so any alkynes or alkenes would be reduced to alkanes, I just thought it was some sort of reagent that is necessary to use. BTW are any non aromatic or anti-aromatic cyclic compounds formed upon pyrolysis of petroleum or is it just aromatic compounds? $\endgroup$ Commented Dec 11, 2023 at 17:00
  • $\begingroup$ Does it not basically vary from compound to compound? I've studied in my 11th year about two different large-sized alkenes and we were expected to remember those two as where the chain broke was quite random and had no logic $\endgroup$ Commented Dec 11, 2023 at 17:53
  • $\begingroup$ I would like to send an image of my book too but the comments section won't help $\endgroup$ Commented Dec 11, 2023 at 17:54
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    $\begingroup$ I think you're talking about hydrocracking. $\endgroup$
    – Mithoron
    Commented Dec 11, 2023 at 19:01

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Pyrolysis of alkanes (cracking) involves breaking of $\ce{C-C}$ and $\ce{C-H}$ bonds to form free radicals. These radicals join to form smaller alkanes. So, there is higher chance of forming straight alkanes rather than unsaturated alkynes. But still there are chances of forming alkynes (where the alkenes undergo dehydrogenation) which are considered undesirable impurities in the cracking process.

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In the last stage of cracking, the alkanes can undergo cycloaddition and aromatization to form cycloalkanes/cycloalkenes and benzene.

Ref.: Gholami, Zahra & Gholami, Fatemeh & Tišler, Zdeněk & Vakili, Mohammadtaghi. (2021). A Review on the Production of Light Olefins Using Steam Cracking of Hydrocarbons. Energies. 14. 8190. DOI: 10.3390/en14238190

My other answer: Is reforming and isomerization the same thing?

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    $\begingroup$ Did you see my comment?^ It appears to be hydrocracking - much different results. $\endgroup$
    – Mithoron
    Commented Dec 12, 2023 at 14:08

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