# Why are high pressures used in cracking of long-chain hydrocarbons?

If we have a long-chain hydrocarbon, such as decane, and we split it through thermal cracking (say in an industrial plant), we use high temperatures, and high pressures. Cracking produces smaller molecules - alkanes, and alkenes. It might look like this:

$$\ce{CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3}$$ Which is cracked into products like this: $$\ce{CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3-> 2CH2=CH2 + C3H6 + C8H18}$$

Why do we use high pressure in this case? Le chateliers principle states that the equilibrium will shift to resist the change - in this case it would move the position of equilibrium to the left, not the right. In cracking we want maximum amount of products (ie. the right) - therefore surely a low pressure would shift the equilibrium that way.

The reaction is not dependent on molecules colliding - merely enough thermal energy has to be provided to enough molecules to cause them to split - homolytic fission - and become free radicals which react to form smaller molecules. Therefore a pressure requirement to increase the collisions doesn't seem necessary.

Why then do we use high pressure when cracking hydrocarbons?

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First of all, cracking is not an equilibrium process, therefore La Chatelier-Brown principle doesn't apply! (irrelevant) Also, the mechanism is much more complicated than this:

• In gas phase, radical reactions generally have extremely complex reaction networks. E.g. burning of methane with oxygen contains thousands if not more elementary reactions. I am pretty sure that gas phase cracking is not a pure first order reaction.

• In industry, cracking is generally done on heterogeneous catalyst, therefore not a gas phase reaction. Heterogeneous catalysis is pretty much controlled by the absorbed amount of molecules on the catalyst surface: high pressure, faster reactions.

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Ethylene is better made otherwise; alpha-olefins overall are assembled catalytically. You do not want straight-chain products. You want a maximally branched ~$\ce{C8}$-cut for gasoline, with maximum octane number. Linear ~$\ce{C10}$-cut and up is cheap diesel and kerosene, with maximum cetane number.