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Alkanes: Cracking

Cracking: Mechanism

The first step in the process is the homolytic cleavage of a hydrocarbon.

Fig.1

Since this process is carried out at high temperatures, it is also called pyrolysis, from the Greek pyro, fire, and lysys, decomposition. The reaction proceeds under exclusion of oxygen. Subsequently, the radicals are recombined to give among others the following potential products:

Fig.2

Movie of the pyrolysis

The formation of a small fraction of long-chain products can be prevented by separating gaseous products. Furthermore, alkenes are formed by disproportionation. During this process, a hydrogen atom is transferred from one hydrocarbon radical to another as shown in Figure 3.

Fig.3

Basically, the above reactions are representative only for thermal cracking, i.e. without catalysts. The mechanism of catalytic cracking has not been fully resolved.

The current theory explaining catalytical cracking is based on ion theory assuming a carbocation intermediate. This is being formed either by removal of a hydride ion or by addition of a proton. Each time the carbocation collides with the catalyst it gains one positive charge. According to the theory, the positive charge finally breaks the carbon-carbon bond and subsequent hydride addition leads to low-molecular fractions. Since the step of transferring the positive charge has not been experimentally shown, proof of the theory is still pending.

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