Thermal cracking has played a large role in petroleum refining for many years. The first technique of thermal cracking was invented and patented by a Russian engineer in 1891. Ever since its invention, Thermal cracking has been used in the petroleum refining industry to “crack” longer, heavier alkane chains into smaller, lighter alkane chains. This is beneficial to the refining process because it allows a larger yield of lighter products to be created from the heavier less desirable products of refining.
Free radicals, are the mechanisms that allow thermal cracking to be possible. It is because of this free radical chemistry that refineries can use the thermal cracking of gas oil to produce higher yields of low octane number gasoline.
There are three main types of reactions involved in thermal cracking. The three types are initiation, propagation and termination reactions which also occur in that order. During the first step, or the initiation reaction, a single molecule is broken into two free radicals. Then during the propagation reaction one of three types can occur, whether it be hydrogen abstraction, radical decomposition or radical addition, all propagation reactions involve the manipulation of a radical into a different radical. Finally, during the termination reaction, two free radicals are essentially terminated, forming a new, shorter, molecule than the one which was originally initiated.
Thermal cracking produces shorter straight-chain alkanes and olefins but lacks the presence of branched iso-alkanes. It is for this reason that catalytic cracking is highly favored over thermal cracking in the production of high octane gasolines.
References:
1. http://en.wikipedia.org/wiki/Cracking_(chemistry)#History_and_patents