Several chemical reactions occur during the cracking.

In the refining process the next step after crude oil distillation is called cracking processes

The objective of cracking processes is to produce from heavier, higher boiling fractions, lighter lower boiling point fuels, like gasoline, kerosene and/or gas oil. These lighter fuels command a higher market price and are in greater demand however, their natural availability in a typical crude oil is lower than the global market demand.

During the cracking several chemical reactions occur

During cracking the following chemical reactions occur, amongst others: breaking of carbon-carbon bonds, dehydrogenation, isomerisation and polymerisation. By breaking a carbon-carbon bond an olefin (alkene) is produced. Paraffins (or alkanes) are more easily cracked compared to naphthenes (cycloalkanes) or even aromatics.

There are three different cracking processes Thermal cracking, Catalytic cracking and Hydrocracking 

Cracking processes at the refinery can be categorised in three main groups: Thermal cracking processes (only heat/high temperatures), Catalytic cracking processes (besides heat also a catalyst is applied) and Hydrocracking processes (besides heat and catalyst also hydrogen gas is applied). Obviously these three different cracking processes will yield different amounts and quality of the lighter products.