Unit 5.12 I can recall the problems associated with the fractional distillation of crude oil

3. a) When crude oil undergoes fractional distillation there are too many long-chain molecules (eg. bitumen) produced and not enough short chain hydrocarbons (eg refinery gases) 
b) To solve these problems the long chain hydrocarbons undergo a chemical reaction called cracking
(page 145 of chem 4 you)