Chemistry [[Module Three]] Petroleum: Cracking

[[Cracking]]

Involves breaking C-C and C-H bonds

Higher Mr Alkanes -------> Smaller Mr Alkanes + Alkenes

Molecules can break up several different ways to form a micture of products, which is then seperated by fractional distiliation.

[[Thermal Cracking]]

Results in formation of a high proportion of alkenes
energy required for bond breaking provieded by heat
Temps range from 400 to 900 degrees at pressures of 7000 KPa

At lower end of temperature range, carbon chains break up near to middle of chain
At higher end, carbon chains break up towards the end keading to a greater % low Mr alkenes

The length of exposure has to be short, to avoid decomposition

1) Initiated by homolytic fission of C-C bond to form to alkyl radicals
2) Each alkyl free radical can abstract a hydrogen molecule from an alkane to produce a different alkyl radical and a shorter alkane:

CH3(CH2)6CH3 ---> CH3CH2CH2CH2CH2* + *CH2CH2CH3

CH3(CH2)6CH3 + CH3CH2CH2CH2CH2* ---> CH3(CH2)5C*HCH3

Radical: Species which results from homolytic fission of a covalent bond. They contain an odd n umber of electrons with one unpaired electron. Written as a dot (* on this blog)

[[Catalytic Cracking]]

Involves the use of ZEOLITE catalysts
Slight excess of pressure
Temperature = 450 degrees

Large alkanes converted to branched chain alkanes, cycloalkanes and aromatic hydrocarbons

C14H30 -------> C8H18 + C6H12

Alkene proportion is small
Catalytic cracking = used for producing motor fuels
Branched Chain burn more smoothly than unbranched alkanes

Branched chain alkanes used as fuel to prevent the problem of knocking when the fuel air mixture tries to ignite before spark is produced.

In catalytic cracking, the catalyst acts as a lewis acid and the method involves the formation of carbocations