GCSE OCR Gateway Organic Chemistry C6.2o Cracking

C6.2o To be able to describe the production of materials that are more useful by cracking and to give the conditions and reasons for cracking and some of the useful materials produced

Cracking

Crude oil is an essential resource at the moment for the production of many fuels, lubricating oils and bitumen for road surfaces.

What cracking does is make some of the larger molecule fractions more useful.

Cracking does that in two ways

Cracking produces more reactive hydrocarbons and cracking produces smaller molecule hydrocarbons.

For example cracking duodecane C₁₂H₂₆ could produce octane C₈H₁₈ and and butene C₄H₈

Both these molecules are more useful superficially than duodecane since octane could be a constituent of higher grade fuel and butene is a reactive intermediate.

C₁₂H₂₆    ➞  C₈H₁₈  +  C₄H₈

From this simple example we can see what cracking essentially does: it breaks up larger hydrocarbon molecules in the crude oil fractions into smaller molecules.

Note that the sum of carbon and hydrogen atoms on the right hand side of the equation equals the number of carbon and hydrogen atoms on the left hand side.

No hydrogen or carbon atoms are theoretically “lost” in the process.

Industrially, catalytic cracking is achieved using a zeolite type catalyst and a high temperature (450^oC) and pressure.

Thermal Cracking by the name uses heat only processing the hydrocarbon fraction at between 700 and 1000K and a high pressure.

In cases of cracking the products are separated using further fractional distillation.

You can run a mock up experiment in the laboratory that mimics the industrial catalytic cracking process.

Here is a diagram of the apparatus you might have used:

Liquid paraffin is usually used as the long chain hydrocarbon that is going to be cracked.  The paraffin is usually absorbed on mineral wool to stop it slopping about in the test-tube.

Porcelain chips, crushed brick or aluminium oxide beads are used for the catalyst. 

The catalyst is held in place using mineral wool.

Heating the catalyst rather than the paraffin generates sufficient heat to warm and boil the paraffin and this expands over the catalyst and cracks up.

A gas issues from the delivery tube.  It is insoluble in water and can be collected over water.

This gas is very flammable unlike paraffin burning with a yellow flame and it also decolorises bromine water unlike paraffin.

The catalyst turns black on heating.

So how do we explain what is going on?

Essentially, alkane hydrocarbons crack into an alkane and an alkene.

It is the alkene that is the more reactive product of cracking because it contains a double carbon-carbon bond.

You can find out more about alkenes here

The catalyst turns black because the cracking process does produce atomistic carbon. 

The bromine water test indicates the presence of a double carbon–carbon bond in a molecule.

These alkenes can be polymerised and plastics of many different varieties result from addition polymerisation.

You can find out more about addition polymerisation here

Summary