Origins of oil
Graphic of hydrocarbons
Simple hydrocarbons.

Crude oil is a mixture of hydrocarbons. They are often chains of carbon atoms with hydrogens attached. The longer chains have higher boiling points, so they can be separated by distillation.

The simplest groups are the alkanes and alkenes. They all end with 'ane' and 'ene' respectively. The first bit of their name depends on the number of carbon atoms.

The second bit of the name depends on the shape of the molecule.

A simple chain of carbons with its full complement of hydrogens is said to be saturated. These saturated, simple, straight chain hydrocarbons are known as alkanes. Their name ends with 'ane'.

So, the alkane with 3 carbon atoms is propane. This is one of the gases known as LPG.

prefix number of carbons
meth 1
eth 2
prop 3
but 4
pent 5
hex 6
hept 7
Hydrocarbons with double bonds in them are said to be unsaturated. Their molecules contain at least one double bond. The molecules of unsaturated hydrocarbons are able to make new bonds with other atoms.
Alkenes are unsaturated hydrocarbons with only hydrogen and carbon atoms in their molecules. So, for example, a molecule of ethene has 2 carbon atoms (joined by a double bond) and 4 hydrogen atoms.

Unsaturated alkenes are useful because they are suitable for polymerisation.

Branching chains
Sometimes two hydrocarbon molecules can have the same numbers of the same atoms but have different arrangements of these atoms. We say they are isomers.

For example, pentane and 2-methylbutane both have 5 carbons and 12 hydrogens. However, one has a straight chain and one has a branching chain. Although they are made of the same atoms, their chemical properties are different.

The branching chain molecule is useful because it increases the octane number of petrol. They are produced in the isomeriser.

Graphic of branching hydrocarbons and aromatics
Branching isomers and aromatics.
Aromatics are hydrocarbons with a ring of hydrocarbons. Again, these improve the octane number of petrol and they are made in the reformer on the refinery. The reformer takes straight chain alkanes and turns some of them into aromatics.