To name an alkane you need to know the number of carbon atoms it contains.

This gives the first part of the alkane’s name.

An alkane is named by adding –ane to the prefix given by its number of carbon atoms.

The number of hydrogen atoms in an alkane is two plus twice its number of carbon atoms.

It helps to know the prefixes given by the number of carbon atoms and the general formula.

The molecular formula shows the number of atoms of each element in the alkane.

The structural formula shows how the atoms are bonded together. For methane, it is the same as the molecular formula.

The displayed formula shows all the atoms and all the bonds in the alkane.

The same information is shown here for ethane, propane and butane.

Notice how the alkane chain increases by a CH 2 unit each time. There are only single bonds in alkanes.

This alkane has four carbon atoms.

It is butane.

This alkane also has four carbon atoms but they are arranged differently from butane.

It has a branch containing one carbon atom. This is called a methyl group.

The longest chain has three carbon atoms, like propane.

So the branched alkane is called methylpropane. It is a chain isomer of butane.

Alkanes can form rings, called cycloalkanes.

They are named like open-chain alkanes, but with cyclo- at the beginning.

A cycloalkane contains two fewer hydrogen atoms than the equivalent open-chain alkane.

The ring is shown in the structural formula.

To make things easier to see, a skeletal formula can be used.

There would be a carbon atom at each corner, bonded to two hydrogen atoms.

When you work out isomers it often helps to draw the carbon atoms without the hydrogen atoms.

This is hexane (but remember to add in the missing bonds and hydrogen atoms later).

Imagine taking one carbon atom away.

You get a five-carbon chain. This is going to form pentane with a branch.

Add the missing carbon atom. But not at the end, as you would get hexane again.

It forms a methyl group at position 2.

This is 2-methylpentane.

See if you could have put the carbon atom somewhere else on the pentane chain.

Add the missing carbon atom in a different place.

It forms a methyl group at position 3.

This is 3-methylpentane.

Now imagine taking two carbon atoms away from hexane.

You get a four-carbon chain. This will form butane with a branch.

Add one missing carbon atom, but not at the ends.

Add the other missing carbon atom, again not at the ends.

There are two methyl groups at position 2.

This is 2,2-dimethylbutane.

See if you could have put the second missing carbon atom somewhere else on the butane chain.

Add it to a different place on the main chain.

It forms a methyl group at position 3.

This is 2,3-dimethylbutane.

There are no more isomers. It does not matter which way up the methyl groups go.

A bent chain is still just a straight chain. You need to find branches.

There are five isomers of hexane. Remember to add in the missing bonds and hydrogen atoms.