Saturated and unsaturated hydrocarbons

This is a compound containing only hydrogen and carbon - a hydrocarbon. The name of this hydrocarbon is ethane. The bond between the carbon atoms is a single bond - each atom shares one electron with the other atom. The carbon atoms can also share two electrons each, in a double bond. When there is a double bond in a hydrocarbon, it changes the ending of its name - instead of ethane, this compound is called ethene.

But it's not just the name that's different. Ethene has fewer hydrogen atoms. Why is that? Each carbon atom can form four bonds to other atoms. In ethane, each carbon atom has used one of its bonds in the carbon chain.

That means it has three bond possibilities left. In ethane, each carbon atom bonds to three hydrogen atoms. In ethene, there are two bonds to the other carbon atom. This means that there's only room for two hydrogen atoms. Double bonds means that the number of other atoms attached to the carbon chain, decreases.

How are the properties of the hydrocarbon affected if there's a double bond instead of a single bond. The double bond is pretty unstable - the compound can easily add more atoms or groups of atoms. For instance, the ethene molecule can add a hydrogen molecule... ... to form ethane. Those hydrocarbons that have the possibility of adding more atoms have a special name.

They are unsaturated - meaning there's room for more. But ethane already has the maximum number of atoms bonded to each carbon atom. Ethane can't add more atoms; it is a saturated hydrocarbon. Let's look at a longer compound: hexane with six carbon atoms. If we change it to hexene, it gets less stable - the double bond means that it is more likely to react with other substances.

If there are three double bonds in the molecule it gets even more unstable. But there is one exception. If we make a ring-shaped hydrocarbon with six carbon atoms, and three double bonds, it will be much more stable than the straight molecule. This substance is called benzene. That the name ends in -ene shows that it contains double bonds - it is an unsaturated compound.

Despite that, the benzene ring is pretty stable, and it is a part of many compounds that are important in biology. So, hydrocarbons with only single bonds have names ending with -ane, for example, butane. They are saturated- and stable - compounds. Hydrocarbons with double bonds have names ending with -ene. They are unsaturated - and unstable - compounds.

They have the ability to pick up more atoms. This makes them more reactive. Benzene is an exception - despite having three double bonds, it is pretty stable.