Hydrocarbons

Carbon is an element that can form millions of different compounds. Some of the simplest compounds only contain carbon and hydrogen - hydrocarbons. They can have different lengths. They can be straight or branched... Or form rings.

They give of a lot of energy during combustion, so petrol, diesel and aircraft fuel are mixtures of different hydrocarbons. The gas in gas stoves is composed of hydrocarbons. And so are most kinds of oil. The basis of all carbon compounds is that the carbon atom wants to form four bonds. This means that the simplest hydrocarbon is: one carbon atom, surrounded by four hydrogen atoms.

That substance is called methane. The second hydrocarbon has two carbon atoms. They can be attached by a single bond... A double bond... Or a triple bond.

We'll keep it simple and stick to single bonds in this film. When the carbon atom has used one of its four available bonds, it has three left. That means there's room for a total of six hydrogen atoms in this hydrocarbon - ethane. We can continue adding carbon atoms, making the chain grow longer and longer. In order to name the hydrocarbons, you start by counting the carbon atoms.

There is a list that shows which name corresponds to which length of chain. We've already seen the first two on the list - methane and ethane. The substances with 3 and 4 carbon atoms are called propane and butane. From five and upwards, we use the Greek words for the numbers. You might recognise some of them from other words: Pentane - penta meaning five, Hexane Heptane Octane Nonane Decane You've noticed that the names of all these compounds end with "-ane"?

A collective name for all these hydrocarbons are alkanes. What about the number of hydrogen atoms? That's easy, you just have to make sure that each carbon atom has four bonds in total. The carbon atoms in the middle have used up two of their bonds to form the carbon chain. That means that they have room for two hydrogen atoms each.

The carbon atoms at each end have only used one bond. They can hold three hydrogen atoms. Let's run through the list again with the first ten alkanes, this time with the chemical formulae as well: The first four are methane, ethane, propane and butane... After that we count in Greek: Pentane, hexane, heptane, octane, nonane and decane. Gas burners usually contain a mixture of propane and butane, while most cigarette lighters use pure butane.

Methane gas is released when bio-waste decomposes in swamps and in puddles. And in cows' stomachs. It is the length of the carbon chain that determines the properties of the substance, like melting point and boiling point. The four shortest ones - methane, ethane, propane and butane - are gases at room temperature and normal pressure. ... chains with between five and fifteen carbon atoms are liquids at room temperature... ...

and longer chains than that are solids, like this octadecane molecule with 18 carbon atoms. What makes the hydrocarbons extra important is that their names are used to name many other organic compounds. Even if some of the hydrogen atoms are replaced with other atoms, these molecules will always have names that contain "penta-", since they have a chain of five carbon atoms. That's why it's a good thing to know the order by heart: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane.