 In this video, you will learn about structural isomers of the alkanes, what structural isomers are and be able to do examples of isomers. Here are the formulae of the first four alkanes, methane, ethane, propane and butane. And here are the structural formulae of the first three. Pause the video and draw out the structural formula for butane. I expect most of you have drawn a straight chain molecule like this. However, there is another way of arranging the atoms. It has the same molecular formula, but a different structural formula. These two forms of the molecule C4H10 are called structural isomers. The longest chain in this molecule is three carbon atoms long. So we say it is a form of propane with a methyl group sticking to the second or middle carbon atom. So its name becomes two methyl propane. Do not be confused, it is still an isomer of butane because of the four carbon atoms. To make it easier to draw the structural formulae, we often omit hydrogen atoms and just show the bonds connecting the carbon atoms, bending the chain so you can see where the carbon atoms are. Before we try to find more isomers, here is the story of lead-free petrol. Petrol for cars is mostly octane, C8H18. In the 1920s, when petrol started to be used for cars, the petrol-air mix in the cylinders used to explode too quickly before the spark, causing the engine to knock or jolt. If an isomer of octane was used, the petrol burnt more smoothly. Look carefully, there are eight carbon atoms, so it is a structural isomer of octane. But what is its formal name? Pause and count the atoms in the longest chain. Whichever way you count, the longest chain is five carbon atoms long, so the name will be pentane. And off the third atom in the chain, there is an ethyl group and also a methyl group, so its name will be, pause and think, three, three, methyl, ethyl, pentane. But this compound is not normally found in crude oil and is very expensive. In 1923, scientists discovered a compound of lead with almost the same structure and did the job perfectly, lead tetraethyl or tetraethyl lead. You might remember that lead is in group four of the periodic table along with carbon. The lead addition helped the petrol to burn smoothly, but the lead atoms combine with oxygen, forming lead oxide. This comes out of the exhaust pipe as a fine dust and is very toxic. It has taken us until this 21st century to get rid of all the lead in petrol globally. But for 80 years, lead from car exhausts has been carried around the world and is even found in the ice in Antarctica. So let's finish with a simpler example. Can you find the other four isomers of hexane C6H14 and give them their formal names? I'll give you a hint. There are two based on pentane and two based on butane. Pause whilst you try to draw them out. Here are the hexane isomers with five carbon chain. So they are named pentane, two methyl pentane, two because the methyl group is on the second carbon atom in the pentane chain. On the atoms, six carbons, 14 hydrogens, it's still hexane. And here's the other isomer based on pentane, three methyl pentane. And there are two more structural isomers based on butane with only four carbon atoms in the longest chain, two three dimethyl butane and two two dimethyl butane. So from this video, you should know that structural isomers have the same molecular formula but different structural arrangement. And that naming is based on the length of the longest carbon chain and on where the groups come off this chain.