in this video you will learn about structural isomers of the alkanes what structural isomers are and be able to examples of isomers here are the formula of the first four alkanes methane ethane propane and butane and here are the structural formula 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 C4 h10 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 methy group sticking to the second or middle carbon atom so its name becomes two methy 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 formula we often omit the hydrogen atoms and just show the bonds connecting the carbon atoms bending the chain so so you can see where the carbon atoms are before we try to find more isomers here is the story of leadfree petrol petrol for cars is mostly octane C8 h18 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 ethy group and also a methy group so its name will be pause and think 33 methyle ethy 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 Tetra ethy or Tetra ethy 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 combind 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 let's finish with a simpler example can you find the other four isomers of hexane C6 h14 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 methy pentane two because the methy group is on the second carbon atom in the pentane chain count the atoms six carbons 14 hydrogens it's still hexane and here's the other isomer based on pentane three methy pentane and there are two more structural isomers based on butane with only four carbon atoms in the longest chain two three dimethy butane and 2 two dimethy 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 for