in this video we're going to talk about how to draw constitutional isomers so let's start with this example how many constitutional isomers can we draw given the molecule c4h10 so this is butane and here's one way we can draw it we can put all four carbons in a straight chain or we can draw three carbons and put a methyl group on carbon two now the reason why these are constitutional isomers is because they have the same chemical formula but they're connected differently now i'm going to draw it out so for the first example on the left you can write it this way ch3 ch2 ch2 ch3 as you can see it has four carbons and 10 hydrogens now for this example we have a ch in the middle surrounded by three methyl groups and so we get the same chemical formula c4h10 now let's move on to pentane let's draw the constitutional isomers of c5h12 so we have five carbons one way we can draw is like this another way is we can go down four carbons and put a methyl group on carbon two or we can go down to three carbons and put two methyl groups on carbon two and so these are the three constitutional isomers of pentane now what about for hexane how many constitutional isomers can we draw for this molecule c6h14 so the first one let's start with a straight chain alkane that's one now let's go down to five carbons and let's add a methyl group on carbon two we could also put a methyl group on carbon 3. now we can't put a methyl group in carbon 4 because these two are identical they're not constitutional isomers in fact if you tried to name it they would have the same name so this would be called 2-methyl pentane and then for the other molecule you have to count any the other direction going from right to left so as you can see it has the same name 2-methylpentane now if we name the structure on the left notice that this is called 3-methylpentane and so it's important to understand that constitutional isomers they have the same molecular formula but they're connected differently and also have different names the nomenclature cannot be the same so let's see what other isomers we can draw so let's go down to four carbons and we need to put two methyl groups so we can place both methyl groups on carbon two or we can spread it out put in one methyl group on carbon two and the other in carbon three so for hexane i got five constitutional isomers now let's see how many we can draw for heptane c7h16 feel free to pause the video if you want to try it so first we can start with seven carbons in a straight chain so that's a total of seven then we can go down to six carbons we could put a methyl group on carbon two or we could put one on carbon three if we place one here it would be the same as putting it on carbon three or if we place it here it's the same as putting it on carbon two from the left just keep that in mind so now let's go down to five carbons and so we need to add two methyl groups we can add both methyl groups on carbon two or we could add two methyl groups on carbon three or we can place one methyl group on carbon two and one on three or we can place one on two and one on four so far we have seven constitutional isomers now let's go down to four carbons in this case we need to add three other carbons so we can place two methyl groups on carbon two one on three and i believe that's it for the four carbon chain but you know what if we go back to the five carbon chain what we could do is we can place an ethyl group on a carbon three and that's going to give us a completely different structure than anything we have seen so for the five carbon chains for this one this is 2 2 dimethyl pentane this one is 3 3 dimethyl pentane this is 2 3 dimethyl pentane and this is 3 ethyl pentane so that's completely different from the others and so we have 9 constitutional isomers for heptane now let's move on to the next example which is going to be octane so for octane there's a total of 18 constitutional isomers so what i want you to do is pause the video and try to draw as many isomets as you can come up with so let's start with a 8 carbon chain so that's simply octane now let's go down to seven and we could put a methyl on carbon two or we could put a methyl on carbon three or we could put a methyl carbon for now let's go down to a six carbon chain so we can either place two methyl groups or an ethyl group based on the last problem so let's start with two methyl groups on the same carbon so we could place it on carbon two or carbon three now we can place two methyl groups on different carbon atoms so let's say carbon two and three two and four or two and five we can also place a method group on carbon three and on carbon 4 because that will be different as well now let's talk about adding the ethyl group where can we add the ethyl group and where should we not add it we cannot add it on carbon two because the longest chain won't be six carbons it's going to be seven carbons and so what we're gonna have is three methyl heptane which is the same as this one so when adding the ethyl group we have to add it to the third carbon or more in order that the parent chain will not have more than six carbons so we can add it on carbon 3 or on carbon 4 which becomes carbon 3 if you count it in the other direction so there's only one location where we can add the ethyl group for a six carbon chain now let's go down to a five carbon chain so we can add three methyl groups or we can do an ethyl and a methyl let's start with three methyl groups we can place two here and one here or we could place two on carbon two one on carbon four we could place two on carbon three and one on carbon two or we can place one on carbon two three and four now let's talk about the ethyl group that we can add so we can add an ethyl group on carbon 3 and then we can add a methyl on carbon 2 or we can add the ether group on carbon 3 and the methyl on carbon 3. so let's count how much we have so far so this is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17. so far we have 17 constitutional isomers we need one more so what do you think the last one could be well we can't go down to four carbons maybe we could go down to four carbons if we go down to four carbons we have to add four more carbons to get eight carbons so we could put two on carbon and we could also put 2 in carbon 3. so that that's 18. so these are the 18 constitutional isomers of octane now what about this one c2h6o how many different constitutional isomers can we draw so we can draw an ether this is dimethyl ether it has two carbons six hydrogens and oxygen or we can represent it as an alcohol now what about this one c4h8 we did c4h10 which was an alkane it had the formula cn h2 two n plus two this one has the formula cnh2n when you see that it's either an alkene with one double bond or it's a ring so for c4h10 we can draw this isomer trans2butine we can also draw cis to butane these are cis trans geometric isomers but they're not constitutional isomers because they're connected the same way but they are isomers though we could also draw one butene we can also draw butane cyclobutane notice that each carbon has two hydrogens we can draw a cyclopropane ring with the methyl group and so those are some different ways in which we can draw a structure with this formula c4h8 now let's talk about cis and trans to butane the reason why they're not constitutional isomers is because they're connected the same way however the groups are arranged in space differently so there are stereoisomers specifically cis trans geometric isomers so if you look at this carbon it's attached to the same thing as this carbon both carbons are attached to a methyl group they're both attached to a hydrogen and to another carbon and if you analyze this carbon it's attached to the same thing the same carbon on the left they're both attached to the same method group and also the same hydrogen however as you can see the orientation of the hydrogen and the methyl group in space differs from this one here the methyl group is on the top here's at the bottom and so because they're connected the same they are not constitutional isomers however because the way the atoms are arranged in three-dimensional space because it's different thus we have stereoisomers but in this particular case specifically cis trans geometric isomers there's many different types of stereoisomers but cis trans isomers are one subcategory of stereoisomers and so we're going to stop it here for this video and now you know how to draw different types of constitutional isomers so that's all i got thanks for watching you