Consider each pair of compounds below and determine whether the pair represent the same compound, constitutional isomers, or different compounds that are not isomeric at all:
Alright guys. Let's take a look at both of our compounds here, we have a 5 membered ring with the double bond and we have a five carbon chain right here. So, 1, 2, 3, 4, 5 carbons, okay? So they both have five carbons, so we can write in that they're both C5. Now, a shortcut to this is if we know they have five carbons can we predict the number of hydrogens and without counting them? yeah guys actually we can, because remember, we have a rule that states alkanes, right? Have double the number of carbons, this is for, let's write it over here, regular alkane is going to be CN H2N plus 2, right? So, double the number of, the hydrogen's will be double the number of carbons plus 2, that's where alkane, this is for alkene and then we know there's alkyne, so if you remember what they are for these CN. and CNH2N, NH2N but this one has minus 2, right? okay. But remember that alkyne, alkyne could be an alkene or just one ring and alkyne could be one ring and one double bond, right? They would follow this formula also and you guys would have two rings or two double bonds, right? okay. But for alkenes it's just a ring or a double one, so we know that our alkyne is going to have two, if you double the number of hydrogens you get 10, it's going to be 2 less than that, we should expect that this would be C5H8, right? Following that formula. Now, let's look at the other one it has a ring and a double bond, so we can go that alkyne around again and the formula CNH2N minus 2, so this one is also going to be C5H8, right? That's just a little shortcut to notice that but if you want to count the hydrogens to just check your work, there's two, three. four, we have five, six, seven, eight and over here we have three, four, five, six, seven there's zero hydrogens there, eight, okay? So these have the same molecular formula but they're connected very differently from each other, right? One is a ring and one is not. So, by definition, what that means is that these two compounds are constitutional isomers to each other, okay, because they have the same molecular formula, right? Same molecular formula but where they differ is in their connectivity is a different connectivity. Alright guys, well, hopefully you don't have any trouble with this, what we did here is just label that we can have a general formula to count the number of hydrogens when we're dealing with alkanes, alkenes and alkynes and of course those other derivatives that we could think about in red, right? A ring or double bond and for alkynes it could include one ring and one double bond which is what we saw here. Alright, guys. So, again the relationship is simply just constitutional isomers, okay? So hopefully that makes sense and let me know if you have any questions.