Alkene nomenclature

- [Instructor] We've already seen how to name alkanes. For example, this alkane is five carbons. One, two, three, four, five. And we know it's called pentane. So, what do we do if we're trying to name alkenes? So this compound has a double bond present but there's also five carbons. One, two, three, four and five. So, we start with pent indicating five carbons present but since there's a double bond we need to change our ending from ane to ene, so let me write in here ene and we'd call this molecule pentene. We want to number it to give the lowest number possible to our double bond. So, we start from the left. This'd be carbon one, this'd be carbon two, three, four and five. So, our double bond starts at carbon one. And we could write one pentene. If you leave the one off, it's implied that the double bond starts at carbon one. What if we tried to number this starting from the right? So, if we said that this was carbon one, that would make this carbon two, carbon three, carbon four and carbon five. So, you're saying the double bond would start at carbon four and this would be four pentene. That's incorrect because our goal is to give the double bond the lowest number possible, so we'd call it one pentene or just pentene. Let's look at this next example here. So, our goal is to find the longest carbon chain that includes the double bond and we want to give the double bond the lowest number possible. So, the longest carbon chain that includes the double bond, we would have to make this carbon one and this would be carbon two, this'd be carbon three, this is four and this is five. So, a five carbon alkene would be pentene. You could put the one in there or you could leave it off. Next, you think about the substituents, in this case we have only one group coming off our carbon chain, it's a two carbon alkyle group and we call that ethyl. So, with an ethyl group coming off carbon two, we would call this two ethyl pentene for the name. But that's not the longest carbon chain. So, someone might look at this molecule and say, wait a second, shouldn't this be carbon one and then this two, three, four, five and six because that's six carbons and six carbons is, of course, more than five? The problem with this is the double bond, the alkene is not a part of our longest carbon chain and that's one of our rules. We need to make the longest carbon chain that includes the double bond and that's why we name this as a pentene derivative and not numbering it according to the way I did it in the right here. So, this is incorrect. So, we find the longest carbon chain that includes our double bond and give the lowest number possible to your double bond. That's true even if you have some alkyl substituents. So, let's look at this next example and let's name this compound. So, first let's try numbering from the left to the right. This'd be carbon one, this'd be carbon two, this is carbon three, carbon four, carbon five and carbon six. A six carbon alkene would be called hexene, so let me write that in here. So, hexene and our double bond starts at carbon two. So, here we can see our double bond starting at carbon two. So, I'm gonna put a two in front of this, so two hexene. Next, let's think about the substituents coming off of that longest carbon chain including the double bond. We have a methyl group coming off carbon two, another methyl group on carbon four and another methyl group on carbon five. So, three methyl groups would be called trimethyl, so let me write that in here. So we have trimethyl and those methyl groups are on two, four and five. So, two, four, five trimethyl two hexene would be the name for this compound. There's another way to name this molecule. Instead of putting this two in front of the hexene, you can put the two between the hex and the ene. So, let me write out that way. So, we'd still have methyl groups at two, four, five. So, two, four, five trimethyl, trimethyl, let's write hex, and then we'll put the two in here, hex two ene. So again, the two right here indicates the double bond starts at carbon two. This first way of naming it, two, four, five trimethyl two hexene is more of the old school way of naming this alkene. Two, four, five trimethyl hex two ene is more of the newer way, but currently most people would accept both ways to name this alkene. What if you tried to number this from the right to the left? If you said this was carbon one and this was carbon two and this was carbon three, carbon four, carbon five and carbon six. You might think this would be correct because this would give you methyl groups at two, three and five and thinking about the first point of difference rule, right, we have two here and a two here but we have a three here versus a four, and we know that three is of course, a lower number. So, you might think that the alkyl groups would lead you to number it starting from the right. The problem with this way is this would give you an alkene that starts at carbon four, a double bond that starts at carbon four and our goal is to give the lowest number possible to the double bonds. And we want to give the double bond the lowest number possible and that's all for any alkyl group which is why we number it the way we did and two, four, five trimethyl two hexene is the correct IUPAC name for this compound. Next, let's look at cyclic examples. So, down here on the left we already know this is cyclohexane with an ane ending. If we put a double bond in, we have cyclohexene. So, let me write that in here, this is called cyclohexene, so ene. So, either one of these carbons could be carbon one. So, this carbon or this carbon, either one of them could be. Let's just say this one is. If that's carbon one, then this must be carbon two and so on around the ring, three, four, five and six. What if you have substituents? Let's look at this example down here on the left first. So, we wanna give our substituent the lowest number possible so here's our double bond, so we can make this carbon one or the one below it carbon one to give this methyl group the lowest number possible, we make this top carbon here carbon one. So, that's carbon one and this is carbon two and so on around the ring. So, this is a cyclohexene ring, so let me write this down here, so cyclohexene and we have this methyl group coming off carbon one, so we would write one methyl cyclohexene. What about this one right here? So again, we have a cyclohexene ring, so let me start by writing that in, so cyclohexene as our parent name here and we have a choice. Here's our double bond. We could make this one carbon one or we could make this one carbon one. Well, if we make the bottom one carbon one, that gives this substituent a lower number, alright. So, this would be carbon one and we have to follow the double bond around the ring. So, if that's carbon one, this is carbon two and that makes this carbon three. So, let's think about those substituents. At carbon three we have two methyl groups, so that's dimethyl, so let's write that in here, so dimethyl cyclohexene and those two methyl groups are both on carbon three, so three, three dimethyl and then we have this ethyl group here coming off of carbon one. So, this is an ethyl group, so let's write that in. So, we have one ethyl, so one ethyl three, three dimethyl cyclohexene and don't forget about the alphabet rule. So ethyl comes before methyl, right, E comes before M which is why we put the ethyl first in the name. So one ethyl three, three dimethyl cyclohexene.