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cis-trans and E-Z naming scheme for alkenes

Video transcript

if I were to come over to your place and say name this molecule the first thing you would do is you would say well look we have a double bond so we're dealing with an alkene we have four carbons in the longest chain there's actually one chain here so it's one two three four carbons so the prefix will be Butte and regardless of which end you count from the double bond starts at the number two carbons so this is beaut beaut - een Butte - een I'd say well that's that's very good and that is correct based on everything we've learned but what about what about this molecule what about the molecule that looks like this the molecule that looks like this and you go through the same process you'd say it has a double bond it's an alkene our longest chain has four carbons one two three four regardless of which end you start counting from the the double bond starts at the number two carbon 1 2 3 4 so once again you say hey Sal that's also beaut beaut - lean and then I'd point out that these are each fundamentally different molecules and the reason why they're fundamentally different molecules is because this double bond can't rotate it's got that PI bond there for that second bond that keeps this carbon-carbon bond rigid so it's not like the molecule can switch configurations between this or light it's not like it can switch confirmations between this and that it cannot rotate around this double bond so these are different molecules these are different molecules different molecules that will behave different chemically and because they behave different we have to have different names for them now there's two different naming conventions one is kind of I guess the simpler naming convention and it works when we have one functional group on each of the carbons in the double bond and then in the next few videos we'll talk about the slightly more advanced naming scheme that will work when we have more than one functional group so the first way to name it if you have your functional group so let me circle the functional group so on this top one our functional groups are on opposite sides they're apart they're on opposite sides of the carbon-carbon double bond this is on the top this is on the bottom in this molecule right here our functional groups are on the same side so when our functional groups are on different sides we could call it trans we could call it trans literally for I believe Latin for apart or Latin for opposite I'm not a lot in scholar so forgive me but with the the functional groups are apart so we call it trans beaut to lean the other convention the other convention to use comes from the German for apart and there we call it an Kagan and instead of writing out and again we write e in parentheses so we would call them unison different colors since this molecule down here is blue so we would call this e e Bute to e these mean the same thing e boot to e but this is now kind of the standard convention we'll see this notation where you use entgegen or e is actually more powerful it can extend to when we have more than one functional group on each carbon so let me just make it clear that the e stands for and again and again which is german for german for apart now or at least I believe it is I can't speak German so I'll just have to take people's words for it now in this situation where our functional groups are on the same side you could call this sis sis beauty queen sis I believe comes from the Latin for together or or you could call it Z or you could call it Z beaut to een Butte to een and you could imagine this comes this Z comes from the German for together and it stands for is su Simon zu some in zusammen which is German for together and that's all you have to do and in the next few videos we'll do a bunch of examples here so we can really figure out whether to label something with either an e or a Z but I wanted to expose you to the sis and trans naming scheme because that sometimes used for simpler molecules where you only have one functional group on each side but I want you to realize that they're really trying to make the same differentiation they're really just trying to decide whether the functional groups on one side of a double bond and on the other side of a double bond whether they're on the same side sis or Z or zusammen or on different sides trans or E or n begin