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cis-trans and E-Z naming scheme for alkenes

Cis-Trans and E-Z Naming Scheme for Alkenes. Created by Sal Khan.

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  • blobby green style avatar for user Tzvi Shumulinskiy
    i was taught that the way you name it is 2-butene, not but-2-ene?
    (30 votes)
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  • blobby green style avatar for user TDAppleDuck
    So (E)-but-2-ene and (Z)-but-2-ene are diastereomers, right?
    (20 votes)
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  • blobby green style avatar for user Mackenzie
    Can you please explain cis and trans a little bit more? Are they only involved in alkenes or can they be involved in alkanes as well? And when should I use E and Z vs. cis and trans?
    (6 votes)
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    • blobby green style avatar for user Moff
      Cis and Trans isomerism only happens around an alkene because they're locked into the positions thanks to the pi structure. You don't get any rotation around a pi bond whereas you do around a sigma bond so theres no cis and trans for alkanes

      So if you had two methyl groups in the 2,4 positions of butane, one above and one below the chain it wouldn't be trans-2,4-dimethyl-butane because the main chain can rotate and essentially bring them into the cis arrangement so it just doesn't work.

      E and Z comes in when you have and alkene with more than two substituents such as CFBR-CClH where you can't say exactly say which is cis and which is trans: The rules for this are, the higher its atomic mass the more priority the atom has so if Br and Cl are on the same side across the double bond the molecule is Z where as if they're on opposite sides across the bond the molecule is E :)
      (11 votes)
  • blobby green style avatar for user Sai Kiran
    The priority order of functional groups for naming hydrocarbons
    (4 votes)
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  • leaf green style avatar for user Humaira Tariq
    In the video Sal says the CH3 group is a functional group. What does he mean by this because I thought hydrocarbon chains aren't functional groups?
    (3 votes)
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  • leaf green style avatar for user Kris Kalavantavanich
    Which is correct:
    cis - 2, 4 - dichloro - 3 - propylhex - 2 - ene or
    2, 4 - dichloro - 3 - propyl - cis - hex - 2 - ene ?
    (2 votes)
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  • blobby green style avatar for user cmyknow
    What is a functional group?
    (1 vote)
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  • male robot donald style avatar for user Shivesh Kumar Singh
    is it necessary for cis/trans to have the same functional group on either side of double bond
    (2 votes)
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    • leaf green style avatar for user Dave M
      No. There is no reason to have the same functional group on both sides because you will never say "cis" or "trans" for an alkene except in conversation. Organic chemists use E or Z exclusively. The ones who don't are....still stuck in freshman biology.
      (2 votes)
  • spunky sam blue style avatar for user Chunmun
    So why we use syn and anti naming system when we already have cis - trans and E-Z naming system?
    (2 votes)
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    • orange juice squid orange style avatar for user awemond
      Alkenes are usually named using cis/trans or E/Z. Syn and anti are reserved to indicate the geometry of addition across a double bond when it undergoes reactions: if two atoms add from the same side of the double bond then it is called syn, and if they add from opposite sides of the double bond, then it is called anti addition. Anti and syn are sometimes used in common names, such as in oximes to indicate the orientation of the OH group; there's no particular reason why oximes can be named that way, that's just how the (non-systematic) naming emerged.
      (2 votes)
  • duskpin ultimate style avatar for user Rodrigo
    Why does 1,2-dimethylbenzene does not have geometric isomery?
    (2 votes)
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    • leaf green style avatar for user Dave M
      Simple answer: There is no way to make geometric isomers of 1,2-dimethylbenzene.

      The molecule rotates, but no more than any other. A geometric isomer (outdated term) is now know as an E/Z double bond. Can you make the double bond between the two methyls anything other than "Z". It's impossible to place an E double bond inside a ring with fewer than 8 carbons...unless the ring gets bigger. That's why. The prior response doesn't include any explanation at all.
      (3 votes)

Video transcript

If I were to come over to your place and say, name this molecule, the first thing you would do is you'd 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 but-, and regardless of which end you count from, the double bond starts at the number two carbon, so this is but-2-ene. I'd say, well, 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? And you'd 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 double bond starts at the number two carbon: one, two, three, four. So once again, you say, hey, Sal, that's also but-2-ene. And 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-- it's not like it can switch conformations between this and that. It cannot rotate around this double bond, so these are different molecules. These are 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 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 two 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 is if you have your functional groups. So let me circle the functional groups. So in 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, I believe, Latin for, apart, or Latin for opposite. I'm not a Latin scholar, so forgive me. But the functional groups are apart, so we call it trans-but-2-ene. The other convention to use comes from the German forepart and there we call it entgegen. And instead of writing out entgegen, we write E in parentheses. So we would call them-- let me just use different colors since this molecule down here is blue. So we would call this (E)-but-2-ene. These mean the same thing. (E)-but-2-ene. 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. Let me just make it clear that the (E) stands for entgegen, which is German for apart, 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 cis-but-2-ene. Cis, I believe, comes from the Latin for together, or you could call it (Z)-but-2-ene. And you could imagine this (Z) comes from the German for together. And it stands for 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 expose you to the cis- and trans- naming scheme, because that's 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, cis-, or (Z), or zusammen, or on different sides, trans- or (E), or entgegen.