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Geometric isomers

  1. May 4, 2005 #1
    Hi everyone... can anyone help me out? I am not sure if i am on the rigth track.

    Why do geometric isomers exist in alkenes, but not in alkynes?

    Does it have something to do with the that alkenes have a double bond and alkynes have a triple bond? Also the pi bond restricts the rotational movement of the molecule and locks the groups in place. But how does that change in alkynes? I understand why geometric isomers cannot exist in alkanes beacuse of the sima (single bond) but why can't they exist in the alkynes?

    Any help is greatly appriciated... thanks
  2. jcsd
  3. May 4, 2005 #2


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    There is restricted rotation with alkynes too, but rotation around the triple bond wouldn't form a different compound as it could with a double bond. Think about the shape of the molecules; alkynes are linear with two substituents, one on each end. Alkenes are not linear: there are four distinct positions where different groups can be. Switching the position of two of the groups at one end of an alkene can form a geometric isomer, but in an alkyne there aren't two distinct groups to switch!
  4. May 4, 2005 #3
    alkynes have a triple bond which ristricts these of having 4 extra bonds (2 per C making the triple bond)which you should have when considering a geometric isotomer
  5. May 5, 2005 #4
    Thanks a lot for clearing that up. It makes perfect sense now.

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