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Identifying Possible Structural Isomers

  1. Aug 30, 2011 #1

    There are 4 structural isomers possible to draw... which I'd like to do :)

    The carbon skeleton isn't of much importance here, as we only have three carbon atoms. The two halides, however, are a pain, I've found.

    The linear sigma bonds may rotate around its axis, meaning that several seemingly different structures basically are identical. Also, we can rotate the entire molecule, so we can that way also acknowledge several seemingly different structures as identical.

    1) 2,2.dibromopropane

    2) 1,2-dibromopropane (Br at the end no bonded at the end of the carbon chain, but either "up" or "down", if you get my point)

    C - C - C

    3) 1,3-dibromopropane (each Br bonded to the ends of the carbon chain)

    4) 1,2-dibromopropane (Br-CH2-C(Br)H-CH3)

    Are these the correct structural isomers? Am I thinking correctly, here? I'm guessing that being able to efficiently identify isomers is a matter of practice and getting a "trained" eye, so to speak. I organic chemistry, they're telling us it's important to develop good 3D interpretations of 2D representations. A molecular building set is on the way from Evil-Bay

    I appreciate any help :) I'll try to be a member of this community, and help where I'm able, and not just a user.

  2. jcsd
  3. Aug 30, 2011 #2


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    I'm not quite sure what you mean here (especially since you also list 1,2-dibromopropane for #4).
  4. Aug 30, 2011 #3


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    There will be no "ups" and "downs". Just try to assign Br atoms to carbons in four different ways.
  5. Aug 31, 2011 #4
    That's what I thought I just had done :P

    Yes, I've basically given two structures the same IUPAC name, which is a little off, perhaps. I'll give it another shot, and upload a picture of what I'm doing soon.

    Thanks, folks!
  6. Sep 1, 2011 #5
    Here's another go. Do you agree with this?

    (don't mind the norwegian spelling of "structural isomers" on the top)

    Attached Files:

  7. Sep 1, 2011 #6


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    1,1 and 2,2 and 1,2 and 1,3 - that's correct.
  8. Sep 1, 2011 #7
    Okay, thanks!

    One question:

    In these structural isomers it doesn't matter which one of carbon's three available bonds the Br atoms are bonded to. What sort of isomers deal with that? The sigma bonds rotate around their axis, and so the "up" and "down" bonds practically are the same, but what about that bond on the end?

    That was poorly formulated! Sorry.
  9. Sep 1, 2011 #8


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    Google chirality.
  10. Sep 3, 2011 #9


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    If you include chirality, there are 5 isomers. Can you determine which of your positional isomers has an enantiomer?
  11. Sep 3, 2011 #10


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    How could I miss the chiral one? :grumpy:
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