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how can this allene be chiral?

Discussion in 'DAT Discussions' started by joonkimdds, Dec 24, 2008.

  1. joonkimdds

    joonkimdds Senior Member
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    Ch3-CH=C=C-CH3(C2H5)

    this is from DAT destroyer. It says this should be chiral and optically active.
    and then it says
    CH2=C=C-CH3(C2H5) is not chiral and not optically active.

    So I think the main focus of these comparison is the carbon on the left that is double bonded to the next carbon.


    I thought chiral is the one with stereocenter meaning carbon has to be attached to 4 different atoms or groups. If it's double bonded, how can that be stereocenter?

    and is Meso the only exception that can have stereocenters but still called achiral? Is it because they cancel each other? If so, what about other non meso structures that have same amount of R and S? Do they cancel each other too and called achiral?
     
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  3. doc3232

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    Meso is not optically active because of the symmetry. The symmetry causes the compound to appear like a racemic mixture. Makes sense right?

    As for these allenes. Just think about the 4 groups as the ones that the OUTSIDE carbons are attached to. The bottom one has 3 different groups (There are 2 hydrogens which only counts as 1 group).

    The allene question is a bit tricky and I don't think the DAT would have it, but probably good to know.
    Hope this helps.
     
  4. Sublimation

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    I have this chem sketch program that can do wonders when u wanna visualize things ill try my best to post both molecules for u.
     
  5. Sublimation

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    sorry for the late response.
     

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  6. joonkimdds

    joonkimdds Senior Member
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    Thank you for drawing it out for me :)

    By the way, does this have nothing to do with the fact that chiral should have 4 different groups attached to a carbon?
     
  7. Sublimation

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    I kno alot of ppl think once they see a asymetrical carbon. A carbon with 4 diff. substituents u think right off the bat its chiral. Not necessarly, those compounds that do have a asymetrical carbon and are achiral are known as meso compounds. Now we know one rule of fact, that has no acceptions. If there is a line of symmetry in the molecule then it is difinetly achiral. If there is no line of symmetry then you have a chiral compound :).
     
  8. joonkimdds

    joonkimdds Senior Member
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    wu wu wu wu wu wu...wait!!!
    I thought
    Chiral = 1(ONE!) carbon that is attached to the 4 different group.

    and now it seems that this question is saying
    C=C=C is one huge carbon thing.
    That's 3(THREE!) carbon but do we count as one big center that can be chiral if 4 different groups are attached to that giant carbon monster? o_O



    and the solution says
    H2C=C=C-C2H5(CH3) is achiral, the groups must be different if bonded to the same carbon.
    I thought all 4 groups are supposed to be different but this is saying that the only 2 groups on the same carbon should be different! o_O;
     
  9. Sublimation

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    to be completely honest, im confused as to wat ur asking. Google chemsketch, and you should find a free download for it. download and draw the molecule and it will give oyu a 3dimensional rep of how it looks. then lookfor a line of symmetry. Look if you have one C atom. Just one. And it has 4 diff. groups attached. Then IT MUST BE CHIRAL. ONLY IF YOU HAVE ONE CARBON ATOM. NOW they said for the sake of i dont know for some weird reason. THey said, "hey lets condense this and treat these 3 Carbons like they were one carbon". <<<<<NOTE:this will only work with allenes. Do you know why? if not let me know, it will take me a bit to explain and im about to pass out and i cant .....stay.....foc....uss......eeeedd. Soory for the tangent lol. Now if you apply the rule <as we are imagining these 3 carbons as if they were one> like this

    (H) (H) [C=C=C] (CH3) (C2H5)
    ^^^^Pretend it was (WTF, imagine the ^^ to be under the [C=C=C] and the [C] under that :)
    [C]

    Thats wat they are saying. Lets play pretend and now if we are playing pretend and [C=C=C] = [C] By the rule we stated above this will have only 3 diff. groups. therefore its achiral. However, the other one has 4 diff. groups making it Chiral :) Hope i cleared the air for u :)
     
  10. joonkimdds

    joonkimdds Senior Member
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    Crystal Clear! :D
     
  11. doc3232

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    Thanks, well explained.
    But I doubt they would put an allene on the O-chem section considering everyone has said it is the fairest section. But i wouldn't chance it :)
     

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