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ochem question

Discussion in 'DAT Discussions' started by ejaehak38, Apr 8, 2007.

  1. ejaehak38

    ejaehak38

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    Dec 1, 2006
    which of the following is capable of exhibiting cis-tran isomerism?

    a. ethene
    b, 1-pentene
    c. 1-butene
    d. 2-butene
    e. cyclohexene














    Can you tell me why the answer is A??
     
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  3. jdmsamurai

    jdmsamurai 2+ Year Member

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    Feb 11, 2007
    are you sure the answer is A? i dont think that is possible...i think the answer is suppose to be D. 2-butene
     
  4. gochi

    gochi 10+ Year Member

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    Nov 24, 2006
    Yea....it looks like 2-butene..however, couldnt all teh answers be correct, b/c all ofthem are ALKENES, and we can draw the H's as comming into the page or going out fo the page, so that will be cis/trans ISOMERISM, right ?
     
  5. DailyDrivenTJ

    DailyDrivenTJ 2+ Year Member

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    Mar 20, 2007
    No.

    cis/trans are designation between different atoms attached to a carbon carbon double bond to clarify their relative positional difference. For the ethene, the 4 hydrogens attached to the double bonded carbons are identical thus cis and trans designation serves no purpose.

    The answer can only be 2-butene because that allows both cis and trans conformation.
     
  6. skyisblue

    skyisblue 2+ Year Member

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    Dec 25, 2006
    I agree. The answer can't be a, b, or c. 2-butene has cis and trans isomerism b/c of the different substituents on the carbon atoms.

    For cyclohexene, it can't have cis and trans isomerism b/c the 2 carbon's on the double bond is sp2-hybridized, thus planar right Daily? So the 2 H's can't be both up out of the plane or on opposite sides?? BUT when you think about it both carbons on the double bond have 2 different substituents...
     
  7. DailyDrivenTJ

    DailyDrivenTJ 2+ Year Member

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    Mar 20, 2007
    The cyclohexene can only exist in cis conformation. However, cycloheptene and any other cyclo alkene with 7 carbons or more can exist in both cis and trans.
     
  8. skyisblue

    skyisblue 2+ Year Member

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    Dec 25, 2006
    Why is cyclohexene cis only?

    How are the > and = to 7 cycloalkenes both cis and trans when the carbons are sp2-hybridized thus is trigonal planar?
     
  9. DailyDrivenTJ

    DailyDrivenTJ 2+ Year Member

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    Mar 20, 2007
    Draw out cyclohexene. You get cis conformation with the carbon carbon double bond.

    Cycloheptene, can be drawn like cyclohexene ring (in cis) and as trans conformation with double bonded carbon with two other substitutent carbons in trans to complete the ring structure. So it won't look like a ring per se but more like a half eaten pie.

    In a nutshell, anything less than 7 carbon will have so much strain in the ring formed in trans conformation, it will not keep its structure. FYI, cyclohexene in trans only exists with half life of 45 sec.

    If you don't understand my half-a$$ed explanation, I suggest you reread your textbook. :)
     
  10. skyisblue

    skyisblue 2+ Year Member

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    Dec 25, 2006
    Hi Daily. Thanks. I read like both chapters on alkenes, but let's see...

    For Cyclohexene would you say that the 2 hydrogens on the carbon-carbon double bond is out of the plane of the double bond and is on the same side which makes it cis?

    I guess one can say that any cycloalkene bigger than and equal to 7 carbons will be "floppy" so the other carbons can be up or down making cis and trans isomers possible...yeah?
     
  11. DailyDrivenTJ

    DailyDrivenTJ 2+ Year Member

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    Mar 20, 2007
    I am not sure about it being out of plane WRT the carbon double bond since sp2 carbons are perceived as flat. However, you are right in that the carbon substituent are facing the same direction, thus it is cis.

    I am not sure if I qualify to answer your second question but I can see how that can be used as an analogy to understand this phenomenon. Thus I agree with your explanation. :) YES.
     

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