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Help with Question-Stereoisomers

  • 16-09-2009 1:25am
    #1
    TeganRainIsGod
    Closed Accounts Posts: 18 ✭


    Lads can someone help me answer this question,

    The correct answer is C structure 3, but i cant quite understand why...

    Any help appreciated

    Thanks


Comments

  • #2
    Creature
    Registered Users, Registered Users 2 Posts: 3,005 ✭✭✭Creature


    It's because no matter how many times you rotate structure III around you can't get it to match any of the other three. It will never be superimposable on any of the others unless you change around the arrangement of atoms on the molecule. It really helps if you get a molecular modelling kit for this sort of thing.


  • #3
    Marvinthefish
    Registered Users, Registered Users 2 Posts: 193 ✭✭Marvinthefish


    The third structure is a different configuration of atoms. It's like 1,2 and 4 have two arms and legs in the right places, whereas 3 has an arm, where a leg should be! (Arms and legs analogy taken from "Org Chem" by Clayden et al.)


  • #4
    Professor_Fink
    Registered Users, Registered Users 2 Posts: 861 ✭✭✭Professor_Fink


    TeganRainIsGod wrote: »
    Lads can someone help me answer this question,

    The correct answer is C structure 3, but i cant quite understand why...

    Any help appreciated

    Thanks

    It's relatively easy to see. Try imagining an axis composed of C and W. This has the other three arranged in a circle on the far side of C from W. Now, if we are looking along the axis with W closest to us, them going clockwise around starting with X we have in:
    i) XZY
    ii) XZY
    iii) XYZ
    iv) XZY
    Since there is no freedom in this standard way of describing the configuration, we see iii) is a fundamentally different configuration to each of the others, and so cannot be superimposed by rotation and translation alone.


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