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Homework Help: Shielding Effect and Chemical Shift

  1. Oct 2, 2008 #1
    1. The problem statement, all variables and given/known data
    Given a molecule of chloroethene (vinyl chloride) or ClCH=CH2. Rank the hydrogen atoms in the molecule in terms of which will have the greatest chemical shift.


    2. Relevant equations
    None


    3. The attempt at a solution
    Since the Cl and the double bond are both electron-rich, this affects the shielding of each proton. So to rank them, I think it could go one of two ways:
    1) The proton on the Cl-CH= is the most blinded (smallest chemical shift) since it is between the two electron-rich groups, followed by the proton in the cis position to the Cl since it is slightly closer to the electronic density than the H in the trans position to the Cl.
    OR
    2) The reverse order so: the H on the Cl-CH= is the least blinded (largest chemical shift) because the Cl is electronegative and is withdrawing the electron density from the H. Then the H in the cis position to the Cl would be 2nd rank in terms of blinding and then followed by the H in the trans position to the Cl (being the least blinded) because all of its electron density is being pulled toward the double bond and the Cl.

    I think I'm a bit mixed up on how electronegativity works, so any help would be greatly appreciated. Thanks in advance!!
     
  2. jcsd
  3. Oct 8, 2008 #2

    chemisttree

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    No. Halogens are electron rich relative to a similar carbon group but they get that way in part because they withdraw electron density from the carbon they are attached to. Therefore, the remaining carbon is electron-deficient so the geminal C-H bond is polarized with more electron density being pulled toward the carbon. This effect is called de-shielding.
     
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