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Homework Help: Which of these is more polar?

  1. Nov 5, 2011 #1
    1. The problem statement, all variables and given/known data

    I'm trying to assign the spots for a TLC plate that I did in lab.


    What is the order of polarity of these three molecules?

    2. Relevant equations

    The more polar something is, the lower it will stay on a TLC plate. The less polar it is, the further it will travel up.

    3. The attempt at a solution

    I have three spots on my plate, I'm trying to figure out what is what.

    I think that 3 is the most polar because of the OH groups and the fact that it forms B- in solution with another reactant that is in there.

    I just don't know about 1 and 2. Br is electronegative, but I don't know if this decreases the polarity because it is para to a group with an electronegative oxygen, or if it increases the polarity (compared to the phenyl ring).
  2. jcsd
  3. Nov 5, 2011 #2
    I agree, molecule 1, even though the caboxyl group is very polar (because of the high electron density) the molecule will no exhibit great polarity because R-group to the left is to big.
    Number 2, Br is not as electronegative as Oxygen (can you explain why? look at the electronegativity trend). In fact these two electronegative atoms are on opposite sides of the molecule. What does this mean?
    molecule 3, B is extremely 'weak' (even hydrogen is more electronegative) so the two oxygen will pull all the electrons away from Boron, since these two pull the electron toward the same direction, i conclude that this molecule is the most polar
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