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Homework Help: Polarity of Tetraphenylporphyrin?

  1. Oct 21, 2012 #1
    1. The problem statement, all variables and given/known data
    What is its polarity of a molecule of Tetraphenylporphyrin (TPP) as a whole?

    2. The attempt at a solution
    Given the structure of TPP, there is a ring inside that contains two N-H bonds across from each other and two nitrogen atoms with lone pairs across from each other like below:


    As thus, I am to believe that these dipoles cancel each other out, leading to a molecule that is overall nonpolar. Can someone justify my argument?

    Attached Files:

    • TPP.png
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    Last edited: Oct 21, 2012
  2. jcsd
  3. Oct 22, 2012 #2
    It is a matter of symmetry. One way to look at it is as follows: the molecule has a centre of symmetry. A dipole is like a vector that points in a single direction. But you cannot have a dipole with this symmetry, because if you inverted the molecule through the centre of symmetry, the molecule would be exactly the same but the dipole would point in the opposite direction. That is a contradiction unless there is a zero dipole.

    Your thinking and your argument was quite correct.
  4. Oct 22, 2012 #3
    Thanks for justifying my answer. Now on a side note, if TPP was to react with copper (II) acetate with the following formula:

    [itex]TPP + Cu(CH_{3}COO)_{2} \leftrightarrow 2(CH_{3}COOH) + \left[Cu^{2+}\right]\left[TPP^{2-}\right][/itex]

    TPP will have its N-H bond, inside the ring, to break, releasing hydrogen ions for acetate to take. The resulting products are acetic acid and a Copper TPP complex. The Copper TPP complex is the TPP molecule, with a Copper (II) ion placed right inside the ring of TPP.

    1. The problem statement, all variables and given/known data
    Now given this new Copper TPP complex, what is its polarity?

    2. The attempt at a solution
    Given that TPP was initially nonpolar, the addition of the copper (II) ion would most likely cause the new complex to be polar? I believe this is due to the electrons inherent within the orbitals of the copper (II) ion (electronic configuration is [Ar] 3d9, granting 27 electrons). As a result, the complex will have negativity within the ring of TPP.

    Can you please justify my new argument? Thanks!
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