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Trichloroacetic acid: electron density and stability

  1. Oct 20, 2009 #1
    "The rather strong acidity of trichloroacetic acid is usually ascribed to the inductive effect of the three chlorine atoms attached to the end of the molecle opposite the acidic proton. Electron density is withdrawn away from the carboxylate group so that the trichloroacetate anion that is formed when the acid dissociates is stabilized." (Skoog, West, Holler & Crouch, 2003)

    Isn't oxygen more electronegative than chlorine? So why does the electron density shift in favour of the chlorine atoms? Is it because they outnumber the oxygen atoms, or is it a consequence of the structure (shape) of the molecule?
    Also, what is the meaning of the term "stabilized" as used here?
    Any input appreciated. I will also continue to search elsewhere; so far it's been to no avail.
  2. jcsd
  3. Oct 20, 2009 #2


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    The chlorine atoms are replacing hydrogen atoms and chlorine is certainly more electronegative than hydrogen. So, while the chlorine atoms are not going to pull all or most of the electron density away from the oxygen, they will pull some electron density away and the oxygen atom will have less electron density than if the chlorines had not been present.
  4. Oct 21, 2009 #3


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    Staff: Mentor

    Stabilization most likely refers to the carboxylic group.

  5. Oct 21, 2009 #4
    Thanks Yggdrasil and Borek. For some reason I took it to mean a change after the proton had departed, but as your explanation makes clear its disucussing the molecule as it stands.

    What does it mean by stable in this usage? If it was unstable, or less stable, what would this mean?
  6. Oct 21, 2009 #5


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    Staff: Mentor

    Think resonance.

  7. Oct 23, 2009 #6
    Thanks Borek.
    I think I understand:
    The electronegativity of the chlorine atoms draws the electrons, to a small extent, as pointed out above, towards themselves. This 'slight' (?) delocalization of the electrons lowers the overall energy of the molecule; whereas, without the chlorine atoms, the electrons would generally reside in a much smaller area (around the oxygen atoms), and therefore the overall energy of the molecule would be higher, and therefore the molecule would be less stable.
    Have I understood resonance correctly? And if so, have I understood it's application here?
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