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Homework Help: Acidity of phenol

  1. Sep 28, 2008 #1
    1. The problem statement, all variables and given/known data
    How do you explain the difference in acidity of the ortho, meta, and para- fluorophenols.

    2. Relevant equations

    3. The attempt at a solution

    I know that EWG enhance the acidity of phenols because they stabilize the delocalized negative charge.
    I also drew out the resonance structures and saw that ortho and para positions have 4 resonance structures, whereas meta has only 3 structures. This means ortho and para would be more stable. However, the pKa show different.
    pKa of phenol = 10.0
    pKa of ortho-fluorophenol = 8.81
    pKa of meta-fluorophenol = 9.28
    pKa of para-fluorophenol = 9.81

    How do you explain this?
  2. jcsd
  3. Sep 28, 2008 #2


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    Science Advisor
    2017 Award

    If the fluorine were affecting the acidity via resonance, fluorine would act as an electron donating group, rather than an electron withdrawing group. The inductive effect is more important here than the resonance effects because resonance structures where the fluorine has a + charge are very small contributors.
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