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Which one is a stronger acid?

  1. Oct 24, 2010 #1
    1. The problem statement, all variables and given/known data
    Which one is a stronger acid- o- Toluic acid or p- Toluic acid?


    3. The attempt at a solution

    I think o-Toluic acid will be less acidic because CH3 - group is nearer to the O-H bond and it will exert more +I effect compared to that in para- position. But at the same time o - Toluic acid increases the electron density of -COOH group by a greater amount than p - Toluic acid.

    What is the correct answer and why?
     
  2. jcsd
  3. Aug 21, 2017 #2

    TeethWhitener

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    Gold Member

    This is an example of what is known as the "ortho effect," and it occurs because of the existence of a somewhat bizarre resonance structure: the carboxyl group can form a weak hydrogen bond-like interaction with the methyl group ortho to it, stabilizing the conjugate base anion and therefore making the molecule more acidic. This interaction is absent when the methyl is para to the carboxyl. Here's a picture, along with a reasonable QTAIM analysis:
    https://chemistry.stackexchange.com...ffect-in-substituted-aromatic-acids-and-bases
     
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