While qualitatively comparing the acidic nature of o-methyl benzoic acid and benzoic acid, we consider the steric effect due to which the $\ce{-COOH}$ group loses its planarity and the +M effect of $\ce{-CH3}$ isn't considered any longer. But still, the +I of $\ce{CH3}$ is functioning, which increases the electron density of the compound and reduces its acidity. Thus o-methyl benzoic acid should be less acidic than benzoic acid. But while confirming my results with the experimental data, I found that $\ce{pK_a}$ of o-methyl benzoic acid is 3.90 while that of benzoic acid is 4.20. How can this be explained using the basic concepts of electronic displacement effects?


  • $\begingroup$ +I is a very weak effect compared to loss of coplanarity due to sterics. $\endgroup$ Feb 20, 2021 at 10:20
  • $\begingroup$ the grouping $ \ce {-CH_3} $ is too far for its + I effect to be felt on the acidity $\endgroup$
    – Nicolas
    Feb 20, 2021 at 10:22


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