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?
