# Combined Effect of Steric Inhibition of Resonance and Inductive Effect

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?

• +I is a very weak effect compared to loss of coplanarity due to sterics. Feb 20 '21 at 10:20
• the grouping $\ce {-CH_3}$ is too far for its + I effect to be felt on the acidity Feb 20 '21 at 10:22