# Acidic strength of organic acids. Substituted PhCOOH acids

Could someone please explain the order of acidity of ortho, para and meta substituted benzoic acids with the substituent as methyl group? Methyl group operates +I and +R effect, I believe.

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• – Faded Giant Apr 16 '16 at 10:54

The methyl group, when at ortho position, causes steric inhibition of resonance, which essentially means that the $\ce{-COOH}$ group goes out of the plane of the benzene ring (due to steric hindrance from the $\ce{-CH3}$ group). Due to this, the benzene's $\pi$ electrons can no longer be transferred to the central carbon of the $\ce{-COOH}$ group by resonance, hence increasing the positive charge on it. This makes the hydrogen of the $\ce{-COOH}$ group more acidic.