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Which is more acidic 2-methyl benzoic acid , 3-methyl benzoic acid or 4-methyl benzoic acid.?

Experimentally the pKa value of 4-Methylbenzoic acid (Pka= 4.362), 3 -Methylbenzoic acid (Pka=4.269), Benzoic acid (pKa=4.19), 2-Methylbenzoic (Pka=3.90).

Hence acidic order is 
2-Methyl Benzoic acid > benzoic acid > 3–Methylbenzoic acid > 4-Methylbenzoic acid 

The methyl group at ortho (2nd position) and para (4-position) destabilises Negative charge of corresponding Carboxylate ion by both +I effect (ERG)) as well as by +H (hyperconjugation). But at meta position only +H effect by methyl  does not affect the carboxylate.(From here only +I effect operates). So we can easily understand that m-isomer is more acidic than para-isomer.

2-Methyl Benzoic acid (or o-toluic acid) is stronger than benzene and it's methyl derivatives because steric inhibition of resonance (SIR effect)  due to this effect the carboxyl group being out of the plane of the aromatic ring.The carboxy group being out of the plane in 2-methyl benzoic acid, which inhibiting +R effect and eliminates the factor of +H and increase acidic character of benzoic acid.

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