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

AIcohols and phenols. borh are acidic in nature because they Iiberate H+ with active metals. But alcohols are weaker acids than phenols because,  

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.

What is Steric Inhibition of Deprotonation effect ? Or SID effect ?

We know that picric acid is weak acid than expected this is fact that when picric acid releases a proton it becomes an anion for which repulsion results between oxygen atoms of phenoxide ion and oxygen of nitrate group, commonly known as Steric Inhibition of Deprotonation (SID effect). This also results in slight unstability of phenoxide ion and also decrease acidity. Thus picric acid is a weak acid than expected ever.

Related Questions:

1. Highly branched carboxylic acids are less acidic than unbranched acids. Explain why?
2. Benzamide is less easily hydrolysed than methyl benzoate. Why ?
3. Alcohols and Phenols both are acidic but phenols are more acidic. Why?
4. Which is more acidic 2-methyl benzoic acid , 3-methyl benzoic acid or 4-methyl benzoic acid.?
5. Why is o-toluic acid (2-methyl benzoic acid) stronger than benzoic acid ?
6. What is the ortho effect? Why are nearly all ortho substitute benzoic acids (except some groups) stronger acid than benzoic acid?
7. Name of groups which does not show ortho effect in benzoic acid?
8. Why Picric acid (2,4,6-trinitrophenol) is weak acid then expected ?
9. What is Steric Inhibition of Deprotonation effect ? Or SID effect ?
10. Alcohols and Phenols both are acidic but phenols are more acidic. Why?
11. Among orthochlorophenol and orthofluorophenol, which will be a stronger acid and why?
12. Why The phenoxide ion is more stable than phenol ?
13. What is correct acidic strengths order of the haloforms acids ? Give correct explanation.
14. Lower aliphatic acids having 1 to 5 carbons are soluble in water but higher fatty acids having six or more than six carbon are insoluble in water but soluble in alcohol. Explain.
15. Phenoxide ion is more stable than an alkaoxide. why?
16. Why does aqueous sodium bicarbonate solution dissolve carboxylic acids but not phenol though they are also acidic ?
17. Chloroform is more acidic than fluoroform why?
18. Fluorine is more electronegative than chlorine even then, p-flurobenzoic acid is weaker acid than p-chlorobenzoic acid explain ?