JEE Main levelSingle correct optionDifficulty 2 of 3g2144
Which is the strongest acid among benzoic acid (4.20), 4-methylbenzoic acid (4.37), 4-methoxybenzoic acid (4.47) and 2,6-dimethylbenzoic acid (3.25)? Choose the correct reason as well.
A4-Methoxybenzoic acid; the +M effect of OCHX3 stabilises the carboxylate
B4-Methylbenzoic acid; the +I effect of methyl stabilises the carboxylate
CBenzoic acid; it has no electron-releasing substituent
D2,6-Dimethylbenzoic acid; two ortho methyl groups twist COOH out of the plane of the ring
Answer: D
The +I and +M groups in the para positions destabilise the carboxylate and lower acidity (4.37, 4.47). Two ortho methyl groups force the COOH group out of the plane, giving the ortho effect and the lowest pKa (3.25).