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JEE Advanced levelSingle correct optionDifficulty 3 of 3g2210

Assertion (A): 2,6-Dimethylbenzoic acid is a stronger acid than 4-methylbenzoic acid. Reason (R): The two methyl groups stabilise the carboxylate ion by their electron-donating (+I) effect, and this is what raises the acidity.

  1. ABoth A and R are true and R is the correct explanation of A
  2. BBoth A and R are true but R is not the correct explanation of A
  3. CA is true but R is false
  4. DA is false but R is true

Answer: C

The assertion is true ( 3.25 against 4.37) and is caused by the ortho effect: the two ortho methyl groups twist out of the ring plane, cutting off conjugation of the carboxyl with the ring and so raising acidity. The reason is false: electron-donating methyl groups destabilise, not stabilise, a carboxylate ion, so by themselves they would weaken the acid. Hence A is true but R is false.
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