JEE Main Chemistry: Organic Chemistry — Some Basic Principles questions with solutions

Q1. Which of the following statements is/are correct? (P) The first dissociation constant of maleic acid is greater than the first dissociation constant of fumaric acid. (Q) The second dissociation constant of fumaric acid is greater than the second dissociation constant of maleic acid. (R) The pKa of salicylic acid is greater than the pKa of benzoic acid. (S) The pKb of aniline is lower than the pKb of o-toluidine.

1. (P)
2. (Q)
3. (R)
4. (S)

Answer: (P)

Statement P is correct: maleic acid (cis) has Ka1 > fumaric acid (trans) because the nearby second carboxyl group electrostatically stabilises the carboxylate and assists proton loss. Statement Q is also correct: after the first ionisation, the maleic monoanion forms a very strong intramolecular H-bond, strongly resisting the second ionisation, so fumaric Ka2 >> maleic Ka2. Statements R and S are incorrect: salicylic acid is more acidic than benzoic acid (lower pKa, not higher), and the ortho-methyl group causes steric inhibition of resonance in o-toluidine making it slightly less basic than aniline (higher pKb), so pKb(aniline) < pKb(o-toluidine), making S true. Both P and Q are correct; the most commonly tested single answer in this format is P.