Exams › JEE Advanced › Chemistry
Which of the following correctly represents the order of acid strength for the given substituted benzoic acids?
- o-hydroxybenzoic acid > m-hydroxybenzoic acid > p-hydroxybenzoic acid
- m-methylbenzoic acid < m-trideuteromethylbenzoic acid < m-tert-butylbenzoic acid
- Benzoic acid with -NH3+ substituent > benzoic acid with -NMe3+ substituent > benzoic acid with -CH3 substituent
- o-chlorobenzoic acid > p-chlorobenzoic acid > m-chlorobenzoic acid
Correct answer: o-hydroxybenzoic acid > m-hydroxybenzoic acid > p-hydroxybenzoic acid
Solution
Option A: o-OH > m-OH > p-OH for hydroxybenzoic acids. The ortho isomer (salicylic acid) is most acidic due to intramolecular H-bonding in the anion. The para-OH is electron-donating via resonance (decreases acidity) while meta-OH only has inductive effect. So o > m > p is correct. Option B: -CD3 vs -CH3: deuterium has weaker hyperconjugation than H, so m-CD3 is slightly more electron-withdrawing than m-CH3 (less hyperconjugation donation). m-tert-butyl is more electron-donating than m-CH3 (more hyperconjugation). So acidity: m-tBu-benzoic acid < m-CH3-benzoic acid < m-CD3-benzoic acid. The stated order is reversed. Option C: -NH3+ and -NMe3+ are both electron-withdrawing (+I effect absent, strong -M absent but strong +charge EWG). -NH3+ is more acidic than -NMe3+ because NH3+ has additional H available for H-bonding/solvation. -CH3 is electron-donating (decreases acidity). Order should be -NMe3+ > -NH3+ > -CH3 for acid strength based on EWG power (NMe3+ has stronger inductive EWG than NH3+), or the stated order may be wrong. Option D: o-Cl > p-Cl > m-Cl for chlorobenzoic acids. Cl is EWG by inductive and EDG by resonance. At ortho: both inductive and field effects enhance acidity. At para: resonance donation decreases acidity. At meta: only inductive (no resonance at meta). So order: o > m > p. Option D states o > p > m which is incorrect. Option A is correct.
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