Question

Compare acidic strength of the following compounds:
 (I) Phenol (II) \(p\)-Nitrophenol (III) \(p\)-Methylphenol (IV) \(p\)-Nitrobenzoic acid (V) Benzoic acid

• A. II \(>\) IV \(>\) V \(>\) I \(>\) III
• B. IV \(>\) V \(>\) III \(>\) I \(>\) II
• C. IV \(>\) V \(>\) II \(>\) I \(>\) III
• D. IV \(>\) II \(>\) I \(>\) V \(>\) III

Answer 1

The Correct Option is C

To compare the acidic strength of the compounds given, we need to consider the structural effects of substituents on acidity. The more stable the conjugate base (anion formed after releasing \(H^+\)), the stronger the acid.

1. Phenol (I): Phenol has a hydroxyl group attached directly to the aromatic ring, making it weakly acidic due to resonance stabilization of the phenoxide ion.
2. \(p\)-Nitrophenol (II): The presence of a nitro group at the para position increases acidity due to its strong electron-withdrawing effect, which stabilizes the conjugate base through resonance and induction.
3. \(p\)-Methylphenol (III): The methyl group is an electron-donating group, decreasing acidity by destabilizing the conjugate base. Thus, it is less acidic than phenol.
4. \(p\)-Nitrobenzoic acid (IV): The nitro group significantly enhances the acidity of the benzoic acid by stabilizing the carboxylate ion. Benzoic acid groups themselves are very acidic due to resonance stabilization.
5. Benzoic acid (V): Benzoic acid is acidic but less acidic than its nitro-substituted counterpart.

Based on the above analysis, the acidic strength order is:

• IV (\(p\)-Nitrobenzoic acid) > V (Benzoic acid) > II (\(p\)-Nitrophenol) > I (Phenol) > III (\(p\)-Methylphenol)

The correct answer is IV > V > II > I > III, which is represented by the option:

IV \(>\) V \(>\) II \(>\) I \(>\) III