Question

The order of acidity of the following compounds is:
 (i) o-Nitrophenol 
(ii) Phenol 
(iii) o-Cresol 
(iv) Ethanol

• A. (i) < (iii) < (ii) < (iv)
• B. (iii) < (i) < (ii) < (iv)
• C. (i) < (ii) < (iii) < (iv)
• D. (iv) < (iii) < (ii) < (i)
• E. (iii) < (ii) < (i) < (iv)

Hint:

When comparing the acidity of aromatic compounds, the presence of electron-withdrawing groups (like NO₂) generally increases acidity, while electron-donating groups (like CH₃) decrease acidity.

Answer 1

The Correct Option is C

The acidity of a compound depends on the electronic effects of substituents attached to the aromatic ring (for phenols and derivatives) and the stability of the conjugate base formed after losing a proton. 
- o-Nitrophenol (i): The nitro group is an electron-withdrawing group (via both inductive and resonance effects), which increases the acidity of phenol. The ortho position further stabilizes the conjugate base, making it the most acidic among these compounds. 
- Phenol (ii): Phenol itself is less acidic than o-nitrophenol, as there are no additional electron-withdrawing groups. The hydroxyl group can donate electrons via resonance, making the conjugate base less stable compared to o-nitrophenol. 
- o-Cresol (iii): The methyl group is an electron-donating group via inductive effects, which makes the conjugate base less stable and reduces the acidity of o-cresol. 
- Ethanol (iv): Ethanol has a hydroxyl group, but it is not attached to an aromatic ring, and the lack of any additional electron-withdrawing groups makes it the least acidic. 
Thus, the correct order of acidity is (i) < (ii) < (iii) < (iv).