The increasing order of for the following phenols is
(A) 2, 4 - Dinitrophenol
(B) 4 - Nitrophenol
(C) 2, 4, 5 - Trimethylphenol
(D) Phenol
(E) 3-Chlorophenol
Choose the correct answer from the option given below:
(A) 2, 4 - Dinitrophenol
(B) 4 - Nitrophenol
(C) 2, 4, 5 - Trimethylphenol
(D) Phenol
(E) 3-Chlorophenol
Choose the correct answer from the option given below:
- A.(C), (E), (D), (B), (A)
- B.(C), (D), (E), (B), (A)
- C.(A), (B), (E), (D), (C)
- D.(A), (E), (B), (D), (C)
Show answer and solution
Correct option: C
Solution
Lower pKa means stronger acid (more stable phenoxide ion). Electron-withdrawing groups (NO₂, Cl) stabilize phenoxide by dispersing negative charge, increasing acidity (lowering pKa), while electron-donating groups (CH₃) destabilize phenoxide, decreasing acidity (raising pKa).
2,4-Dinitrophenol (A) has TWO nitro groups (one ortho, one para), giving maximum EWG effect → strongest acid, lowest pKa (~4.0). 4-Nitrophenol (B) has one para-nitro group → strong acid, pKa ~7.1. 3-Chlorophenol (E) has a meta-Cl (inductive EWG only, no resonance) → moderately acidic, pKa ~9.0. Phenol (D) is the reference with pKa ~10.0. 2,4,5-Trimethylphenol (C) has three EDG methyl groups raising electron density on the ring, destabilizing phenoxide → weakest acid, highest pKa (~10.6).
So increasing order of pKa: A < B < E < D < C, which is option 3.
2,4-Dinitrophenol (A) has TWO nitro groups (one ortho, one para), giving maximum EWG effect → strongest acid, lowest pKa (~4.0). 4-Nitrophenol (B) has one para-nitro group → strong acid, pKa ~7.1. 3-Chlorophenol (E) has a meta-Cl (inductive EWG only, no resonance) → moderately acidic, pKa ~9.0. Phenol (D) is the reference with pKa ~10.0. 2,4,5-Trimethylphenol (C) has three EDG methyl groups raising electron density on the ring, destabilizing phenoxide → weakest acid, highest pKa (~10.6).
So increasing order of pKa: A < B < E < D < C, which is option 3.
[JEE Main 2023 · 29 Jan · Shift 1]