The increasing order of pKa for the following phenols is: (1) 2,4-Dinitrophenol (2) 4-Nitrophenol (3)…

Step 1: Rank by acidity (most acidic = lowest pKa)

• (1) 2,4-Dinitrophenol: Two $\ce{-NO2}$ groups (EWG) at ortho and para → very strong stabilization of phenoxide → pKa ≈ 4.0 — most acidic.

• (2) 4-Nitrophenol: One $\ce{-NO2}$ at para → resonance + inductive stabilization → pKa ≈ 7.1.

• (5) 3-Chlorophenol: $\ce{-Cl}$ at meta → only inductive effect (EWG -I) → slightly increases acidity. pKa ≈ 9.0.

• (4) Phenol: No substituent → pKa ≈ 10.0.

• (3) 2,4,5-Trimethylphenol: Three $\ce{-CH3}$ groups (EDG, +I effect) → destabilize phenoxide → least acidic. pKa ≈ 10.6.

Step 2: Increasing pKa order (least acidic to most acidic in terms of pKa value)

Increasing pKa = starting from lowest pKa (most acidic) to highest pKa (least acidic): 1 < 2 < 5 < 4 < 3

(pKa: 4.0 < 7.1 < 9.0 < 10.0 < 10.6)

Answer: 1, 2, 5, 4, 3 ✓

Key Points to Remember:

• Increasing pKa = decreasing acidity (remember: lower pKa = stronger acid).
• EWG (NO2 > Cl > F at o/p) decrease pKa (increase acidity).
• EDG (CH3, OCH3, NH2 at o/p) increase pKa (decrease acidity).
• Multiple EWG have additive effect on acidity.