For the given compounds, the correct order of increasing pKa value is:

Step 1: Understand pKa and acidity

Lower pKa = stronger acid. Increasing pKa order = increasing in least-to-most acidic order (i.e., decreasing acidity).

Step 2: Effect of substituents on pKa of phenol

• EWG (like -NO2) at ortho/para: stabilizes phenoxide → lowers pKa → more acidic.
• EDG (like -OCH3) at ortho/para: destabilizes phenoxide → raises pKa → less acidic.
• More EWG = lower pKa; more EDG = higher pKa.

Step 3: Rank by pKa (ascending = least acidic to most acidic)

Most acidic (lowest pKa) → least acidic (highest pKa):

• 2,4-Dinitrophenol (2 EWG groups, pKa ≈ 4.0) → most acidic, lowest pKa
• p-Nitrophenol (1 EWG at para, pKa ≈ 7.1)
• Phenol (no substituent, pKa ≈ 10.0)
• p-Methoxyphenol (EDG at para, pKa ≈ 10.2)
• Methoxy-substituted phenol with more EDG (highest pKa, least acidic)

Increasing pKa: B < D < A < C < E

Key Points to Remember:

• EWG at o/p: lowers pKa (more acidic).
• EDG at o/p: raises pKa (less acidic).
• The pKa order is the REVERSE of acidity order.
• 2,4-dinitrophenol: pKa ≈ 4 (nearly as acidic as acetic acid!).