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Question:

The correct decreasing order of pKa in the given compound is:

II>IV>I>III

III>II>IV>I

IV>II>III>I

IV>I>II>III

Solution:

Weaker acids have higher pKa. -OCH3 at meta-position exerts only -I effect, hence increases the acidity. -I effect order: -NO2 > -OCH3 > -Cl. -CH3 has +I effect. pKa is inversely proportional to acidity. So, option A is correct.