Question

Consider the following four compounds I, II, III and IV. Choose the correct statement(s)

• A. The order of basicity is II > I > III > IV.
• B. The magnitude of $pK _{ b }$ difference between I and II is more than that between III and IV.
• C. Resonance effect is more in III than in IV.
• D. Steric effect makes compound IV more basic than III.

Answer 1

Answer: (C), (D)

Resonance effect is more in III than in IV.

Answer 2

• The correct order of basic strength for the given compounds is $(IV) > (II) > (I) > (III).$
• Compound IV is a stronger base than compound III due to the SIR effect (steric inhibition of resonance), while the difference in basic strength between compound I and compound II is minimal.
• In compound IV, the SIR effect causes both the -NO2 and N(CH3)2 groups to be out of plane, resulting in reduced resonance effects compared to other compounds.
• (III) is the weakest base due to the presence of three -NO2 group, which exert strong -M effect.
• Difference of pKa values is highest between (III) and (IV).
• $pK_b; (I) ⇢ 9.42; (II) ⇢ 8.94, (IV) ⇢ 4.60$