General Organic Chemistry MCQ Quiz - Objective Question with Answer for General Organic Chemistry - Download Free PDF

CONCEPT:

Basicity and pK_b Relationship

• pK_b is the negative logarithm of the base dissociation constant (K_b).
• Lower pK_b → Stronger base
• Higher pK_b → Weaker base
• The basicity of nitrogen-containing heterocycles depends on:
  • Availability of the lone pair on nitrogen
  • Resonance delocalization
  • Aromaticity and hybridization

EXPLANATION:

• Compound I (imidazole):
  • One nitrogen has a lone pair not involved in aromaticity; the other is basic.
  • More basic due to lone pair availability.
• Compound II (pyridine):
  • Lone pair on sp² hybridized N is not involved in aromaticity → easily available.
  • Less basic than imidazole due to resonance.

• Compound III (pyrrole):
  • Lone pair is involved in aromaticity → not available for protonation.
  • Very weak base.
• Compound IV (indole):
  • Pyrrole-like nitrogen, lone pair involved in aromaticity → very low basicity.
  • Slightly more basic than pyrrole, but still weak.

• Order of basicity:
  • II > I > IV > III → pK_b order: I > II and III > IV

Therefore, the correct pK_b order is I > II and III > IV.