Source: Aniline structure — Wikimedia Commons (CC BY-SA)

SMILES: Nc1ccccc1 (aniline)

Labeled Key Points:

1. Nitrogen lone pair — Located on the –$NH_{2}$ group; normally available for protonation to act as a base
2. Resonance delocalization — Lone pair enters π system of benzene ring via resonance; shown by curved arrows from N into ring
3. Quinoid structures — Two resonance forms place positive charge on N and negative charge at ortho/para positions; these reduce electron density on N
4. C–N bond — Partial double-bond character due to resonance (bond length ~1.40 Å, shorter than typical C–N single bond ~1.47 Å)
5. Reduced basicity — Because lone pair is delocalized, aniline pKb ≈ 9.38 vs methylamine pKb ≈ 3.38
6. Electrophilic substitution — Resonance makes ortho/para positions electron-rich → aniline undergoes EAS preferentially at ortho/para
7. Effect of substituents — Electron-withdrawing groups (–$NO_{2}$) at para pull more electron density, further weakening N lone pair → even weaker base (e.g., p-nitroaniline pKb ≈ 13)