Q1(a). Azulene: resonance structures and dipole moment

Introduction. Azulene (C₁₀H₈) is a blue hydrocarbon isomeric with naphthalene: a seven-membered ring fused to a five-membered ring. Its ten π electrons run round the periphery (4n+2, n=2), so it is aromatic, but unlike naphthalene it has a dipole moment of about 1.0 D.

Resonance structures

Scheme: I and II: covalent Kekulé forms. III: tropylium cation (7-ring) fused to cyclopentadienide anion (5-ring); the + and - are spread over each ring.

Forms I and II are the two covalent Kekulé structures. A shift of the shared π pair towards the five-membered ring gives the dipolar form III, and further forms place the + on C-4, C-6, C-8 and the - on C-1 and C-3.

Why azulene has a dipole moment

1. In III each ring holds six π electrons: the seven-membered ring is a tropylium cation (6π, aromatic) and the five-membered ring a cyclopentadienide anion (6π, aromatic).
2. Because the charge-separated form gives two aromatic sextets, it contributes strongly to the hybrid. Electron density is drawn from the seven-membered ring into the five-membered ring.
3. The result is a permanent charge separation: the five-membered ring is the negative end and the seven-membered ring the positive end, so μ≈1.0 D.
4. Naphthalene has two identical six-membered rings; no such shift is possible and μ = 0.

Chemical evidence for the polarisation: electrophiles (nitration, acylation, azo coupling) attack C-1 and C-3 of the electron-rich five-membered ring, while nucleophiles (organolithium reagents) add at C-4, C-6 or C-8 of the seven-membered ring.

Conclusion. Azulene is a resonance hybrid of two covalent forms and a dipolar form in which a tropylium cation is fused to a cyclopentadienide anion. Both rings are then aromatic sextets, so the dipolar form contributes heavily; the electron drift towards the five-membered ring gives μ≈1.0 D.

Application. The same charge separation lowers the HOMO–LUMO gap, so azulene absorbs in the visible and is blue; the azulene ring is found in chamomile oil (chamazulene), used as an anti-inflammatory.