Q7(a). Aromatic, anti-aromatic or non-aromatic

Reading of the question: “cyclooctatrienyl dianion” is the C₈H₈²⁻ ion (cyclooctatetraene dianion), written as a triene with two carbanion centres.

Introduction. Test for each species: is it cyclic, planar and fully conjugated (a p orbital on every ring atom)? If yes, count the π electrons: 4n+2 = aromatic, 4n = anti-aromatic. If the ring is not fully conjugated or not planar, it is non-aromatic.

Scheme: Structures; the circle in (ii) stands for 10 delocalised π electrons.

1. Tetrahydrofuran (C₄H₈O, oxolane): all four carbons are sp³; there is no π system round the ring. Non-aromatic (an ordinary cyclic ether).
2. Cyclooctatetraene dianion (C₈H₈²⁻): neutral cyclooctatetraene (8π) is tub-shaped and non-aromatic; with potassium it takes two electrons and becomes a planar regular octagon with 10π electrons (n = 2) spread over all eight carbons (equal C–C bonds, one ¹H NMR signal). Aromatic.
3. Azulene (C₁₀H₈): a planar, fully conjugated 5–7 fused system with 10π electrons round the periphery (n=2). Its dipolar form, a tropylium (6π) ring fused to a cyclopentadienide (6π) ring, explains its dipole moment and its blue colour. Aromatic (substitution at C-1 and C-3, not addition).
4. Tropolone (2-hydroxycyclohepta-2,4,6-trien-1-one): the polar C=O gives a dipolar form, a hydroxytropylium ring with 6π electrons on seven sp² carbons. Nearly equal C–C bonds, pKₐ about 7 (more acidic than phenol, 10), substitution rather than addition: aromatic.

Conclusion. (i) THF: non-aromatic. (ii) C₈H₈²⁻: aromatic (10π, planar). (iii) Azulene: aromatic (10π). (iv) Tropolone: aromatic (6π tropylium form). None of the four is anti-aromatic.

Application. The C₈H₈²⁻ ion is the ligand in uranocene, U(C₈H₈)₂; the azulene chromophore gives the blue of chamomile oil (chamazulene), and the tropolone ring occurs in colchicine.