Q1(a). [10]Annulene isomers; sydnone and tropolone

Introduction. A ring is aromatic when it is cyclic, planar and fully conjugated and holds (4n+2) π electrons (Hückel rule). [10]Annulene (cyclodecapentaene, C₁₀H₁₀) has 10π electrons (n=2), so on electron count alone it should be aromatic; geometry decides the matter.

(i) Three geometrical isomers of [10]annulene

Figure: Three geometrical isomers of [10]annulene; none of them is planar.

• All-cis: a planar regular decagon needs bond angles of 144^°, far from 120^° for sp² carbon. The angle strain is too large, so the ring puckers.
• Mono-trans (azulene-shaped): angle strain is smaller, but one H points into the ring and the ring twists to make room for it.
• Di-trans (naphthalene-shaped): angles are close to 120^°, but the two inner H atoms (positions 1 and 6) would be about 0.8 Å apart if the ring were flat. This transannular H/H repulsion forces the ring out of plane.

In every isomer the p orbitals are twisted out of line, the π system cannot delocalise round the ring, and the molecule behaves as a reactive non-aromatic polyene (it is isolable only below about -80°C). When the inner hydrogens are replaced by a CH₂ bridge, 1,6-methano[10]annulene is nearly planar and is aromatic: it shows a diamagnetic ring current (ring H near δ 7, bridge CH₂ near δ -0.5) and undergoes substitution.

(ii) Sydnone and tropolone

A sydnone is a mesoionic 1,2,3-oxadiazolium-5-olate. No neutral covalent structure can be written for it; every structure carries a + in the ring and a - on the exocyclic oxygen. The five ring atoms are planar and share six π electrons (two from the ring O, one each from N-2 and N-3 and the C=C, with the + spread over the ring), so the ring is aromatic.

Figure: Left: 3-phenylsydnone (mesoionic, 6π). Right: tropolone.

The evidence: sydnones are stable crystalline solids, have large dipole moments (about 6 D), and undergo electrophilic substitution (bromination, nitration) at C-4 instead of addition.

Tropolone (2-hydroxycyclohepta-2,4,6-trienone) is aromatic through its dipolar form. The ring C=O is polarised towards oxygen, and the C⁺–O⁻ form makes the seven-membered ring a hydroxytropylium ion: seven sp² carbons with six π electrons. Tropolone shows a large dipole moment, nearly equal C–C bond lengths, acidity close to phenol (pKₐ≈ 7), no normal ketone derivatives, and substitution (azo coupling, nitration) instead of addition. An intramolecular O–H…O hydrogen bond adds stability.

Conclusion. All three [10]annulene isomers have 10π electrons but are non-planar (angle strain or inner H/H repulsion), so they are non-aromatic. Sydnone (mesoionic) and tropolone (dipolar tropylium form) each hold a planar 6π ring and are aromatic.

Application. Sydnones are used as 1,3-dipoles in click-type cycloadditions to make pyrazoles; the tropolone ring is present in the gout drug colchicine and in the antifungal hinokitiol.