Q1(a). Is [14]annulene aromatic?

Introduction. Hückel rule: a cyclic, planar, fully conjugated ring with (4n+2) π electrons is aromatic. [14]Annulene, C₁₄H₁₄ (cyclotetradecaheptaene), has seven C=C bonds and 14 π electrons, which is 4n+2 with n=3. On electron count it is aromatic; the question is whether the ring can be flat enough.

1. Electron count. 14 π electrons fill the seven bonding π MOs of the ring completely (closed shell), as Hückel's rule requires for n=3.
2. Geometry. A 14-membered ring can take bond angles near 120° only with some trans double bonds. In the stable form four H atoms point into the ring (C-3, C-6, C-10, C-13).
3. Strain. These inner H atoms crowd each other (transannular H/H repulsion), so the ring is twisted slightly out of plane.

Figure: [14]Annulene drawn on the pyrene outline: four H point into the ring.

Evidence that it is still aromatic

• Diamagnetic ring current in the 1H NMR spectrum: the ring current shields the four inner protons, which appear far upfield (δ about -0.6), while the ten outer protons are deshielded and appear in the aromatic region. A non-aromatic polyene would show all its CH signals near δ 5–6.
• No bond alternation: X-ray bond lengths lie between 1.35 and 1.41 Å, close to the benzene value, not alternating 1.34/1.46 Å as in a localised polyene.
• The distortion from planarity is small and is concentrated at the four inner-H carbons, so the p orbitals still overlap round the ring.

The steric strain lowers the aromatic stabilisation: [14]annulene is less stable than benzene, it isomerises between two geometric forms on the NMR time scale, and it is weakly aromatic. When the inner hydrogens are replaced by a bridge (e.g. trans-15,16-dimethyl-15,16-dihydropyrene, a [14]annulene built round a saturated core), the periphery becomes almost planar and the system is strongly aromatic: equal bond lengths and typical substitution reactions.

Conclusion. Yes. [14]Annulene has 4n+2 = 14 π electrons (n=3) in a cyclic conjugated ring that is nearly planar. The inner-H repulsion twists it a little, so the aromatic character is weaker than in benzene, but the NMR ring current (inner H near δ -0.6) and equalised bond lengths show that it is aromatic.

Application. Ring-current shielding of protons inside or above an aromatic ring is used in NMR to recognise aromatic macrocycles such as porphyrins (inner N–H far upfield) and bridged annulenes.