Q1(a). Aromatic, anti-aromatic or non-aromatic
Classify the following examples into aromatic, anti-aromatic and non-aromatic. Justify your answer.
[(i)] [(ii)] [(iii)]
tabular[c]@c@
-1pt] (iv)tabular [(v)]
Answer in brief
Aromatic: (i) styrene, (iii) azulene, (v) pyrrole. Anti-aromatic: (iv) cyclopropenyl anion. Non-aromatic: (ii) cyclooctatetraene (tub-shaped).
Key points
- Introduction. A compound is aromatic when it is cyclic, planar, fully conjugated and holds (4n+2) π electrons; anti-aromatic when it meets the first three conditions with 4n π electrons (destabilised); non-aromatic when the…
- Figure: π levels: with 4 electrons the degenerate pair is half filled (diradical, destabilised); with 6 every bonding level is full.
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
Introduction. A compound is aromatic when it is cyclic, planar, fully conjugated and holds (4n+2) π electrons; anti-aromatic when it meets the first three conditions with 4n π electrons (destabilised); non-aromatic when the ring is not planar or not fully conjugated.
| Compound | π e⁻ | Reason and class | |
| (i) | styrene | 6 (+2) | Benzene ring: planar, cyclic, 6π (n=1). The vinyl group is only conjugated with it. Aromatic. |
| (ii) | cyclooctatetraene | 8 | 4n (n=2). A planar ring would be anti-aromatic, so the molecule takes a tub shape; the C=C bonds are isolated (alternating long and short bonds) and it adds Br₂ like an alkene. Non-aromatic. |
| (iii) | azulene | 10 | Planar bicyclic; the 10π periphery obeys Hückel (n=2). Polar form: cyclopentadienide (6π) fused to tropylium (6π), so azulene has a dipole and is blue. Aromatic. |
| (iv) | cyclopropenyl anion | 4 | Planar, cyclic, each carbon sp² with the lone pair in a p orbital: 4π (n=1). Anti-aromatic; the anion is far less stable than the open allyl anion. |
| (v) | pyrrole | 6 | Two C=C (4e) plus the N lone pair in the ring p orbital (2e) =6π; planar. Aromatic: substitution at C-2, weak base. |
Figure: π levels: with 4 electrons the degenerate pair is half filled (diradical, destabilised); with 6 every bonding level is full.
Conclusion. Aromatic: (i) styrene, (iii) azulene, (v) pyrrole. Anti-aromatic: (iv) cyclopropenyl anion. Non-aromatic: (ii) cyclooctatetraene (tub-shaped).
Application. The anti-aromaticity of planar 4n rings explains why COT readily takes two electrons (K metal) to form the planar, aromatic 10π C₈H₈²- dianion, and why cyclopropenyl cation salts (2π) are isolable.
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All parts of Question 1
- 1(a) Aromatic, anti-aromatic or non-aromatic10 marks
- 1(b) E2 of 2-bromo-2-methylpentane: Zaitsev and Hofmann10 marks
- 1(c) Diethylamine and 2-fluoro-1-acetylcyclohexene: addition–elimination10 marks
- 1(d) LDA at -78°C: kinetic enolate of 2-phenylcyclohexanone10 marks
- 1(e) Electrocyclic closure of (2E,4E)-hexa-2,4-diene: FMO10 marks