Q4(b). Aromatic, non-aromatic or anti-aromatic
Classify the following as aromatic, non-aromatic and anti-aromatic. Give reasons:
tabularccccc
[structure] & [structure] & [structure] & [structure] & [structure]
(i) & (ii) & (iii) & (iv) & (v)
tabular
Answer in brief
Aromatic: (iii), (v); anti-aromatic: (iv); non-aromatic: (i), (ii).
Key points
- (i) has two C=C and an O lone pair, (ii) has three C=C, but in each ring one sp³ carbon interrupts conjugation; they behave as ordinary dienes/trienes (non-aromatic). (ii) becomes aromatic only on losing H⁻ to give (v).
- (iii) The anion carbon is sp²; its lone pair is in a p orbital and becomes part of the ring π system: 6π. Cyclopentadiene is unusually acidic (pKₐ 15) because this aromatic anion forms.
- (iv) Cyclobutadiene has 4π electrons; two occupy degenerate non-bonding MOs (open shell). It is destabilised compared with butadiene, rectangular and reactive (it dimerises unless trapped at very low temperature): anti-aromatic.
- (v) The tropylium ion: seven sp² carbons, an empty p orbital on the cationic carbon and six π electrons; the charge is spread equally over seven carbons, and tropylium bromide is an ionic, water-soluble salt.
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 ring is aromatic when it is cyclic, planar and fully conjugated (a p orbital on every ring atom) with (4n+2) π electrons; anti-aromatic when it meets the first three conditions with 4n π electrons; non-aromatic when the cycle of p orbitals is broken (an sp³ atom) or the ring is not planar.
| Compound | Reason | Class | |
| (i) | 4H-pyran | C-4 is an sp³ CH₂: no continuous ring of p orbitals | non-aromatic |
| (ii) | cyclohepta-1,3,5-triene | one sp³ CH₂ breaks the cycle; the ring is non-planar (6π but not cyclic) | non-aromatic |
| (iii) | cyclopentadienyl anion | planar, every C sp²; 4 + 2 = 6π (n=1) | aromatic |
| (iv) | cyclobutadiene | planar, cyclic, conjugated; 4π (n=1) | anti-aromatic |
| (v) | cycloheptatrienyl (tropylium) cation | planar, every C sp² (empty p on C⁺); 6π | aromatic |
Figure: Frost circles (sketch). Dashed line: non-bonding level. The 6π ions fill every bonding MO; cyclobutadiene has two unpaired electrons at the non-bonding level.
- (i) has two C=C and an O lone pair, (ii) has three C=C, but in each ring one sp³ carbon interrupts conjugation; they behave as ordinary dienes/trienes (non-aromatic). (ii) becomes aromatic only on losing H⁻ to give (v).
- (iii) The anion carbon is sp²; its lone pair is in a p orbital and becomes part of the ring π system: 6π. Cyclopentadiene is unusually acidic (pKₐ 15) because this aromatic anion forms.
- (iv) Cyclobutadiene has 4π electrons; two occupy degenerate non-bonding MOs (open shell). It is destabilised compared with butadiene, rectangular and reactive (it dimerises unless trapped at very low temperature): anti-aromatic.
- (v) The tropylium ion: seven sp² carbons, an empty p orbital on the cationic carbon and six π electrons; the charge is spread equally over seven carbons, and tropylium bromide is an ionic, water-soluble salt.
Conclusion. Aromatic: (iii), (v); anti-aromatic: (iv); non-aromatic: (i), (ii).
Application. Aromatic ions explain the high acidity of cyclopentadiene (ferrocene is made from the cyclopentadienide ion) and the easy hydride loss from cycloheptatriene.
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All parts of Question 4
- 4(a) Stobbe condensation; Fries rearrangement15 marks
- 4(b) Aromatic, non-aromatic or anti-aromatic10 marks
- 4(c) Periodic acid oxidation of glucose and fructose10 marks
- 4(d) Cyclohexanone to nylon-6; phenylhexatriene to biphenyl15 marks