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SolutionsAromaticity

1. Aromatic, nonaromatic or antiaromatic

Indicate whether each of the following compounds is aromatic, nonaromatic or antiaromatic.

Six compounds: a) cyclooctatetraene; b) cyclopentadienyl cation; c) cyclopentadienyl anion; d) [10]-annulene; e) azulene; f) [18]-annulene
  1. Cyclooctatetraene: nonaromatic. It would have 8 π electrons (4n), but it adopts a tub shape to avoid being antiaromatic and behaves as a polyene.
  2. Cyclopentadienyl cation: antiaromatic. It is cyclic, planar and conjugated, but has only 4 π electrons.
  3. Cyclopentadienyl anion: aromatic, with 6 π electrons.
  4. [10]-Annulene: nonaromatic. It has 10 π electrons (4n + 2), but the inner hydrogens clash and prevent the ring from being planar.
  5. Azulene: aromatic. It is a planar bicyclic system with 10 π electrons.
  6. [18]-Annulene: aromatic. It is planar and has 18 π electrons (4n + 2, with n = 4).

2. Lone pairs and aromaticity

For the following compounds, determine which lone pairs are involved in aromaticity.

Eight compounds: a) furan; b) 1H-azepine; c) thiazole; d) pyrylium cation; e) 1,4-dioxin; f) caffeine; g) moxonidine; h) a pyrido[1,2-a]pyrimidinone derivative bearing a tetrazole
  1. Furan: one of the two oxygen lone pairs is part of the aromatic sextet; the other lies in the plane of the ring.
  2. 1H-Azepine: if the nitrogen lone pair were delocalized the ring would have 8 π electrons (antiaromatic); the ring is not planar and azepine is not aromatic.
  3. Thiazole: one sulfur lone pair is involved; the nitrogen lone pair (as in pyridine) is in an sp2 orbital and is not involved.
  4. Pyrylium cation: the oxygen contributes one electron to the π system, like the nitrogen of pyridine; its lone pair lies in the plane and is not involved.
  5. 1,4-Dioxin: with one lone pair from each oxygen there would be 8 π electrons; it is not aromatic.
  6. Caffeine: in the imidazole ring the lone pair of the methylated nitrogen is involved; that of the doubly bonded nitrogen is not.
  7. Moxonidine: the pyrimidine nitrogens, like that of pyridine, do not contribute their lone pairs; the imidazoline ring is not aromatic.
  8. In the bicyclic system the lone pair of the bridgehead nitrogen is involved; in the tetrazole the lone pair of the N–H nitrogen is involved, and not those of the other three.

3. Hückel rule

How many π electrons do the cyclopropenyl cation, the cyclopropenyl anion and the cycloheptatrienyl (tropylium) cation have? Which of them are aromatic?

  • Cyclopropenyl cation: 2 π electrons (4n + 2, with n = 0): aromatic.
  • Cyclopropenyl anion: 4 π electrons: antiaromatic and very unstable.
  • Tropylium cation: 6 π electrons: aromatic.

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