Which one of the following is NOT a heterocyclic aromatic compound?

Correct — A, Tropolone. A heterocyclic compound has at least one atom other than carbon inside the ring. Tropolone's ring is a seven-membered carbon ring carrying a hydroxyl and a carbonyl group on the outside, with no heteroatom in the ring itself, so it is carbocyclic — aromatic in the non-benzenoid sense, but not heterocyclic. Furan, thiophene and pyridine each carry a heteroatom in the ring and are the standard textbook examples of aromatic heterocycles.

• (b)Furan — Furan is a five-membered aromatic ring with an oxygen atom in it, and that oxygen is exactly what makes it heterocyclic.
• (c)Thiophene — Thiophene has the same five-membered aromatic ring with sulphur in place of furan's oxygen.
• (d)Pyridine — Pyridine is a six-membered aromatic ring in which one of benzene's carbon atoms has been replaced by nitrogen — the classic aromatic heterocycle.

Aromaticity and heterocyclicity are two separate labels and a compound can carry either, both or neither. Aromatic means a planar cyclic system with a delocalised set of electrons satisfying the 4n + 2 rule. Heterocyclic simply means the ring contains an atom other than carbon. Benzene is aromatic and carbocyclic; pyridine is aromatic and heterocyclic; cyclohexane is neither.

The item is testing whether that pair of labels has been kept apart, because all four options are aromatic and three are also heterocyclic. Tropolone is the interesting case: its seven-membered ring is aromatic without being a benzene ring, which is why it is called non-benzenoid aromatic, and the natural products colchicine and the thujaplicins are built on the same skeleton. A candidate who reasons only from unfamiliarity will still land on it, but the reason to choose it is the absence of a heteroatom rather than the strangeness of the name.

• A heterocyclic compound has at least one non-carbon atom within the ring.
• Furan contains ring oxygen, thiophene ring sulphur and pyridine ring nitrogen; all three are aromatic heterocycles.
• Tropolone has a seven-membered all-carbon ring and is aromatic in the non-benzenoid sense.
• Aromaticity requires a planar cyclic conjugated system obeying Huckel's 4n + 2 rule.
• Pyrrole, imidazole and quinoline are further aromatic heterocycles worth recognising.

• Treating aromatic and heterocyclic as the same label.
• Assuming an aromatic ring must have six members; furan and thiophene have five and tropolone has seven.
• Counting an oxygen attached outside the ring as making the compound heterocyclic — the atom has to be in the ring.

A classification odd-one-out. The reliable method is to name the ring atoms of each option rather than to judge by familiarity.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following is a five-membered aromatic heterocycle containing sulphur?

  • (a)Furan
  • (b)Pyridine
  • (c)Thiophene
  • (d)Benzene

  Answer(c) Thiophene — furan has oxygen in the same ring position, and pyridine has nitrogen in a six-membered ring.

• practice — not a real PYQ

  According to Huckel's rule, a planar cyclic conjugated system is aromatic if the number of delocalised electrons equals

  • (a)4n
  • (b)4n + 2
  • (c)2n + 1
  • (d)n + 2

  Answer(b) 4n + 2 — benzene with six such electrons is the standard case, taking n as 1.