CAPF AC 2020 Q25: Science & Technology – Chemistry
Which one of the following is NOT a heterocyclic aromatic compound?
- (a) Tropolone
- (b) Furan
- (c) Thiophene
- (d) Pyridine
Answer: (a) Tropolone
Explanation
Heterocyclic aromatic compounds
How to think: hetero means different.
A heterocyclic ring must have an atom other than carbon inside the ring.
So ask of each option: what is the odd atom in the ring?
| Compound | Ring size | Atom other than carbon in the ring | Heterocyclic? |
|---|---|---|---|
| Furan | 5 | Oxygen | Yes |
| Thiophene | 5 | Sulphur | Yes |
| Pyridine | 6 | Nitrogen | Yes |
| Tropolone | 7 | None; all seven ring atoms are carbon | No |
Tropolone does have oxygen, but the oxygen atoms hang outside the ring.
It is a non-benzenoid aromatic compound.
Answer: (a) Tropolone.
Why each option is right or wrong
- (a) CORRECT. Tropolone has a ring of seven carbon atoms; its oxygen is outside the ring.
- (b) Incorrect. Furan is heterocyclic; its ring contains an oxygen atom.
- (c) Incorrect. Thiophene is heterocyclic; its ring contains a sulphur atom.
- (d) Incorrect. Pyridine is heterocyclic; its ring contains a nitrogen atom.
Key point
Heterocyclic aromatic = ring with an atom other than carbon: furan (oxygen), thiophene (sulphur), pyridine (nitrogen). Tropolone is a non-benzenoid carbon ring.
Remember
Furan: O. Thiophene: S. Pyridine: N. Tropolone: carbon ring only.
Common mistakes
- Calling tropolone heterocyclic because its formula contains oxygen.
- Forgetting which atom goes with which ring: thio means sulphur, pyr goes with nitrogen.
- Thinking only benzene rings can be aromatic.
Exam tip
Educated guess: furan, thiophene and pyridine are the three NCERT examples, so the odd one is tropolone.
Sources
- NCERT Class XI Chemistry — Organic Chemistry: Some Basic Principles and Techniques (classification of organic compounds)
- Encyclopaedia Britannica — heterocyclic compound