Question

Which of the following statements about tropone is true?

• A. It is heterocyclic having molecular formula $C_5H_{10}O$.
• B. It is benzenoid having molecular formula $C_{10}H_8O$.
• C. It is non-benzenoid having molecular formula $C_7H_6O$.
• D. It is heterocyclic having molecular formula $C_7H_6O$.

Hint:

Remember that "hetero-" means a non-carbon atom is part of the ring loop itself. Since the oxygen in tropone is part of an external carbonyl group (C=O) and not inside the ring, the ring remains strictly carbocyclic!

Answer 1

The Correct Option is C

Step 1: Read what the name tells us.
Tropone is the common name of $2,4,6$-cycloheptatrien-1-one. The prefix cyclohepta- already signals a seven-membered ring, and -trien-1-one tells us there are three C=C double bonds plus one C=O group. So before drawing anything, we expect 7 ring carbons.
Step 2: Decide heterocyclic or carbocyclic.
A heterocyclic ring must contain at least one non-carbon atom (like N, O, S) inside the ring itself. In tropone the oxygen sits outside the ring as a C=O, not inside it, so the ring is built only from carbon. That makes it carbocyclic, and we can already drop options (1) and (4).
Step 3: Decide benzenoid or non-benzenoid.
Benzenoid compounds contain one or more fused benzene (six-membered) rings. Tropone has a seven-membered ring, so it cannot be benzenoid. It is still aromatic, because the C=O polarises to $C^{+}-O^{-}$, leaving a 6 $\pi$-electron tropylium-like cation that obeys Huckel's rule, but it is a non-benzenoid aromatic. That removes option (2).
Step 4: Count the carbon atoms.
Six carbons carry the three double bonds and one carbon carries the carbonyl, giving $6 + 1 = 7$ carbons. So the formula begins $C_7$.
Step 5: Count hydrogen and oxygen.
Each of the six C=C carbons bears exactly one H, giving 6 hydrogens; the carbonyl carbon has no H. One oxygen is double bonded to the ring. So we have $C_7H_6O$.
Step 6: Match with the options.
Non-benzenoid, formula $C_7H_6O$ - this is precisely option (3).
\[ \boxed{\text{Non-benzenoid, } C_7H_6O \text{ (option 3)}} \]