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

A heterocyclic compound is defined as a cyclic molecule where the ring contains carbon and at least one heteroatom such as oxygen, nitrogen, or sulfur [1]. Furan (oxygen), thiophene (sulfur), and pyridine (nitrogen) are classic examples of heterocyclic aromatic compounds because they possess heteroatoms within their ring structures and satisfy Hückel's rule for aromaticity. In contrast, Tropolone (2-hydroxy-2,4,6-cycloheptatrien-1-one) is a non-benzenoid aromatic compound. While it contains oxygen in its functional groups (hydroxyl and carbonyl), these oxygen atoms are substituents attached to the ring, not part of the ring skeleton itself. The ring in tropolone consists entirely of seven carbon atoms, making it a carbocyclic (homocyclic) aromatic compound rather than a heterocyclic one. Therefore, tropolone does not fit the definition of a heterocyclic aromatic compound.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > 4.2.3 Will you be my Friend? > p. 66