Q.34 Which of the following compound(s) is/are aromatic?

The aromatic compounds in this question are options (B) and (D); options (A) and (C) are non‑aromatic.​​

Introduction

Aromaticity questions are common in competitive exams, and understanding which cyclic or heterocyclic structures are aromatic, non‑aromatic, or anti‑aromatic is essential. This IIT JAM aromaticity problem asks, “Which of the following compound(s) is/are aromatic?” and tests planarity, conjugation, and Hückel’s $4n+2$ $\pi$‑electron rule.​

Option (A): Non‑aromatic cationic five‑membered ring

• In option (A), the five‑membered ring contains one nitrogen with a positive charge and two N–H bonds drawn, making nitrogen sp3sp3 with a lone pair involved in the cationic center rather than in conjugation.​

• Because nitrogen is not sp2sp2‑hybridized, the ring does not have a continuous cyclic overlap of $p$ orbitals, so it fails the conjugation requirement and becomes non‑aromatic despite being cyclic.​

Option (B): Aromatic six‑membered diazinium ring

• Option (B) shows a six‑membered benzene‑like ring bearing a positively charged nitrogen, analogous to a pyridinium or diazinium cation, with alternating double bonds around the ring.​

• Every ring atom is sp2sp2‑hybridized, the $\pi$ electrons are fully conjugated, and the ring holds $6\pi$ electrons (which fits Hückel’s $4n+2$ rule with $n=1$), so it is aromatic.​

Option (C): Non‑aromatic four‑membered ring

• The four‑membered ring in option (C) is drawn with only one double bond and no continuous conjugation over all four atoms.​

• As the ring lacks a completely delocalized $\pi$ system and does not satisfy the $4n+2$ $\pi$‑electron requirement, it is classified as non‑aromatic (neither aromatic nor anti‑aromatic under the drawn conditions).​

Option (D): Aromatic benzene ring

• Option (D) is simply benzene, a planar six‑membered carbon ring with three conjugated double bonds, giving $6\pi$ electrons.​

• Benzene fulfills all aromaticity criteria—cyclic, fully conjugated, planar, and satisfying Hückel’s $4n+2$ rule—so it is aromatic.​

Final Answer

For this aromatic compounds IIT JAM question, the aromatic species are (B) and (D), while (A) and (C) are non‑aromatic.​