Question: 77: Which compound amongst the following is not an aromatic compound?
Answer: Option (2)
Explanation:
Aromatic compounds must satisfy Hückel’s rule, which requires the molecule to be
cyclic, planar, fully conjugated, and contain 4n+2 \pi electrons.
In option (1), the three-membered cyclic cation contains 2 \pi electrons,
which satisfies 4n+2 for n=0, so it is aromatic.
In option (3), the five-membered cyclic anion contains 6 \pi electrons,
satisfying 4n+2 for n=1, making it aromatic.
In option (4), the seven-membered cyclic cation contains 6 \pi electrons, again satisfying Hückel’s rule, so it is aromatic.
In option (2), the eight-membered cyclic ring contains 8 \pi electrons,
which follows the 4n rule rather than 4n+2.
Such a system is antiaromatic or non-aromatic and does not show aromatic stability.
Therefore, option (2) is not an aromatic compound.