JEE Main 2025 Apr 2 Shift 1, Chemistry Q1: Molecular Orbital Theory

A planar, fully conjugated, cyclic species is aromatic when its $\pi$-electron count satisfies Huckel's rule, that is $(4n+2)$ $\pi$-electrons with $n=0,1,2,\dots$

Applying the rule to each structure:

(a) Cyclopentadienyl anion: 6 $\pi$-electrons $\Rightarrow$ aromatic.

(c) Cyclobutadiene dication: 2 $\pi$-electrons $\Rightarrow$ aromatic.

(d) Cyclobutadiene dianion behaves as a 6 $\pi$-electron system in the relevant Huckel count $\Rightarrow$ aromatic.

(e) Benzene-type 6 $\pi$-electron ring $\Rightarrow$ aromatic.

(h) Cyclopropenyl cation: 2 $\pi$-electrons $\Rightarrow$ aromatic.

(b) Cyclopentadienyl cation: 4 $\pi$-electrons, fails Huckel's rule $\Rightarrow$ not aromatic.

(f) Cyclooctatetraene: 8 $\pi$-electrons and non-planar (tub shape) $\Rightarrow$ not aromatic.

(g) Cyclobutadiene: 4 $\pi$-electrons $\Rightarrow$ not aromatic.

$\therefore$ The aromatic set is a, c, d, e, h while b, f, g are non-aromatic. Option (4) is correct.