JEE Main 2025 Jan 22 Shift 2, Chemistry Q21: Naming of Alkynes

The molecular formula $C_6{H}_6$ has degree of unsaturation $=\frac{2(6)+2-6}{2}=4$. The compound consumes 4 moles of $H_2$ on complete hydrogenation, so all 4 degrees of unsaturation are $\pi$-bonds (no rings remain after exhaustive hydrogenation, or each ring counts toward the $H_2$ uptake as a $\pi$ equivalent only if it is a $\pi$ bond; benzene, for instance, only takes 3 moles of $H_2$, not 4).

A $C_6{H}_6$ isomer that (i) takes 4 moles of $H_2$ and (ii) gives only one monobromo product (i.e. has all hydrogens equivalent) is hexa-2,4-diyne.

Counting $\pi$ electrons: each $\pi$ bond contributes 2 electrons. With 4 $\pi$ bonds, total $\pi$ electrons $=4\times 2=8$.

$\therefore$ Number of $\pi$ electrons $=8$.