Question

What is the degree of unsaturation for the most unstable compound among the compounds shown below?

Answer 1

The most unstable compound is cyclobutadiene $\left(C_4{H}_4\right)$. It contains $4$ pi electrons.

Hence, it is antiaromatic. (Antiaromatic compounds are unstable)

Also, a large amount of ring strain and angle strain is present.

The degree of unsaturation is $3$ as two $C=C$ double bonds and one ring is present.

The degree of unsaturation $=\frac{2n_C+2-n_H}{2}=\frac{2\times 4+2-4}{2}=\frac{6}{2}=3$

Here, $n_C$ and $n_H$ represent the number of C atoms and the number of H atoms present in one molecule of cyclobutadiene respectively.

Hence, the degree of unsaturation in cyclobutadiene is $3$.