2-butyne and 1,3-butadiene are:

Functional isomerism is the type of isomerism in which molecules have the same chemical formula but having different functional groups.

$$\begin{gathered} \underset{1,3-\mathrm{butadiene}}{\underbrace{{\mathrm{CH}}_2=\mathrm{CH}-\mathrm{CH}={\mathrm{CH}}_2}} \\ \underset{2-\mathrm{butyne}}{\underbrace{{\mathrm{CH}}_3-\mathrm{C}\equiv \mathrm{C}-\mathrm{C}{\mathrm{H}}_3}} \end{gathered}$$

Both have same chemical formula but having different functional groups.