$2$-Hexyne gives trans-$2$-hexene on treatment with

Identify X in the following sequence of reactions

A compound $\mathrm{X}\hspace{0.17em}\left({\mathrm{C}}_5{\mathrm{H}}_8\right)$ reacts with ammoniacal ${\mathrm{AgNO}}_3$ to give a white precipitate, and on oxidation with hot alkaline ${\mathrm{KMnO}}_4$ gives the acid, $({\mathrm{CH}}_3{)}_2\mathrm{CHCOOH}$. Therefore, $\mathrm{X}$ is