When acetylene is passed through red hot iron tube, benzene is formed as
$3 \mathrm{C}_2 \mathrm{H}_2 \stackrel{\text { red hot tube }}{\longrightarrow} \mathrm{C}_6 \mathrm{H}_6$(X)$
Reaction (a) also gives $\mathrm{C}_6 \mathrm{H}_6$ (or $X$ ) as
$\mathrm{C}_6 \mathrm{H}_5 \mathrm{OH}+\mathrm{Zn} \stackrel{\text { distillation }}{\longrightarrow} \mathrm{C}_6 \mathrm{H}_6+\mathrm{ZnO}$