$\begin{array}{*{20}{c}}
{C{H_3}} \\
|\,\,\,\,\,\, \\
{C{H_{_3}} - C - C{H_2}Br} \\
|\,\,\,\,\,\, \\
H\,\,\,\,
\end{array}\,\xrightarrow[{C{H_3}OH}]{{C{H_3}{O^ - }}}$
${C_2}{H_5}I\,\xrightarrow{{Alc.\,KOH}}X\xrightarrow{{B{r_2}}}Y\xrightarrow{{KCN}}Z$
$(I)$ $CrO_4^{2-}$
$(II)$ $MnO_4^-$
$(III)$ $Cr_2O_7^{2-}$ ?