$\begin{matrix}
O \\
|| \\
R-C-Cl \\
\end{matrix}$ $+\,{{R}_{1}}MgBr\,\xrightarrow[(2){{H}^{\oplus }}]{}$ $\begin{matrix}
\begin{matrix}
\,\,\,OH \\
|\,\, \\
\end{matrix} \\
R-C-{{R}_{1}} \\
|\,\, \\
{{R}_{1}} \\
\end{matrix}$
$(d)$ has three unlike group : it cannot be prepared.

$(I)$ $CrO_4^{2-}$
$(II)$ $MnO_4^-$
$(III)$ $Cr_2O_7^{2-}$ ?


