$(b)$ $H_4P_2O_7$ $\begin{array}{*{20}{c}}
{\begin{array}{*{20}{c}}
{\,O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O} \\
{\,||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||}
\end{array}} \\
{OH - P - O - P - OH} \\
{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|\,} \\
{\,\,\,\,OH\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,OH\,}
\end{array}$
$(c)$ $H_4P_2O_6$ $H\begin{array}{*{20}{c}}
{\begin{array}{*{20}{c}}
{\,O\,\,\,\,\,\,\,O\,\,} \\
{\,||\,\,\,\,\,\,\,\,\,||\,\,\,\,}
\end{array}} \\
{O - P - P - OH} \\
{\,\,|\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,} \\
{\,\,\,\,OH\,\,\,\,OH\,\,\,\,}
\end{array}$
$C _{2} H _{5} OH + PCl _{3} \rightarrow C _{2} H _{5} Cl + A$
$A + PCl _{3} \rightarrow B$