a
\(\begin{array}{*{20}{c}}
  {C{H_3} - CH - C{H_3}} \\ 
  |\,\,\,\,\,\,\, \\ 
  {OH\,\,} 
\end{array}\xrightarrow{{PB{r_3}}}{\mkern 1mu} \mathop {\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,C{H_3}} \\ 
  | \\ 
  {C{H_3} - CH - Br} 
\end{array}}\limits_{(X)} {\mkern 1mu} \xrightarrow{{Mg}}\)

\(\mathop {\begin{array}{*{20}{c}}
  {\,\,\,C{H_3}} \\ 
  {|\,\,\,} \\ 
  {C{H_3} - CH - MgBr} 
\end{array}}\limits_{(Y)} {\mkern 1mu} \,\,\xrightarrow{{\begin{array}{*{20}{c}}
  {C{H_2} - C{H_2}} \\ 
  {\mathop {\begin{array}{*{20}{c}}
  \backslash &/ 
\end{array}}\limits_O } 
\end{array}}}\)

\(\mathop {\begin{array}{*{20}{c}}
  {C{H_3}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {C{H_3} - CH - C{H_2} - C{H_2} - OMgBr} 
\end{array}}\limits_{(Z)} {\mkern 1mu} {\mkern 1mu} \,\xrightarrow[\Delta ]{{{H_2}O}}\,\)

\(\mathop {\begin{array}{*{20}{c}}
  {C{H_3}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {C{H_3} - CH - C{H_2}C{H_2}OH} 
\end{array}}\limits_{} +\,Mg(OH)\)