$\begin{array}{*{20}{c}}
  {C{H_3} - CHC{H_2}CHO} \\ 
  {\,|\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {OH\,\,\,\,\,\,} 
\end{array}$

This product is obtained when $2$ molecules of $CH_3CHO$ are heated with concentrated alkali $(NaOH)$

$\mathop {C{H_3}CHO\,}\limits_{'X'} \, + \,\mathop {C{H_3}CHO\,}\limits_{'Y'} \xrightarrow[{'Z'}]{{NaOH}}$ $\begin{array}{*{20}{c}}
  {C{H_3}CH - C{H_2} - CHO} \\ 
  {\,|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {OH\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} 
\end{array}$

[Note : The aldehydes having $\alpha $ - hydrogen atom on heating with concentrated alkali give brown resinous mass; aldol condensation ]