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

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 ]