a
$CH_3CHO$ $+$ $H_3CCHO$ $\xrightarrow {NaOH}$ $\mathop {\begin{array}{*{20}{c}}
{OH\,\,\,\,\,\,\,\,\,\,\,\,\,} \\
{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\
{C{H_3} - CH - C{H_2} - CHO}
\end{array}}\limits_{\beta \,\, - \,\,hydroxy\,\,butanal} $ $\xrightarrow {-H_2O,\,\,\Delta ,\,\,dil acid}$ $\mathop {C{H_3}CH = CH - CHO}\limits_{(krotonaldyhide)} $