Propanal (2 mol) is treated with dilute $\mathrm{NaOH}$ and heated. The product obtained after dehydration is

The enolate of propanal adds to a second propanal to give 3-hydroxy-2-methylpentanal. Its alpha carbon still has one H, so E1cB loss of $\mathrm{OH}{}^{-}$ gives the conjugated aldehyde $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}=\mathrm{C}(\mathrm{CH}{}_3)\mathrm{CHO}$, 2-methylpent-2-enal.