Propanal has two α-carbon, so it undergoes aldol condensation. So, the product in the reaction will 2-methyl-pent-2-enaol. Propanal, react with dil. base (NaOH) to give β-hydroxy aldehyde (3-hydroxy-2-methylpentanal) which on heating undergoes dehydration to give α−β-unsaturated aldehyde (2-methylpent-2-enal). The reaction is as follows :