When two molecules of an aldehyde or a ketone condense in presence of a dilute alkali (dil. NaOH,Na2CO3,Ba(OH)2 etc.) to form a β-hydroxyaldehyde or a β-hydroxyketone respectively, these β-hydroyaldehydes or ketones are callectively called aldols and the reaction is called aldol condensation. The aldol reaction requires an aldehyde or ketone that contains at least one alpha-hydrogen in order to form enol or enolate. Thus, acetaldehyde, propanaldehyde and acetone undergo aldol condensation since they contain α-hydrogen atom.