The aldol reaction can be defined in the broad sense as a reaction in which a nucleophile-generated alpha to an electron-withdrawing functional group (in the large majority of cases, carbonyl groups are responsible for the α-hydrogen acidity, although they are not required) adds to a carbonyl group (it may be self-condensation as in the example of acetaldehyde, but it may also involve attack on another carbonyl containing species, if it is present). The Knoevenagel reaction as another condensation possessing a mechanism similar to that of the aldol reaction.