Benzaldehyde, whose formula is (C6H5CHO), does not undergo aldol condensation, because it lacks alpha-hydrogen within its structure. This alpha-hydrogen is the hydrogen attached or adjacent to the carbon in the carbonyl group.
Aldol condensation is a reaction between an aldehyde or a ketone and an enolate that subsequently leads to the formation of a new carbon-carbon double bond that assumes a planar shape-like structure; for this aldol condensation to take place, an acid, a base and a catalyst medium in an aqueous solution must be present. Aldol condensation is very much reversible in the presence of the above three conditions; however, in their absence, the Aldol products are completely stable.
Benzalaldehyde does not undergo aldol condensation, but it undergoes a process known as Claisen-Schmidt condensation; this reaction simply refers to the nucleophilic attack on the carbonyl without an alpha-hydrogen with an enolate derived from a ketone. Subsequently, elimination of water follows the creation of a conjugated carbonyl group.
Benzaldehyde acts as an electriphilic carbonyl; it also reacts with an acetone to produce two products. The absence of the alpha-hydrogen in benzaldehyde makes it impossible for it to enolize. Reactivity of the aldehydes is considerably higher than that of ketones. However, these processes are known as condensation processes because one molecule of water is formed from two reactants.