Aldol condensation cannot occur between

Aldol condensation is an organic reaction where an enol or enolate ion reacts with a carbonyl compound (aldehyde or ketone) to form a ̢-hydroxyaldehyde or ̢-hydroxyketone, often followed by dehydration [t1][t3]. This reaction specifically requires the presence of at least one alpha-hydrogen on the reacting carbonyl species to form the necessary nucleophilic enolate [t2][t6]. While 'Crossed Aldol' reactions can occur between two different aldehydes, two different ketones, or an aldehyde and a ketone [t2][t7], the reaction between an aldehyde and an ester is classified differently. Specifically, the condensation involving esters is known as the Claisen condensation, which results in ̢-keto esters or ̢-diketones rather than the aldol structural unit [t1][t4]. Therefore, aldol condensation cannot occur between an aldehyde and an ester as it falls under a different reaction mechanism and product category.