Aldol condensation does not occur between

Aldol condensation does not occur between an aldehyde and an ester.

Explanation:

Aldol condensation typically occurs between two aldehydes, two ketones, or an aldehyde and a ketone with at least one compound containing alpha-hydrogens. However, when an ester is involved, it does not undergo aldol condensation with an aldehyde because esters are less reactive toward nucleophilic addition due to the electron-donating nature of the alkoxy group (−OR).

1. Aldehydes are highly reactive and readily undergo aldol condensation when combined with another aldehyde or a ketone.
2. Esters, due to their structure, do not participate in aldol condensation with aldehydes. Instead, esters are more likely to undergo other reactions like Claisen condensation under the right conditions.

Thus, aldehydes and esters cannot participate in aldol condensation together.