A crossed aldol reaction is an aldol condensation reaction between two different carbonyl compounds, at least one of which has an α hydrogen atom, with one compound acting as the nucleophile and the other as the electrophile. If both carbonyl compounds can enolize, this reaction yields a mixture of products: two formed via self-condensation, and two via crossed-condensation. In this reaction, preventing self-condensation is key to reaction efficiency by minimizing the number of products formed. For example, formaldehyde, with no α hydrogen atoms, behaves solely as an electrophile while reacting in a base with another aldehyde that does contain α hydrogen atoms to yield a single crossed aldol product. Other popular reactions that exhibit a similar phenomenon are Claisen–Schmidt condensation, the directed aldol reaction, and the Reformatsky reaction.