In cross aldol condensation between butanal and propanal, either of these compounds can form enolate ion and hence four different products will be formed. Attack of enolate of butanal on propanal will give a different product from the product formed by the attack of enolate of propanal on butanal.
1. Taking one molecule each of propanal and butanal in which propanal acts as a nucleophile and butanal acts as an electrophile.
2. Taking one molecule each of propanal and butanal in which propanal acts as an electrophile and butanal acts as a nucleophile.