It is an example of Self Condensation reaction as both the nucleophile and the electrophile are the same compounds. When Butanal is reacted with base, the hydrogen at alpha position is abstracted due to its mild acidity and enolate is formed. Enolate attacks second Butanal molecule and forms a compound containing both Hydroxyl group and carbonyl group (Beta hydroxy aldehyde). Reaction is shown below and the name of product is specified below its structure,
