Aldehydes containing at least one $\alpha$-hydrogen atom undergo a reaction in presence of dilute aq. NaOH to form $\beta$-hydroxyl aldehyde. Identify the reaction.

Aldehydes containing at least one $\alpha$-hydrogen atom undergo self-addition in the presence of a dilute base like aqueous NaOH to form $\beta$-hydroxy aldehydes. This reaction is known as the Aldol reaction.

For example, two molecules of ethanal react in the presence of dilute NaOH to form 3-hydroxybutanal (an aldol).

Cannizzaro reaction is given by aldehydes lacking $\alpha$-hydrogen atoms.

Reformatsky reaction involves the reaction of an $\alpha$-halo ester with a carbonyl compound in the presence of zinc.

Wolff-Kishner reaction is the reduction of carbonyl compounds to alkanes using hydrazine and a strong base.

Answer: Aldol reaction