Final answer:
Intramolecular cyclization of a hydroxy aldehyde forms a cyclic hemiacetal, which is a more stable form than the reactants.
Step-by-step explanation:
Intramolecular cyclization of a hydroxy aldehyde forms a cyclic hemiacetal. When an alcohol and carbonyl group react with each other within the same molecule, they form a five- or six-membered cyclic hemiacetal. This cyclic form is more stable than the reactants.
For example, glucose can interconvert between its open chain aldehyde form and six-membered cyclic hemiacetal form.