Final answer:
Only the hemiacetal OH group reacts to form an acetal in the presence of alcohol and acid because hemiacetals are more reactive than typical alcohols, especially so in cyclic structures found in monosaccharides, allowing a second alcohol molecule to convert it into an acetal.
Step-by-step explanation:
The reason only the hemiacetal OH group reacts to form an acetal when a compound containing both an alcohol OH group and a hemiacetal OH group is treated with an alcohol and acid is due to the reactivity of the hemiacetal group itself. Hemiacetals have an -O-R group and an -OH group bonded to the same carbon atom. Typically, hemiacetals are unstable and exist in a small proportion at equilibrium with their open-chain forms. However, when an excess alcohol is present, the formation of an acetal is favored as a second alcohol molecule can react with the hemiacetal's OH group in an acid-catalyzed H2O elimination, yielding an acetal that has two -O-R groups attached to the same carbon atom.
In the case of cyclic hemiacetals, such as those found in monosaccharides (e.g., glucose), the five- or six-membered ring structure offers enhanced stability compared to acyclic hemiacetals. Therefore, in cyclic forms, the hemiacetal hydroxy group is more reactive than a typical alcohol hydroxy group, facilitating the acetal formation under acidic conditions.