What is the major difference between cyclic hemiacetal and cyclic acetal?

The major difference between cyclic hemiacetals and cyclic acetals lies in their structure and stability.

Formation: Cyclic hemiacetals are formed when a hydroxyl group (-OH) and a carbonyl group (C=O) on the same molecule react to form a cyclic structure with an oxygen atom bridging the carbon atom of the carbonyl group and the carbon atom adjacent to it. Cyclic acetals, on the other hand, are formed when a molecule with multiple hydroxyl groups reacts with a molecule containing a carbonyl group, resulting in the substitution of the hydroxyl groups by alkyl or aryl groups, forming a cyclic structure.
Stability: Cyclic acetals are more stable than cyclic hemiacetals. This is because in cyclic acetals, both oxygen atoms attached to the carbon atom are bonded to alkyl or aryl groups, which are electron-donating and stabilize the positive charge that develops on the carbon atom. In contrast, cyclic hemiacetals have one oxygen atom attached to a hydrogen atom, which is less electron-donating compared to alkyl or aryl groups, leading to less stabilization of the positive charge.
Reactivity: Cyclic hemiacetals are more reactive than cyclic acetals due to the presence of the labile hydroxyl group. The presence of the hydroxyl group makes cyclic hemiacetals prone to undergo further reactions, such as intramolecular or intermolecular nucleophilic substitution, leading to the formation of cyclic acetals or other products.
In summary, the main difference lies in the stability and reactivity of these compounds, with cyclic acetals being more stable and less reactive compared to cyclic hemiacetals.