Questions

Acetals are compounds with the general structure R2C(OR')2 (where R and R' can be hydrogen or alkyl groups), formed from aldehydes or ketones by reaction with alcohols. They have two -OR groups attached to the same carbon atom.

Let's analyze the given structures:

• Structure (i): This is a cyclic ether, specifically a dioxane. It has an oxygen in the ring but doesn't match the acetal structure.
• Structure (ii): This molecule has a hydroxyl group (-OH), so it is a hemiketal (specifically a cyclic alcohol), not an acetal.
• Structure (iii): This structure has two -OR groups attached to the same carbon atom, fitting the definition of an acetal.
• Structure (iv): This structure also matches the definition of an acetal, as it has two -OR groups attached to adjacent (same) carbon atoms.

Therefore, the correct structures that are acetals are:

1. Structure (iii)
2. Structure (iv)

So the correct answer is: 2. III and IV.