Which of the following compounds is a hemiacetal?

• A. A molecule containing an $OH$ group and a carbonyl group on adjacent carbons
• B. A molecule containing two $OR$ groups attached to the same carbon, with no $OH$ group present
• C. A molecule containing a carbonyl group ($C=O$) bonded to two alkyl groups
• D. A molecule containing both an $OH$ group and an $OR$ group attached to the same carbon, where that carbon is also bonded to a hydrogen and an alkyl group

Answer: (D)

Verified step by step guidance

1. Recall the definition of a hemiacetal: it is a functional group where a single carbon atom is bonded to both an -OH group and an -OR group, along with one hydrogen and one alkyl (or other carbon) group.
2. Analyze the first option: a molecule with an -OH group and a carbonyl group on adjacent carbons. This describes a molecule with separate hydroxyl and carbonyl groups, not both on the same carbon, so it is not a hemiacetal.
3. Analyze the second option: a molecule with two -OR groups attached to the same carbon and no -OH group. This describes an acetal, not a hemiacetal, because hemiacetals have one -OH and one -OR on the same carbon.
4. Analyze the third option: a molecule with a carbonyl group bonded to two alkyl groups. This describes a ketone, which is neither a hemiacetal nor an acetal.
5. Identify the correct description of a hemiacetal: a molecule containing both an -OH group and an -OR group attached to the same carbon, where that carbon is also bonded to a hydrogen and an alkyl group. This matches the definition of a hemiacetal.