Which of the following structures is classified as a hemiacetal?

• A. A carbon atom bonded to one $OH$ group, one $O$ bonded to an alkyl group, one hydrogen, and one alkyl group
• B. A carbon atom bonded to two $O$ atoms each attached to alkyl groups and two alkyl groups
• C. A carbon atom bonded to one $O$ double bond and two alkyl groups
• D. A carbon atom bonded to two $OH$ groups and two alkyl groups

Answer: (A)

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 (where R is an alkyl group), along with two other substituents (usually hydrogen and/or alkyl groups).
2. Analyze each option by identifying the groups attached to the carbon atom: look for the presence of one hydroxyl group (-OH) and one alkoxy group (-OR) attached to the same carbon.
3. Exclude options where the carbon is bonded to two alkoxy groups (-OR and -OR), as this corresponds to an acetal, not a hemiacetal.
4. Exclude options where the carbon is bonded to a carbonyl group (C=O), as this represents an aldehyde or ketone, not a hemiacetal.
5. Exclude options where the carbon is bonded to two hydroxyl groups (-OH and -OH), as this corresponds to a geminal diol, not a hemiacetal.