AAMC FL Practice Exam

What type of group does a hemiacetal contain?

1. One -OR group and one -OH group

2. Two -OH groups

3. Three -OR groups

4. Only carbon chains

The correct answer is: One -OR group and one -OH group

A hemiacetal is characterized by the presence of one -OR group (an alkoxy group, where R is usually a hydrocarbon chain) and one -OH group (a hydroxyl group). The formation of a hemiacetal occurs when an aldehyde or a ketone reacts with an alcohol. Specifically, the carbon atom that was part of the carbonyl group becomes a new chiral center, resulting in the hemiacetal structure where one substituent is derived from the carbonyl (the -OR group) and the other substituent is from the alcohol (the -OH group). In this structure, the aldehyde or ketone provides the carbonyl carbon, which is then bonded to one oxygen from the alcohol (creating the -OR group) and retains the hydroxyl (-OH) as part of the structure. This combination is key to the stability and reactivity of hemiacetals, particularly in biological systems and in the context of carbohydrate chemistry. The presence of two -OH groups, three -OR groups, or only carbon chains would not correspond to the structure of a hemiacetal. The distinctive combination of one -OR and one -OH group is what defines the hemiacetal ability to participate in further reactions such as conversion