Textbook Question
Draw the structures of the compound α-D-allopyranose.
Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.
28. Carbohydrates
Monosaccharides - Forming Cyclic Hemiacetals
Multiple Choice
In the formation of a cyclic hemiacetal from a monosaccharide, what type of reaction occurs?
A
Condensation reaction
B
Hydrolysis reaction
C
Intramolecular nucleophilic addition
D
Oxidation reaction
Verified step by step guidance1
Understand the structure of a monosaccharide: Monosaccharides are simple sugars that contain an aldehyde or ketone group and multiple hydroxyl groups. In the context of forming a cyclic hemiacetal, focus on the aldehyde group and the hydroxyl group that will participate in the reaction.
Identify the functional groups involved: In a monosaccharide, the aldehyde group (or ketone group) can react with a hydroxyl group within the same molecule. This is an intramolecular reaction, meaning it occurs within a single molecule.
Recognize the type of reaction: The formation of a cyclic hemiacetal involves an intramolecular nucleophilic addition reaction. The hydroxyl group acts as a nucleophile, attacking the electrophilic carbonyl carbon of the aldehyde or ketone group.
Visualize the mechanism: The nucleophilic hydroxyl group donates a pair of electrons to the carbonyl carbon, forming a new covalent bond. This results in the opening of the carbonyl group and the formation of a new ring structure, which is the cyclic hemiacetal.
Consider the stereochemistry: The formation of a cyclic hemiacetal can lead to different stereoisomers, depending on the orientation of the hydroxyl group and the resulting ring structure. This is important in understanding the diversity of cyclic forms that monosaccharides can adopt.
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