Textbook Question
Indicate whether each of the following structures is D-glyceraldehyde or L-glyceraldehyde, assuming that the horizontal bonds point toward you and the vertical bonds point away from you (Section 4.7):
a.
b.
c.
28. Carbohydrates
Monosaccharides - D and L Isomerism
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How many stereoisomers are possible for a 2-ketohexose, considering D and L isomerism?
A
8
B
16
C
4
D
32
Verified step by step guidance1
Understand the structure of a 2-ketohexose: A hexose is a six-carbon sugar, and a 2-ketohexose has a ketone group at the second carbon. The remaining carbons will have hydroxyl groups, which can be arranged in different configurations.
Identify the chiral centers: In a 2-ketohexose, the chiral centers are located at carbons 3, 4, and 5. Each chiral center can have two configurations (R or S), leading to multiple stereoisomers.
Calculate the number of stereoisomers: The formula for calculating the number of stereoisomers is 2^n, where n is the number of chiral centers. For a 2-ketohexose, n = 3, so the number of stereoisomers is 2^3.
Consider D and L isomerism: D and L isomerism refers to the configuration of the chiral center farthest from the ketone group, typically the last chiral center in the chain. Each stereoisomer can exist in both D and L forms, doubling the number of possible stereoisomers.
Combine the calculations: Multiply the number of stereoisomers from the chiral centers by 2 to account for D and L isomerism. This gives the total number of stereoisomers possible for a 2-ketohexose.
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