Aldohexoses A and B both undergo Ruff degradation to give aldopentose C. On treatment with warm nitric acid, aldopentose C gives an optically active aldaric acid. B also
reacts with warm nitric acid to give an optically active aldaric acid, but A reacts to give
an optically inactive aldaric acid. Aldopentose C is degraded to aldotetrose D, which
gives optically active tartaric acid when it is treated with nitric acid. Aldotetrose D is
degraded to (+)@glyceraldehyde. Deduce the structures of sugars A, B, C, and D, and use
Figure 23-3 to determine the correct names of these sugars.
The Correct Answer and Explanation is :
Based on the given information, we can deduce the structures of sugars A, B, C, and D step by step using the information about the reactions and their outcomes.
Step 1: Understand the Ruff degradation
Ruff degradation is a reaction where aldoses (like hexoses) are converted to pentoses by the oxidative cleavage of the aldehyde group, followed by a decarboxylation reaction. The result is the formation of a pentose sugar. Aldohexoses A and B both undergo Ruff degradation to give the same aldopentose C. Therefore, both A and B are aldoses, and they must be epimers (sugars that differ in the configuration of only one stereocenter).
Step 2: Reaction of aldopentose C with warm nitric acid
Aldopentose C reacts with warm nitric acid to give an optically active aldaric acid. Aldaric acid is formed by the oxidation of both the aldehyde group and the hydroxyl group on the terminal carbon, creating a dicarboxylic acid. The fact that the aldaric acid is optically active indicates that aldopentose C is a chiral molecule (it has at least one asymmetric center). This suggests that C is not a meso compound.
Step 3: Reactions of A and B with nitric acid
- Sugar A gives an optically inactive aldaric acid, suggesting that it is a meso compound, meaning A has symmetry in its structure (likely a sugar with two chiral centers that are mirror images of each other). A likely corresponds to D-glucose.
- Sugar B, which also reacts with nitric acid to give an optically active aldaric acid, likely corresponds to D-mannose, which is an epimer of D-glucose.
Step 4: Aldopentose C and its degradation
Aldopentose C is degraded to aldotetrose D. This suggests that C is a pentose sugar, and it undergoes further oxidation under nitric acid treatment. Tetrose D gives optically active tartaric acid upon treatment with nitric acid, indicating that D is also chiral and likely a sugar with two chiral centers.
Step 5: Degradation of D to glyceraldehyde
Aldotetrose D is further degraded to (+)-glyceraldehyde. This suggests that D is a tetrose with a chiral center at its second carbon. The structure of D corresponds to D-erythrose.
Summary of Structures
- Sugar A is D-glucose (meso compound).
- Sugar B is D-mannose (epimer of glucose).
- Sugar C is D-xylose (pentose).
- Sugar D is D-erythrose (tetrose).
These deductions are supported by known sugar structures and their reactions in Ruff degradation and oxidation processes.