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Which of the following statements about carbohydrates and their derivatives is correct?

1. D(+)-glucose and D(+)-mannose are enantiomers.
2. D(+)-glucose and D(-)-fructose yield the same osazone on reaction with phenylhydrazine.
3. Reduction of D(-)-fructose with sodium amalgam gives only one product.
4. D(+)-glucose and D(-)-fructose are epimers of each other.

Correct answer: D(+)-glucose and D(-)-fructose yield the same osazone on reaction with phenylhydrazine.

Solution

D(+)-glucose and D(-)-fructose have identical configurations at C3, C4, C5, and C6; since osazone formation destroys the stereochemistry at C1 and C2, both give the same osazone (glucosazone). Reduction of fructose gives two products (sorbitol and mannitol) because a new chiral centre is created at C2.

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