Which of the following is a reducing sugar?

(1) A cyclic sugar with free anomeric –OH (hemiacetal form)
(2) A sugar with –OCH₃ at the anomeric carbon (methyl glycoside — acetal form)
(3) A disacchari

Question

Which of the following is a reducing sugar?

(1) A cyclic sugar with free anomeric –OH (hemiacetal form)
(2) A sugar with –OCH₃ at the anomeric carbon (methyl glycoside — acetal form)
(3) A disaccharide with both anomeric carbons involved in glycosidic linkage
(4) A sugar with two –OCH₃ groups blocking both anomeric positions

Canvas Classes · Accepted Answer

Reducing sugar definition:

A reducing sugar has a free anomeric –OH group (hemiacetal or hemiketal) that can open to give a free aldehyde or ketone group, which then reduces oxidizing agents (Fehling's, Tollens', Benedict's reagents).

Key principle:
- Hemiacetal (free –OH at anomeric C) → reducing sugar ✓
- Acetal (–OR at anomeric C, e.g., methyl glycoside) → non-reducing sugar ✗
- Glycosidic bond at anomeric C → non-reducing (if both anomeric carbons are blocked, as in sucrose)

Analysis of o

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