Which of the following statements describes a glycoside?

Question

BlackTom AI · Accepted Answer

A glycoside is an acetal formed when the anomeric hydroxyl of a sugar is converted into an OR group by reaction with an alcohol (a glycosidic bond).

1. Relevant concepts:
- Hemiacetal vs acetal: a hemiacetal has one –OR and one –OH on the same carbon (anomeric carbon in sugars); an acetal has two –OR groups (no free –OH).  
- Anomeric carbon: the carbon derived from the carbonyl (C=O) when a sugar cyclizes; its substituents determine hemiacetal vs acetal character.  
- Glycosidic bond: the C–O–R linkage at the anomeric carbon produced when the anomeric OH reacts with an alcohol to give an acetal (glycoside).

2. Step-by-step reasoning:
- Start with a cyclic sugar (e.g., glucose) in its hemiacetal form: the anomeric carbon bears an –OH and an –OR (ring oxygen) arrangement typical of a hemiacetal.  
- Under acid-catalyzed conditions, the anomeric –OH can be protonated and leave as water, allowing nucleophilic attack by an external alcohol (R–OH).  
- The result is substitution of the anomeric –OH by –OR, producing an acetal functional group at that carbon — this C–O–R linkage is the glycosidic bond.  
- Because the anomeric OH is replaced, the product no longer shows free mutarotation and is classed as an acetal (glycoside).

3. Intermediate steps (mechanistic outline):
- Protonation of anomeric OH → departure as H2O → formation of an oxocarbenium-like intermediate → nucleophilic attack by R–OH → deprotonation → acetal (glycoside) formation.

4. Why the correct answer is correct:
- Definitionally, a glycoside is the acetal derivative of a sugar (the sugar’s anomeric OH is converted to OR). That matches option D exactly.

5. Why other options are incorrect:
- A) enol-keto form: refers to tautomerism between enol and keto forms of carbonyl compounds, not to glycosides.  
- B) mirror image of a sugar: describes an enantiomer, not a glycoside.  
- C) hemiacetal of a sugar: a sugar in its cyclic form is a hemiacetal; a glycoside is the next stage (acetal), so C is wrong.  
- E) enantiomer of a sugar: same as B, incorrect.

6. Conclusion:
This reasoning confirms answer D) It is the acetal of a sugar.

Which of the following statements describes a glycoside?单项选择题

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