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Can someone please explain this to me? So confused!?
Okay this is what the textbook says. I feel like it is contradicting because first it says Tollens reagent cannot be reduced by glycosides, but then it says it can. Which is it?
"Tollens reagent is a basic reagent that detects aldehydes. Aldoses have an aldehyde on their open-chain form and reduce Tollens reagent. Tollens reagent promotes enediol rearrangement of ketoses so that ketoses also reduce Tollens reagent. Since Tollens reagent must react with the open-chain form of a sugar, glycosides (which are closed ring acetals) do NOT reduce Tollens reagent, while nonglycosides do.
Disaccharides and polysaccharides are glycosides where the aglycone is anothe rsugar. The anomeric carbon of a sugar can react with any of the hydroxyl groups of another sugar, but there are only three bonding arrangements that are common: a 1,4' link; a 1,6' link; and a 1,1' link. The numbers refer to the carbon numbers on the sugars. The linkages are glycosidic linkages. A disaccharide or polysaccharide will only react with Tollens reagent if there is an anomeric carbon that is not involved in a glycosidic bond and is free to react."
2 Answers
- steve_geo1Lv 79 years agoFavorite Answer
First off, look at the molecular structures of sugars in your textbook. All of the common sugars that you know reduce Tollen's reagent (also Fehling's, Benedict's, Barfoed's reagents). This means glucose, fructose, maltose, lactose, all of the aldohexoses. They are all capable of existing in their open-chain aldehyde or ketose forms.
The one common exception is sucrose. The anomeric carbon of glucose is closed as an unreactive acetal. The anomeric carbon of fructose is closed as an unreactive ketal. Study the structure of sucrose.
For an examination, this may be enough. Look at the structures of maltose and lactose, however. Each contains a sugar that is locked up in an unreactive acetal. The other is an open-chain aldehyde, however.
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