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Ch. 23 - Carbohydrates and Nucleic Acids
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo
Capitolo 23, Problema 39

Is gentiobiose a reducing sugar? Does it mutarotate? Explain your reasoning

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Gentiobiose is a disaccharide composed of two glucose molecules linked by a β(1→6) glycosidic bond. To determine if it is a reducing sugar, examine whether it has a free anomeric carbon (a carbon attached to a hydroxyl group and part of a hemiacetal or hemiketal structure).
In gentiobiose, one of the glucose units has its anomeric carbon involved in the glycosidic bond, but the other glucose unit retains a free anomeric carbon. This free anomeric carbon allows gentiobiose to act as a reducing sugar.
Mutarotation occurs when a sugar with a free anomeric carbon interconverts between its α and β anomers in aqueous solution. Since gentiobiose has a free anomeric carbon, it can undergo mutarotation.
The process of mutarotation involves the opening of the cyclic form of the sugar to form a linear aldehyde or ketone intermediate, followed by reclosure to form either the α or β anomer. This equilibrium is facilitated by the free anomeric carbon in gentiobiose.
In summary, gentiobiose is a reducing sugar because it has a free anomeric carbon, and it can mutarotate due to the ability of this free anomeric carbon to participate in the interconversion between α and β forms.

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Reducing Sugars

Reducing sugars are carbohydrates that can donate electrons to other molecules, typically due to the presence of a free aldehyde or ketone group. This property allows them to reduce certain chemical reagents, such as Benedict's or Fehling's solution. Gentiobiose, a disaccharide composed of two glucose units, has a free anomeric carbon that can participate in this reduction, classifying it as a reducing sugar.
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Reducing Sugars

Mutarotation

Mutarotation is the change in optical rotation that occurs when an anomeric carbon in a sugar molecule interconverts between its alpha and beta forms in solution. This process is significant for sugars with a free anomeric carbon, as it leads to a dynamic equilibrium between different anomeric forms. Gentiobiose can undergo mutarotation due to its free anomeric carbon, resulting in a change in its optical activity over time.
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Mechanism

Anomeric Carbon

The anomeric carbon is the carbon atom in a sugar that is bonded to two oxygen atoms, one of which is part of a hydroxyl group and the other is part of a ring structure. This carbon is crucial in determining the reducing properties of sugars and their ability to mutarotate. In gentiobiose, the anomeric carbon is involved in the glycosidic bond formation, influencing its reactivity and behavior in solution.
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Carbonation of Grignard Reagents
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