Textbook Question
Predict the products obtained when d-galactose reacts with each reagent.
(b) NaOH, H2O
28. Carbohydrates
Monosaccharides - Aldose-Ketose Rearrangement
3:04 minutesProblem 36
Textbook Question
Ketohexoses form from aldohexoses. Suggest a mechanism for the acid-catalyzed version of this reaction. What type of reaction is this? [You’ve seen it before.]
Verified step by step guidance1
Identify the starting aldohexose and the resulting ketohexose. The starting compound is an aldohexose with an aldehyde group at the top, and the product is a ketohexose with a ketone group on the second carbon.
Recognize that the reaction involves an isomerization process where the aldehyde group is converted into a ketone group. This is a type of tautomerization reaction known as an aldose-ketose isomerization.
In the presence of an acid catalyst (H3O+), the reaction begins with the protonation of the carbonyl oxygen of the aldehyde group, increasing its electrophilicity.
A hydride shift occurs where a hydrogen atom from the adjacent carbon (C2) migrates to the carbonyl carbon, forming a carbocation at C2. This step is crucial for the rearrangement of the carbonyl group.
Finally, deprotonation of the hydroxyl group at C2 leads to the formation of the ketone group, completing the conversion from an aldohexose to a ketohexose.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Aldohexoses and Ketohexoses
Aldohexoses are six-carbon sugars with an aldehyde group, while ketohexoses contain a ketone group. The conversion from aldohexoses to ketohexoses involves the rearrangement of functional groups, specifically the migration of the carbonyl group from the end of the chain to an internal position. This transformation is crucial in carbohydrate chemistry and is often seen in metabolic pathways.
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Acid-Catalyzed Reactions
Acid-catalyzed reactions involve the use of an acid to increase the rate of a chemical reaction. In the context of converting aldohexoses to ketohexoses, the acid protonates the carbonyl oxygen, making it more electrophilic and facilitating the rearrangement. This type of catalysis is common in organic reactions, particularly in carbohydrate chemistry, where protonation can stabilize intermediates.
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Rearrangement Reactions
Rearrangement reactions involve the structural reorganization of a molecule, leading to the formation of isomers. In the case of aldohexoses converting to ketohexoses, the reaction is a type of rearrangement where the carbon skeleton is altered. This process is significant in organic synthesis and biochemistry, as it allows for the interconversion of different sugar forms, impacting their biological functions.
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