Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

28. Carbohydrates

Monosaccharides - Aldose-Ketose Rearrangement

Organic Chemistry

28. Carbohydrates

Monosaccharides - Aldose-Ketose Rearrangement

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3:04 minutes

Problem 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 guidance

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.

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.

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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Related Practice

Textbook Question

Predict the products obtained when d-galactose reacts with each reagent.

(b) NaOH, H₂O

Textbook Question

When D-tagatose is added to a basic aqueous solution, an equilibrium mixture of monosaccharides is obtained, two of which are aldohexoses and two of which are ketohexoses. Identify the aldohexoses and ketohexoses.

Textbook Question

In glycolysis, why must glucose-6-phosphate isomerize to fructose-6-phosphate (Section 22.12 ) before the cleavage reaction with aldolase occurs?

Textbook Question

Write the mechanism for the base-catalyzed conversion of D-fructose to D-glucose and D-mannose.

Textbook Question

Propose a mechanism for the formation of d-allose from d-glucose in a basic solution.

Textbook Question

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

(b) Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has two –OH protons, and removing one or the other gives two different enolate ions.

Multiple Choice

Which of the following monosaccharides can undergo aldose-ketose rearrangement to form fructose?

Textbook Question

Predict the products obtained when d-galactose reacts with each reagent.

(b) NaOH, H₂O

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