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

Mutarotation

Organic Chemistry

28. Carbohydrates

Mutarotation

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4:52 minutes

Problem 22

Textbook Question

Suggest a mechanism by which α-d-glucopyranose is converted to β-d-glucopyranose in acid. [See Figure 27.18.]

Verified step by step guidance

Identify the starting and ending structures: α-d-glucopyranose and β-d-glucopyranose. Notice that the difference is the position of the hydroxyl group at the anomeric carbon (C1), which is axial in α and equatorial in β.

Recognize that the conversion involves the opening of the pyranose ring to form the open-chain form of glucose, followed by reclosure to form the β-anomer.

In acidic conditions, the reaction begins with the protonation of the oxygen in the hemiacetal linkage of α-d-glucopyranose, making it a better leaving group.

The ring opens to form the open-chain form of glucose, which is an aldehyde. This step involves the breaking of the C1-O bond, resulting in a linear form of glucose.

The open-chain form can then undergo ring closure again, but this time the hydroxyl group can attack the carbonyl carbon from the opposite side, leading to the formation of β-d-glucopyranose, where the hydroxyl group at C1 is now equatorial.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Anomeric Carbon

The anomeric carbon is the carbon atom in a sugar that is derived from the carbonyl carbon of the open-chain form. In cyclic sugars, it is the carbon that determines the alpha (α) or beta (β) configuration based on the orientation of the hydroxyl group. Understanding the role of the anomeric carbon is crucial for explaining the interconversion between α-d-glucopyranose and β-d-glucopyranose.

Acid-Catalyzed Equilibrium

In an acid-catalyzed reaction, the presence of acid facilitates the conversion between different forms of a compound by protonating functional groups, making them more reactive. For the conversion of α-d-glucopyranose to β-d-glucopyranose, the acid helps to open the cyclic structure, allowing the molecule to equilibrate between its anomeric forms before closing again in the β configuration.

Mutarotation

Mutarotation is the process by which the specific rotation of a carbohydrate changes over time as it equilibrates between its anomeric forms. In the case of α-d-glucopyranose and β-d-glucopyranose, mutarotation involves the interconversion of these two forms in solution, which is facilitated by the presence of acid, leading to a dynamic equilibrium between the two anomers.

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Related Practice

Textbook Question

(a) Suggest an arrow-pushing mechanism for the mutarotation of α-d-glucose to β-d-glucose in the presence of acid. Acid isn’t necessary, but it does increase the rate of the process.

(b) What is the role of acid in your mechanism?

Textbook Question

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of α-D-galactopyranose is +150.7°, and that of the β anomer is +52.8°. When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.2°. Determine the percentages of the two anomers present at equilibrium.

Textbook Question

Calculate the percentages of $α$ -D-glucose and $β$ -D-glucose present at equilibrium from the specific rotations of $α$ -D-glucose, $β$ -D-glucose, and the equilibrium mixture. Compare your values with those given in Section 20.10. (Hint: The specific rotation of the mixture equals the specific rotation of $α$ -D-glucose times the fraction of glucose present in the a-form plus the specific rotation of $β$ -D-glucose times the fraction of glucose present in the $β$ -form.)

Textbook Question

The specific rotation of α-D-galactose is 150.7 and that of β-D-galactose is 52.8. When an aqueous mixture that was initially 70% α-D-galactose and 30% β-D-galactose reaches equilibrium, the specific rotation is 80.2. What is the percentage of α-D-galactose and β-D galactose at equilibrium?

Textbook Question

Suggest a mechanism by which α-d-glucopyranose is converted to β-d-glucopyranose in base. [See Figure 27.18.]

Textbook Question

(a) Suggest an arrow-pushing mechanism for the mutarotation of α-d-glucose to β-d-glucose in the presence of acid. Acid isn’t necessary, but it does increase the rate of the process.

(b) What is the role of acid in your mechanism?

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