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

5. Chirality

Enantiomeric Excess

Organic Chemistry

5. Chirality

Enantiomeric Excess

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Problem 69

Textbook Question

Upon dissolving α-D-glucose or β-D-glucose in water, the specific rotation gradually changes, eventually reaching +52.6° for both solutions. Explain what is happening here.

Verified step by step guidance

Understand the concept of mutarotation: Mutarotation is the change in the optical rotation of a solution due to the interconversion between different anomers of a sugar (α and β forms) in aqueous solution. This occurs because the sugar exists in equilibrium between its cyclic and open-chain forms.

Recognize the structure of α-d-glucose and β-d-glucose: Both are cyclic forms of glucose, differing in the orientation of the hydroxyl group (-OH) on the anomeric carbon (C1). In α-d-glucose, the -OH group is on the opposite side of the ring relative to the CH2OH group, while in β-d-glucose, the -OH group is on the same side as the CH2OH group.

Describe the equilibrium process: When α-d-glucose or β-d-glucose is dissolved in water, the cyclic forms open up to form the linear chain structure. The linear chain then closes back into cyclic forms, allowing interconversion between the α and β anomers. This process establishes an equilibrium mixture of α-d-glucose, β-d-glucose, and a small amount of the open-chain form.

Explain the change in specific rotation: The specific rotation of a solution depends on the proportions of the α and β anomers present. Initially, the specific rotation reflects the predominant form (α or β) in the solution. As equilibrium is reached, the specific rotation stabilizes at +52.6°, which corresponds to the equilibrium mixture of α-d-glucose and β-d-glucose in water.

Conclude with the significance of the equilibrium: The final specific rotation of +52.6° is a characteristic property of the equilibrium mixture of α-d-glucose and β-d-glucose in water. This demonstrates the dynamic nature of mutarotation and the ability of glucose to interconvert between its anomeric forms in aqueous solution.

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Key Concepts

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

Anomeric Effect

The anomeric effect refers to the preference of certain anomers (α and β forms of sugars) to adopt specific conformations in solution. In the case of glucose, the α and β anomers can interconvert in water, leading to a mixture of both forms. This phenomenon affects the optical rotation of the solution as the equilibrium shifts between the two anomers.

Optical Activity

Optical activity is the ability of a substance to rotate the plane of polarized light. This property is significant in carbohydrates like glucose, where the specific rotation is a measure of how much the light is rotated by a solution of the sugar. The specific rotation can change as the concentration of different anomers in solution varies, reflecting the dynamic equilibrium between them.

Mutarotation

Mutarotation is the process by which the optical rotation of a sugar solution changes over time as the anomers interconvert. For glucose, this involves the conversion between α-d-glucose and β-d-glucose in water, leading to a gradual change in specific rotation until it stabilizes at a constant value. This process is crucial for understanding the behavior of sugars in solution.

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Related Practice

Textbook Question

A student wanted to measure the specific rotation of the following propionate derivative (density = 1.12 g/ml).

A sample of the pure compound was placed in a 10.0-cm polarimeter tube, and using the sodium D line, the observed rotation at 20°C was determined to be +46.6° . What is the specific rotation of the propionate derivative?

Textbook Question

Using a 1.0-cm polarimeter cell and a solution prepared by dissolving 2.54 g of glucose in 1 L of H₂O, the specific rotation of d-glucose was calculated to be +52.7° at 20 °C using the sodium D line as the source of light. What was the observed rotation of the solution?

Textbook Question

Cholesterol (50 mg) was dissolved in 10 mL of chloroform and placed in a 1.0-cm polarimeter cell. This solution produced an observed rotation (using the sodium D line at 20°C ) of - 1.58° . What is the specific rotation of cholesterol?

Textbook Question

The α- and β-anomers of glucose are shown here. In solution, these two epimers can interconvert through a process called mutarotation.

Given that α-D-glucose has a specific rotation of + 112.2° , why is the specific rotation of β-D-glucose not -112.2°? What molecule would have a specific rotation of -112.2°?

Textbook Question

Questions (a)–(d) all refer to the following reaction, which has been engineered to produce one enantiomer to the exclusion of the other.

Textbook Question

How might you separate enantiomers of 2-phenylpropionic acid?

Textbook Question

A chemist prepared a racemic mixture of the enantiomeric sulfonic acids shown here. Suggest two ways that these might be separated.

Open Question

When 0.200 g of lactose is dissolved in 10.0 ml of water and placed in a sample cell 10.0 cm in length, the observed rotation is +2°. Calculate the specific rotation of lactose.

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