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 - Forming Cyclic Hemiacetals

Organic Chemistry

28. Carbohydrates

Monosaccharides - Forming Cyclic Hemiacetals

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2:42 minutes

Problem 47b

Textbook Question

Without referring to the chapter, draw the chair conformations of
(b) α-D-allopyranose (the C3 epimer of glucose).

Verified step by step guidance

Understand the structure of α-D-allopyranose: It is a six-membered cyclic sugar (pyranose form) and the C3 epimer of glucose. This means the hydroxyl group (-OH) on the third carbon (C3) is inverted compared to α-D-glucopyranose.

Recall the rules for drawing chair conformations: In a chair conformation, substituents on the ring alternate between axial (perpendicular to the ring plane) and equatorial (parallel to the ring plane) positions. The α-anomer indicates that the anomeric hydroxyl group (-OH on C1) is in the axial position (down) for D-sugars.

Assign the stereochemistry for each carbon: Start with C1 (anomeric carbon) and place the -OH group in the axial position (down). For C2, place the -OH group in the equatorial position (up). For C3, since it is the epimer of glucose, place the -OH group in the axial position (down). Continue assigning the stereochemistry for C4 and C5 based on the D-configuration of the sugar.

Draw the chair conformation: Begin by sketching the six-membered chair structure. Place the substituents (hydroxyl groups and hydrogens) on each carbon according to the stereochemistry determined in the previous step. Ensure that the axial and equatorial positions alternate correctly.

Double-check the structure: Verify that the hydroxyl groups and hydrogens are placed correctly based on the α-anomer and the C3 epimer of glucose. Confirm that the chair conformation is drawn accurately with proper stereochemistry.

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Key Concepts

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

Chair Conformation

Chair conformation is a three-dimensional representation of cyclohexane and its derivatives, which minimizes steric strain and torsional strain. In this conformation, the carbon atoms are arranged in a staggered manner, resembling a chair, allowing for more stable interactions between substituents. Understanding chair conformations is crucial for predicting the stability and reactivity of cyclic compounds.

Epimers

Epimers are a specific type of diastereomer that differ in configuration at only one stereogenic center. In the case of a-D-allopyranose, it is an epimer of glucose at the C3 position, meaning that while they share the same molecular formula and most stereocenters, they differ in the orientation of the hydroxyl group at that specific carbon. Recognizing epimers is essential for understanding carbohydrate chemistry and their biological roles.

Pyranose Structure

Pyranose refers to a six-membered ring structure formed by the cyclization of monosaccharides, typically involving five carbon atoms and one oxygen atom. In the case of a-D-allopyranose, the ring structure is stabilized by the formation of an acetal linkage between the anomeric carbon and a hydroxyl group. Familiarity with pyranose structures is vital for analyzing the properties and reactions of sugars in organic chemistry.

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Related Practice

Textbook Question

Two structures for the sugar glucose are shown on page 914. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base.

(a) Propose a mechanism for the cyclization, assuming a trace of acid is present.

Textbook Question

Two structures for the sugar glucose are shown on page 914. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base.

(b) The cyclic hemiacetal is more stable than the open-chain form, so very little of the open-chain form is present at equilibrium. Will an aqueous solution of glucose reduce Tollens reagent and give a positive Tollens test? Explain.

Textbook Question

Draw the structures of the compound α-D-allopyranose.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

Textbook Question

Without referring to the chapter, draw the chair conformations of

(a) β-D-mannopyranose (the C2 epimer of glucose).

Textbook Question

Without referring to the chapter, draw the chair conformations of

(d) N-acetylglucosamine, glucose with the C2 oxygen atom replaced by an acetylated amino group.

Multiple Choice

In the formation of a cyclic hemiacetal from a monosaccharide, what type of reaction occurs?

Multiple Choice