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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7:02 minutes

Problem 63

Textbook Question

Two structures of the sugar fructose are shown next. The cyclic structure predominates in aqueous solution.

(a) Number the carbon atoms in the cyclic structure. What is the functional group at C2 in the cyclic form?
(b) Propose a mechanism for the cyclization, assuming a trace of acid is present.

Verified step by step guidance

Step 1: Number the carbon atoms in the cyclic structure of fructose. Start with the anomeric carbon (the carbon attached to two oxygen atoms) as C1, and proceed sequentially around the ring to number the remaining carbons.

Step 2: Identify the functional group at C2 in the cyclic form. In the cyclic structure of fructose, C2 is part of a hemiketal functional group, which forms when a ketone reacts with an alcohol group within the same molecule.

Step 3: To propose a mechanism for the cyclization, recognize that fructose contains a ketone group at C2 and hydroxyl groups on other carbons. In the presence of a trace of acid, the ketone group at C2 becomes protonated, making it more electrophilic.

Step 4: A nucleophilic attack occurs when the hydroxyl group on C5 attacks the electrophilic carbon of the protonated ketone at C2. This forms a new C-O bond, creating a six-membered ring (a hemiketal).

Step 5: The final step involves deprotonation of the oxygen atom that participated in the nucleophilic attack, stabilizing the cyclic structure. This results in the formation of the predominant cyclic form of fructose in aqueous solution.

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Key Concepts

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

Cyclic Structures of Sugars

Cyclic structures of sugars, such as fructose, are formed when the carbonyl group reacts with a hydroxyl group within the same molecule. This intramolecular reaction leads to the formation of a ring, which is more stable in aqueous solutions. Understanding the specific carbon atoms involved in this cyclization is crucial for identifying functional groups and their positions.

Functional Groups in Sugars

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In the case of fructose, the functional group at C2 in its cyclic form is a hydroxyl group (-OH), which plays a significant role in the sugar's reactivity and properties. Identifying functional groups is essential for understanding the behavior of organic compounds.

Acid-Catalyzed Cyclization Mechanism

The cyclization of sugars like fructose can be catalyzed by acids, which protonate the carbonyl oxygen, increasing its electrophilicity. This makes the carbonyl carbon more susceptible to nucleophilic attack by a hydroxyl group, leading to ring formation. Understanding this mechanism is vital for predicting the behavior of sugars in acidic conditions and for grasping broader concepts in organic reaction mechanisms.

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