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

Reducing Sugars

Organic Chemistry

28. Carbohydrates

Reducing Sugars

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3:36 minutes

Problem 60a

Textbook Question

When the gum of the shrub Sterculia setigera is subjected to acidic hydrolysis, one of the water-soluble components of the hydrolysate is found to be tagatose. The following information is known about tagatose:
(1) Molecular formula C₆H₁₂O₆
(2) Undergoes mutarotation.
(3) Does not react with bromine water.
(4) Reduces Tollens reagent to give D-galactonic acid and D-talonic acid.
(5) Methylation of tagatose (using excess CH₃I and Ag₂O) followed by acidic hydrolysis gives 1,3,4,5-tetra-O-methyltagatose.
(a) Draw a Fischer projection structure for the open-chain form of tagatose.

Verified step by step guidance

Step 1: Analyze the molecular formula (C6H12O6) and determine that tagatose is a hexose (a six-carbon sugar) with the general formula of a carbohydrate (CnH2nOn). This indicates it is a monosaccharide.

Step 2: Note that tagatose undergoes mutarotation, which implies it is a reducing sugar and exists in equilibrium between its open-chain form and cyclic forms (α and β anomers). This also suggests the presence of a free aldehyde or ketone group in the open-chain form.

Step 3: Observe that tagatose does not react with bromine water. Bromine water selectively oxidizes aldehydes to carboxylic acids, so the lack of reaction indicates that tagatose is a ketose (contains a ketone group rather than an aldehyde group).

Step 4: Consider the reduction of Tollens reagent to form d-galactonic acid and d-talonic acid. This indicates that tagatose is a reducing sugar and can tautomerize to form an aldehyde group in its open-chain form, which reacts with Tollens reagent.

Step 5: Use the information from methylation (1,3,4,5-tetra-O-methyltagatose) to deduce the hydroxyl group positions. Combine this with the stereochemistry of d-galactose and d-tagatose to draw the Fischer projection of the open-chain form of tagatose, ensuring the ketone group is at C2 and the stereochemistry matches the given data.

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Key Concepts

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

Fischer Projection

The Fischer projection is a two-dimensional representation of a molecule that shows the configuration of its stereocenters. In this format, the vertical lines represent bonds that are oriented away from the viewer, while horizontal lines represent bonds that are coming towards the viewer. This method is particularly useful for visualizing carbohydrates, as it allows for easy identification of the orientation of hydroxyl groups and the anomeric carbon in sugars.

Mutarotation

Mutarotation is the change in optical rotation that occurs when an α-anomer and a β-anomer of a sugar interconvert in solution. This process is significant for sugars like tagatose, as it indicates the presence of an anomeric carbon that can exist in two different configurations. The ability to undergo mutarotation is a key characteristic of reducing sugars, which can affect their reactivity and interactions in biochemical processes.

Reducing Sugars

Reducing sugars are carbohydrates that can donate electrons to other molecules, typically due to the presence of a free aldehyde or ketone group. In the case of tagatose, its ability to reduce Tollens reagent demonstrates its reducing properties, which are crucial for various chemical reactions. Understanding the reactivity of reducing sugars is essential for predicting their behavior in organic reactions and their role in metabolic pathways.

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Related Practice

Textbook Question

Which of the following are reducing sugars? Comment on the common name sucrose for table sugar.

(e)

(f)

Textbook Question

Name the following compounds and indicate whether or not each is a reducing sugar:

Textbook Question

Name the following compounds and indicate whether or not each is a reducing sugar:

Textbook Question

A hexose is obtained when the residue of a shrub Sterculia setigeria undergoes acid-catalyzed hydrolysis. Identify the hexose from the following experimental information: it undergoes mutarotation; it does not react with Br₂; and D-galactonic acid and D-talonic acid are formed when it reacts with Tollens’ reagent.

Textbook Question

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(a) 4-O-(α-D-glucopyranosyl)-D-galactopyranose

Textbook Question

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(b) α-D-fructofuranosyl-β-D-mannopyranoside

Textbook Question

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

Textbook Question

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

(d) Ag(NH₃)₂^{+ –}OH

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