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 - Acylation

Organic Chemistry

28. Carbohydrates

Monosaccharides - Acylation

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Problem 52f

Textbook Question

Predict the products obtained when d-galactose reacts with each reagent.
(f) excess Ac₂O and pyridine

Verified step by step guidance

Step 1: Understand the structure of D-galactose. D-galactose is an aldohexose, meaning it contains six carbon atoms with an aldehyde group at the first carbon and hydroxyl groups (-OH) on the remaining carbons.

Step 2: Recognize the role of the reagents. Ac2O (acetic anhydride) in the presence of pyridine acts as an acetylating agent. It reacts with hydroxyl groups (-OH) to form acetate esters (-OCOCH3). Pyridine serves as a base to neutralize the acidic byproducts of the reaction.

Step 3: Identify the reactive sites on D-galactose. The hydroxyl groups on carbons 2, 3, 4, and 6, as well as the hydroxyl group of the hemiacetal (if in cyclic form), are all potential sites for acetylation. The aldehyde group (if in open-chain form) does not react under these conditions.

Step 4: Predict the reaction outcome. In the presence of excess Ac2O, all hydroxyl groups on D-galactose will be acetylated, resulting in a fully acetylated derivative. If D-galactose is in its cyclic form, the hydroxyl group on the anomeric carbon (C1) will also be acetylated.

Step 5: Write the general structure of the product. The product will be a penta-acetate derivative of D-galactose, where each hydroxyl group is replaced by an acetate group (-OCOCH3). The stereochemistry of the sugar remains unchanged during this reaction.

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Key Concepts

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

Acetylation

Acetylation is a chemical reaction that introduces an acetyl group (C2H3O) into a molecule. In the context of carbohydrates like d-galactose, this process typically involves the reaction of the hydroxyl groups with acetic anhydride (Ac2O) in the presence of a base such as pyridine. This modification can enhance the solubility and stability of sugars, making them more amenable to further chemical transformations.

Pyridine as a Catalyst

Pyridine is often used as a catalyst or solvent in organic reactions due to its basicity and ability to stabilize charged intermediates. In the acetylation of d-galactose, pyridine helps to deprotonate the hydroxyl groups, facilitating the nucleophilic attack on the acetyl group from acetic anhydride. This results in the formation of acetylated derivatives of the sugar.

Stereochemistry of Sugars

Stereochemistry refers to the spatial arrangement of atoms in molecules and is crucial in understanding the behavior of sugars like d-galactose. The specific configuration of the hydroxyl groups in d-galactose influences how it reacts with reagents. When acetylation occurs, the stereochemical integrity of the sugar can affect the final product's properties and reactivity, making it essential to consider during product prediction.

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