Ch. 20 - The Organic Chemistry of Carbohydrates

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 20 - The Organic Chemistry of Carbohydrates

Problem 54

Chapter 21, Problem 54

Draw the mechanism for the formation of β-lactose from ⍺-D-galactose and β-D-glucose in dilute HCl.

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Identify the reactants: α-D-galactose and β-D-glucose. These are monosaccharides that will undergo a condensation reaction to form β-lactose, a disaccharide. The reaction occurs in the presence of dilute HCl, which acts as an acid catalyst.

Protonation of the hydroxyl group on the anomeric carbon of α-D-galactose occurs due to the acidic environment. This makes the anomeric carbon more electrophilic, facilitating the formation of a galactosyl oxocarbenium ion intermediate.

The β-D-glucose molecule acts as a nucleophile. The hydroxyl group on the anomeric carbon of β-D-glucose attacks the electrophilic anomeric carbon of the galactosyl oxocarbenium ion, forming a glycosidic bond. This step links the two monosaccharides together.

The newly formed glycosidic bond is between the C1 of galactose and the C4 of glucose, resulting in a β-1,4-glycosidic linkage. The stereochemistry of the β configuration is retained at the anomeric carbon of glucose.

Finally, the reaction is completed by deprotonation of the intermediate to regenerate the acid catalyst (HCl). The product, β-lactose, is formed as a disaccharide with a β-1,4-glycosidic bond between galactose and glucose.

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Key Concepts

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

Carbohydrate Structure

Understanding the structure of carbohydrates, particularly monosaccharides like a-D-galactose and b-D-glucose, is essential. These sugars have specific configurations and functional groups that influence their reactivity and ability to form glycosidic bonds. The anomeric carbon, which is the carbon that was part of the carbonyl group in the open-chain form, plays a crucial role in determining the type of glycosidic linkage formed.

Glycosidic Bond Formation

The formation of a glycosidic bond is a key reaction in carbohydrate chemistry, where two monosaccharides combine to form a disaccharide. This process typically involves the nucleophilic attack of the hydroxyl group of one sugar on the anomeric carbon of another, resulting in the release of a water molecule. In the case of b-lactose, the specific orientation of the hydroxyl groups on the sugars determines the type of glycosidic bond formed (1→4 linkage).

Acid-Catalyzed Reactions

In the presence of dilute HCl, the reaction mechanism for forming b-lactose involves acid-catalyzed activation of the hydroxyl groups. The acid protonates the hydroxyl group, making it a better leaving group and facilitating the nucleophilic attack. This catalytic role of acid is crucial for enhancing the reaction rate and ensuring the proper orientation and reactivity of the reactants during the formation of the glycosidic bond.

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Acid Catalyzed

Related Practice

Textbook Question

The aldaric acid of D-glucose forms two five-membered-ring lactones. Draw their structures.

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Textbook Question

How many aldaric acids are obtained from the 16 aldohexoses?

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Textbook Question

Draw the mechanism for the interconversion of α-D-glucose and β-D-glucose in dilute HCl.

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Textbook Question

A hexose was obtained after (+)-glyceraldehyde underwent three successive Kiliani–Fischer syntheses. Identify the hexose from the following experimental information: oxidation with nitric acid forms an optically active aldaric acid; a Wohl degradation followed by oxidation with nitric acid forms an optically inactive aldaric acid; and a second Wohl degradation forms erythrose.

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Textbook Question

Hyaluronic acid, a component of connective tissue, is the fluid that lubricates joints. It is a polymer of alternating N-acetyl-D-glucosamine and D-glucuronic acid subunits joined by β-1,3′-glycosidic linkages. Draw a short segment of hyaluronic acid.

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Textbook Question

A D-aldopentose is oxidized by nitric acid to an optically active aldaric acid. A Wohl degradation of the aldopentose leads to a monosaccharide that is oxidized by nitric acid to an optically inactive aldaric acid. Identify the D-aldopentose.

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