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 59

Chapter 21, Problem 59

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

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Understand the problem: Aldaric acids are dicarboxylic acids derived from aldoses by oxidizing both the aldehyde group and the terminal primary alcohol group to carboxylic acids. For D-glucose, this means both the C1 and C6 carbons are oxidized to carboxylic acids. Lactones are cyclic esters formed when a hydroxyl group reacts with a carboxylic acid group within the same molecule.

Identify the functional groups in D-glucose: D-glucose has hydroxyl (-OH) groups on carbons C2, C3, C4, and C5. After oxidation to aldaric acid, the molecule will have carboxylic acid groups at C1 and C6, and hydroxyl groups at the other carbons.

Determine the possible lactone formations: A five-membered ring lactone (γ-lactone) forms when the hydroxyl group on C4 or C5 reacts with the carboxylic acid group on C1. This creates two possible structures depending on which hydroxyl group participates in the reaction.

Draw the first lactone structure: For the first lactone, the hydroxyl group on C4 reacts with the carboxylic acid group on C1. This forms a five-membered ring with the following atoms: C1 (carboxylic acid carbon), O (from the hydroxyl group on C4), C4, C3, and C2. The remaining hydroxyl groups and the carboxylic acid group on C6 remain unchanged.

Draw the second lactone structure: For the second lactone, the hydroxyl group on C5 reacts with the carboxylic acid group on C1. This forms a five-membered ring with the following atoms: C1 (carboxylic acid carbon), O (from the hydroxyl group on C5), C5, C4, and C3. The remaining hydroxyl groups and the carboxylic acid group on C6 remain unchanged.

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Key Concepts

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

Aldaric Acid

Aldaric acids are a type of dicarboxylic acid derived from aldoses, where both the aldehyde and the terminal alcohol group are oxidized to carboxylic acids. In the case of d-glucose, the aldaric acid formed is d-glucaric acid. Understanding the structure of aldaric acids is crucial for recognizing how they can undergo cyclization to form lactones.

Lactone Formation

Lactones are cyclic esters formed when a hydroxyl group reacts with a carboxylic acid group within the same molecule, leading to the loss of water. In the case of d-glucose aldaric acid, two different five-membered lactones can be formed depending on which hydroxyl group participates in the reaction. This concept is essential for predicting the structures of the resulting lactones.

Five-Membered Ring Structures

Five-membered ring structures, such as furanoses, are common in carbohydrate chemistry. They are formed when a sugar molecule cyclizes, creating a stable ring that can influence the molecule's reactivity and properties. Recognizing the formation of these rings from aldaric acids is key to drawing the correct structures of the lactones derived from d-glucose.

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Related Practice

Textbook Question

How many aldaric acids are obtained from the 16 aldohexoses?

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Textbook Question

Draw each of the following:

a. β-D-talopyranose

b. α-D-idopyranose

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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.

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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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Textbook Question

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

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