Ch. 23 - Carbohydrates and Nucleic Acids

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 23 - Carbohydrates and Nucleic Acids

Problem 10a

Chapter 23, Problem 10a

Figure 23-2 shows that the degradation of D-glucose gives D-arabinose, an aldopentose. Arabinose is most stable in its furanose form. Draw D-arabinofuranose.

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Understand the problem: The degradation of D-glucose to D-arabinose involves the loss of one carbon atom, resulting in an aldopentose (a five-carbon sugar with an aldehyde group). The problem asks us to draw D-arabinofuranose, the furanose form of D-arabinose.

Recall the furanose form: A furanose is a five-membered ring structure formed when the hydroxyl group on the penultimate carbon (C-4 in this case) reacts with the aldehyde group on C-1, creating a cyclic hemiacetal.

Determine the stereochemistry: D-arabinose is a D-sugar, meaning the hydroxyl group on the penultimate carbon (C-4) is on the right in the Fischer projection. This stereochemistry must be preserved in the furanose form.

Draw the ring structure: Start by drawing a five-membered ring (furanose). Place the oxygen atom as one of the ring members. Attach the aldehyde-derived carbon (C-1) and the penultimate carbon (C-4) to the ring, ensuring the correct stereochemistry for the hydroxyl groups on C-2, C-3, and C-4.

Add substituents: Place the hydroxyl groups and hydrogen atoms on the appropriate carbons in the ring, ensuring the stereochemistry matches that of D-arabinose. The hydroxyl group on C-1 will be part of the hemiacetal linkage, and the remaining hydroxyl groups should be positioned according to the D-arabinose configuration.

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Key Concepts

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

Aldopentose Structure

Aldopentoses are monosaccharides that contain five carbon atoms and an aldehyde functional group. D-arabinose, specifically, is an aldopentose with a specific configuration at each chiral center. Understanding the structure of aldopentoses is crucial for recognizing their properties and reactivity in biochemical processes.

Furanose Form

The furanose form refers to a cyclic structure of sugars that resembles a five-membered ring, typically formed when a hydroxyl group reacts with the carbonyl group. In the case of D-arabinose, the furanose form is the most stable configuration, which influences its chemical behavior and interactions in biological systems.

Cyclization of Sugars

Cyclization is the process by which linear forms of sugars convert into cyclic structures, significantly affecting their stability and reactivity. This transformation involves the nucleophilic attack of a hydroxyl group on the carbonyl carbon, leading to the formation of either pyranose (six-membered) or furanose (five-membered) rings, which are essential for understanding carbohydrate chemistry.

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Monosaccharides - Cyclization

Related Practice

Textbook Question

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of α-D-galactopyranose is +150.7°, and that of the β anomer is +52.8°. When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.2°. Determine the percentages of the two anomers present at equilibrium.

Textbook Question

The carbonyl group in D-galactose may be isomerized from C1 to C2 by brief treatment with dilute base (by the enediol rearrangement). The product is the C4 epimer of fructose. Draw the furanose structure of the product.

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Textbook Question

Talose is the C4 epimer of mannose. Draw the chair conformation of D-talopyranose.

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Textbook Question

Allose is the C3 epimer of glucose. Draw the cyclic hemiacetal form of D-allose, first in the chair conformation and then in the Haworth projection.

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Textbook Question

Draw the Haworth projection for the cyclic structure of D-mannose by laying down the Fischer projection.

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Textbook Question

Ribose, the C2 epimer of arabinose, is most stable in its furanose form. Draw D-ribofuranose.

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