Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids

Problem 68

Chapter 26, Problem 68

The α- and β-anomers of glucose are shown here. In solution, these two epimers can interconvert through a process called mutarotation.

Given that α-D-glucose has a specific rotation of + 112.2° , why is the specific rotation of β-D-glucose not -112.2°? What molecule would have a specific rotation of -112.2°?

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Understand the concept of mutarotation: Mutarotation is the process by which the α- and β-anomers of a sugar interconvert in aqueous solution through the open-chain form of the sugar. This results in a change in the specific rotation of the solution until equilibrium is reached.

Recognize that the specific rotation of a compound is an intrinsic property that depends on its molecular structure. The specific rotation of α-d-glucose is +112.2°, but the specific rotation of β-d-glucose is not simply the negative of this value because the two anomers are not mirror images of each other (they are epimers, differing only at the anomeric carbon).

Recall that a molecule with a specific rotation of -112.2° would need to be the enantiomer of α-d-glucose. Enantiomers are non-superimposable mirror images of each other, and they have specific rotations that are equal in magnitude but opposite in sign.

Identify the enantiomer of α-d-glucose: The enantiomer of α-d-glucose is α-l-glucose. This molecule would have a specific rotation of -112.2° because it is the mirror image of α-d-glucose.

Conclude that β-d-glucose is not the enantiomer of α-d-glucose, but rather an epimer. Therefore, its specific rotation is not simply the negative of +112.2°. Instead, β-d-glucose has its own specific rotation value, which is determined experimentally.

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Key Concepts

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

Anomers

Anomers are a type of stereoisomer found in carbohydrates, specifically cyclic forms of sugars. They differ at the anomeric carbon, which is the carbon atom that becomes a new chiral center when a sugar cyclizes. In the case of glucose, the α- and β-anomers differ in the orientation of the hydroxyl group (-OH) at the anomeric carbon, leading to distinct physical properties, including specific rotation.

Specific Rotation

Specific rotation is a measure of how much a chiral compound can rotate plane-polarized light, expressed in degrees. It is influenced by the molecular structure and the concentration of the solution. Each anomer of glucose has a unique specific rotation due to differences in their spatial arrangement, which affects how they interact with light. The specific rotation is not simply the negative of the other anomer's value because it depends on the overall molecular configuration.

Mutarotation

Mutarotation is the process by which anomeric forms of a sugar interconvert in solution, leading to a change in optical rotation over time. This occurs as the sugar equilibrates between its α and β forms, which can be influenced by factors such as temperature and concentration. The specific rotation of a mixture of anomers will stabilize at a value that reflects the proportions of each form present, rather than being a simple average of their individual rotations.

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Mechanism

Related Practice

Textbook Question

Starting with a Fischer projection of d-glucose (and other sugars), switch only the stereocenter that gave it the designation of d. What new sugar have you made? [Hint: The answer is not l-glucose.]

Textbook Question

Suggest a mechanism by which TXB₂ might be formed from TXA₂ in an acid-catalyzed hydrolysis reaction. [The structure has been simplified.]

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Textbook Question

(a) Suggest an arrow-pushing mechanism for the mutarotation of α-d-glucose to β-d-glucose in the presence of acid. Acid isn’t necessary, but it does increase the rate of the process.

(b) What is the role of acid in your mechanism?

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Textbook Question

The α- and β-anomers of glucose are shown here.

In solution, these two epimers can interconvert through a process called mutarotation. What is the ratio of anomers (α:β) when the specific rotation is +52.6°

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Textbook Question

Upon dissolving α-D-glucose or β-D-glucose in water, the specific rotation gradually changes, eventually reaching +52.6° for both solutions. Explain what is happening here.

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Textbook Question

Identify the isoprene units in the terpenes shown. In cross-linked or ring-containing terpenes, linkages can be formed between more than just C₁ and C₄ of isoprene.

(e)

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