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 19b

Chapter 23, Problem 19b

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

(b) Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has two –OH protons, and removing one or the other gives two different enolate ions.

Verified step by step guidance

Identify the starting ketose structure and recognize that the isomerization involves the conversion of a ketose to an aldose through an enediol intermediate. This process occurs under basic aqueous conditions.

Understand that the first step involves deprotonation of the α-hydrogen (adjacent to the carbonyl group) of the ketose by a base (e.g., OH⁻). This forms an enolate ion, which is stabilized by resonance.

The enolate ion undergoes protonation at the oxygen atom of the carbonyl group, leading to the formation of the enediol intermediate. The enediol contains two hydroxyl (-OH) groups attached to adjacent carbons.

Recognize that the enediol intermediate can lose a proton from either of its hydroxyl groups. If the proton is removed from the hydroxyl group on the carbon adjacent to the original carbonyl carbon, it forms a new enolate ion that can rearrange to an aldose structure.

Finally, the enolate ion is protonated at the carbonyl carbon to regenerate a carbonyl group, completing the isomerization and forming the aldose. This step restores the neutral molecule and concludes the mechanism.

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Key Concepts

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

Isomerization

Isomerization is a chemical process where a compound is transformed into another compound with the same molecular formula but a different arrangement of atoms. In the context of sugars, isomerization can occur between ketoses and aldoses, affecting their reactivity and properties. Understanding this process is crucial for predicting how sugars behave under different conditions, particularly in aqueous environments.

Enediol Intermediate

An enediol intermediate is a species that contains a carbon-carbon double bond adjacent to a hydroxyl group. This structure is pivotal in the isomerization of sugars, as it allows for the interconversion between ketoses and aldoses. The presence of two hydroxyl protons in the enediol enables the formation of different enolate ions, which are key to the mechanism of isomerization.

Enolate Ion

An enolate ion is a resonance-stabilized anion formed by the deprotonation of a carbon adjacent to a carbonyl group. In the context of sugar isomerization, the formation of enolate ions from the enediol intermediate is essential for the conversion between ketoses and aldoses. The choice of which hydroxyl proton to remove leads to different enolate structures, influencing the pathway and outcome of the isomerization reaction.

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Related Practice

Textbook Question

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

(a) Under basic conditions, the proton alpha to the aldehyde (or ketone) carbonyl group is reversibly removed, and the resulting enolate ion is no longer asymmetric. Reprotonation can occur on either face of the enolate, giving either the original structure or its epimer. Because a mixture of epimers results, this process is called epimerization. Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture of glucose and its C2 epimer, mannose.

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Textbook Question

Draw the structures of the compound methyl α-D-galactopyranoside.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

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Textbook Question

Which of the following are reducing sugars? Comment on the common name sucrose for table sugar.

(e)

(f)

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Textbook Question

Two sugars, A and B, are known to be glucose and galactose, but it is not certain which one is which. On treatment with nitric acid, A gives an optically inactive aldaric acid, while B gives an optically active aldaric acid. Which sugar is glucose, and which is galactose?

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Textbook Question

Draw and name the products of nitric acid oxidation of

(a) D-mannose

(b) D-galactose

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Textbook Question

Which of the following are reducing sugars? Comment on the common name sucrose for table sugar.

(d) ethyl β-D-ribofuranoside

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