Ch. 20 - Carboxylic Acids

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

Wade 9th Edition

Ch. 20 - Carboxylic Acids

Problem 10

Chapter 20, Problem 10

Write the mechanism for the base-catalyzed conversion of D-fructose to D-glucose and D-mannose.

Verified step by step guidance

Identify the reaction type: This is an example of an isomerization reaction known as the Lobry de Bruyn–van Ekenstein transformation, where d-fructose (a ketose) is converted into d-glucose and d-mannose (aldoses) under basic conditions.

Deprotonation of d-fructose: Under basic conditions, a hydroxide ion (OH⁻) abstracts the proton from the hydroxyl group adjacent to the carbonyl group (C2), forming an enolate intermediate. Represent this step using MathML:

{HO}_{-} + d-fructose → enolate

Formation of d-glucose: The enolate intermediate undergoes protonation at the oxygen atom of the enolate, leading to the formation of an aldehyde group at C1. This results in the formation of d-glucose. Represent this step using MathML:

enolate + H_{+} → d-glucose

Formation of d-mannose: The enolate intermediate can also undergo a different protonation pathway, where the proton is added to the opposite face of the enolate. This leads to the formation of d-mannose, an epimer of d-glucose at C2. Represent this step using MathML:

enolate + H_{+} → d-mannose

Conclude the mechanism: Summarize that the base-catalyzed isomerization of d-fructose involves the formation of an enolate intermediate, which can be protonated in two different ways to yield d-glucose and d-mannose as products.

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Key Concepts

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

Base-Catalyzed Mechanism

A base-catalyzed mechanism involves the use of a base to facilitate a chemical reaction. In this context, the base abstracts a proton from the sugar, increasing the nucleophilicity of the molecule. This enhanced reactivity allows for the rearrangement of the sugar's structure, leading to the formation of different isomers, such as d-glucose and d-mannose from d-fructose.

Sugar Isomerization

Isomerization is the process by which one molecule is transformed into another molecule with the same atoms but a different arrangement. In the conversion of d-fructose to d-glucose and d-mannose, the carbon skeleton of the sugar undergoes rearrangement. This process is crucial for understanding how different sugars can be interconverted through specific reaction pathways.

Enolization and Tautomerization

Enolization refers to the conversion of a carbonyl compound into its enol form, which contains a double bond and an alcohol group. Tautomerization is the rapid interconversion between the enol and keto forms. In the context of d-fructose conversion, enolization plays a key role in facilitating the rearrangement to produce d-glucose and d-mannose, highlighting the importance of these structural forms in sugar chemistry.

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Tautomerization Mechanisms

Related Practice

Textbook Question

Oxidation of a primary alcohol to an aldehyde usually gives some over-oxidation to the carboxylic acid. Assume you have used PCC to oxidize pentan-1-ol to pentanal.

(a) Show how you would use acid–base extraction to purify the pentanal.

(b) Which of the expected impurities cannot be removed from pentanal by acid–base extractions? How would you remove this impurity?

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Textbook Question

How many stereoisomers are possible for c. a ketotriose?

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Textbook Question

Show how you would synthesize the following carboxylic acids, using the indicated starting materials.

(a) oct-4-yne → butanoic acid

(b) trans-cyclodecene → decanedioic acid

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Textbook Question

Show how you would synthesize the following carboxylic acids, using the indicated starting materials.

(e) p-xylene → terephthalic acid

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Textbook Question

The IR spectrum of trans-oct-2-enoic acid is shown. Point out the spectral characteristics that allow you to tell that this is a carboxylic acid, and show which features lead you to conclude that the acid is unsaturated and conjugated.

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Textbook Question

Show how you would synthesize the following carboxylic acids, using the indicated starting materials.

(d) butan-2-ol → 2-methylbutanoic acid