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 67

Chapter 21, Problem 67

Draw the mechanism for the elimination step in the Wohl degredation.

Verified step by step guidance

Understand the Wohl degradation: It is a reaction used to shorten an aldose sugar by one carbon atom. The elimination step involves the removal of hydrogen cyanide (HCN) from an intermediate imine compound.

Identify the intermediate: The intermediate in the Wohl degradation is an imine formed by the reaction of the aldose sugar with hydroxylamine (NH2OH) and subsequent dehydration. This imine is then treated with a base to initiate the elimination step.

Deprotonation step: A strong base (e.g., NaOH or KOH) deprotonates the hydrogen atom on the carbon adjacent to the imine group, forming a carbanion. This step is crucial for initiating the elimination process.

Formation of a double bond: The carbanion formed in the previous step donates its lone pair of electrons to form a double bond between the carbon and nitrogen atoms, leading to the elimination of the cyanide ion (CN⁻).

Final product: The elimination step results in the formation of an aldose sugar with one fewer carbon atom, along with the release of HCN as a byproduct. Ensure to draw the structure of the shortened aldose sugar and the eliminated HCN.

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Key Concepts

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

Wohl Degradation

The Wohl degradation is a chemical reaction that involves the conversion of a sugar into a smaller sugar through a series of reactions, including oxidation and elimination. This process is particularly important in organic chemistry for understanding carbohydrate chemistry and the manipulation of sugar structures. The reaction typically starts with the formation of an intermediate that undergoes elimination to yield the final product.

Elimination Reactions

Elimination reactions are a class of organic reactions where two substituents are removed from a molecule, resulting in the formation of a double bond or a ring structure. In the context of the Wohl degradation, the elimination step involves the removal of a leaving group and a hydrogen atom, leading to the formation of a double bond. Understanding the mechanisms of elimination reactions, such as E1 and E2 pathways, is crucial for predicting the products of these reactions.

Reaction Mechanism

A reaction mechanism is a step-by-step description of the process by which reactants are converted into products in a chemical reaction. It outlines the sequence of elementary steps, including bond breaking and formation, and provides insight into the transition states and intermediates involved. For the Wohl degradation, drawing the mechanism requires knowledge of the specific steps involved in the elimination process, including the formation of intermediates and the final product.

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Related Practice

Textbook Question

An unknown disaccharide gives a positive Tollens' test. A glycosidase hydrolyzes it to D-galactose and D-mannose. When the disaccharide is treated with methyl iodide and Ag2O and then hydrolyzed with dilute HCl, the products are 2,3,4,6-tetra-O-methylgalactose and 2,3,4-tri-O-methylmannose. Propose a structure for the disaccharide.

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Textbook Question

Trehalose, C₁₂H₂₂O₁₁, is a nonreducing sugar that is only 45% as sweet as sugar. When hydrolyzed by aqueous acid or the enzyme maltase, it forms only D-glucose. When it is treated with excess methyl iodide in the presence of Ag₂O and then hydrolyzed with water under acidic conditions, only 2,3,4,6-tetra-O-methyl-d-glucose is formed. Draw the structure of trehalose.

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Textbook Question

The specific rotation of α-D-galactose is 150.7 and that of β-D-galactose is 52.8. When an aqueous mixture that was initially 70% α-D-galactose and 30% β-D-galactose reaches equilibrium, the specific rotation is 80.2. What is the percentage of α-D-galactose and β-D galactose at equilibrium?

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Textbook Question

Predict whether D-altrose exists preferentially as a pyranose or a furanose. (Hint: In the most stable arrangement for a five-membered ring, all the adjacent substituents are trans.)

Textbook Question

Draw each of the following:

a. β-D-talopyranose

b. α-D-idopyranose

Textbook Question

Calculate the percentages of $α$ -D-glucose and $β$ -D-glucose present at equilibrium from the specific rotations of $α$ -D-glucose, $β$ -D-glucose, and the equilibrium mixture. Compare your values with those given in Section 20.10. (Hint: The specific rotation of the mixture equals the specific rotation of $α$ -D-glucose times the fraction of glucose present in the a-form plus the specific rotation of $β$ -D-glucose times the fraction of glucose present in the $β$ -form.)

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