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 80

Chapter 26, Problem 80

A derivative of chitin, called chitosan, is sometimes incorporated into bandages to aid healing. Propose a reaction to convert the chitin dimer into a chitosan dimer. [Note: The glycoside linkage is unstable in acid.]

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Identify the structural difference between chitin and chitosan. Chitin has an acetamido group (NHCOCH3) on the C-2 position of the glucosamine unit, while chitosan has an amino group (NH2) at the same position.

Recognize that the conversion from chitin to chitosan involves the removal of the acetyl group from the acetamido group, which is a deacetylation reaction.

Propose a reaction condition that can achieve deacetylation. This typically involves treating chitin with a strong base, such as sodium hydroxide (NaOH), which can cleave the acetyl group.

Consider the reaction mechanism: The base will attack the carbonyl carbon of the acetamido group, leading to the removal of the acetyl group and formation of the free amino group.

Ensure that the reaction conditions are mild enough to prevent the hydrolysis of the glycosidic linkage, as it is unstable in acidic conditions. Therefore, using a basic environment is suitable for this transformation.

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Key Concepts

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

Chitin and Chitosan Structure

Chitin is a biopolymer made of N-acetylglucosamine units linked by β(1→4) glycosidic bonds. Chitosan is derived from chitin through deacetylation, where the acetyl groups are removed, resulting in a polymer of glucosamine units. Understanding the structural differences between these two polymers is crucial for proposing a reaction that converts chitin to chitosan.

Deacetylation Reaction

Deacetylation is a chemical reaction that involves the removal of an acetyl group from a molecule. In the case of converting chitin to chitosan, this reaction typically occurs under alkaline conditions, where chitin is treated with a strong base, such as sodium hydroxide. This process is essential for transforming the chitin dimer into a chitosan dimer by breaking the acetyl groups off the N-acetylglucosamine units.

Stability of Glycosidic Linkages

Glycosidic linkages, such as those found in chitin, can be sensitive to acidic conditions, leading to hydrolysis. The question notes that the glycosidic linkage is unstable in acid, which implies that any proposed reaction should avoid acidic conditions to prevent degradation of the polymer. Understanding the stability of these linkages is vital for successfully proposing a reaction that maintains the integrity of the chitin dimer during conversion to chitosan.

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Related Practice

Textbook Question

α-Pinene is a terpene found in pine trees. Starting from geranyl diphosphate (a terpene), draw the arrow-pushing mechanism to produce α-pinene. Assume that an enzyme active site will provide whatever acids and bases you might need.

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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

Is the compound d or l?

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