Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space

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

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Mullins 1st Edition

Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space

Problem 63

Chapter 5, Problem 63

A chemist prepared a racemic mixture of the enantiomeric sulfonic acids shown here. Suggest two ways that these might be separated.

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Understand the problem: A racemic mixture contains equal amounts of two enantiomers, which are non-superimposable mirror images of each other. The goal is to separate these enantiomers into their pure forms. This process is called resolution.

Step 1: Use a chiral resolving agent. React the racemic mixture with a chiral base or chiral alcohol to form diastereomeric salts or esters. Diastereomers have different physical properties (e.g., solubility, melting point), allowing them to be separated by physical methods such as crystallization.

Step 2: Perform chromatographic separation using a chiral stationary phase. In this method, the racemic mixture is passed through a column containing a chiral stationary phase. The enantiomers interact differently with the chiral environment, leading to their separation as they elute at different rates.

Step 3: After separation, regenerate the original enantiomers. For example, if diastereomeric salts were formed, treat them with an acid or base to recover the individual enantiomers of the sulfonic acids.

Step 4: Verify the purity of the separated enantiomers. Use techniques such as polarimetry to measure optical rotation or NMR spectroscopy with a chiral shift reagent to confirm the enantiomeric excess and purity of the separated compounds.

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Key Concepts

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

Racemic Mixture

A racemic mixture contains equal amounts of two enantiomers, which are molecules that are mirror images of each other. These compounds often exhibit different properties, particularly in chiral environments, such as biological systems. Understanding the nature of racemic mixtures is crucial for developing methods to separate enantiomers effectively.

Chiral Separation Techniques

Chiral separation techniques exploit the differences in the behavior of enantiomers in chiral environments. Common methods include chromatography using chiral stationary phases and resolution through diastereomer formation. These techniques are essential for isolating enantiomers from racemic mixtures, allowing for the study and application of each enantiomer's unique properties.

Enantiomers and Optical Activity

Enantiomers are compounds that are non-superimposable mirror images of each other and often exhibit different optical activities. This means that they rotate plane-polarized light in opposite directions, a property that can be used to distinguish between them. Understanding optical activity is vital for identifying and separating enantiomers in a racemic mixture.

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Related Practice

Textbook Question

Chromatography using a chiral stationary phase is able to separate a pair of enantiomers because one of the enantiomers forms a more tightly bound complex with the stationary phase. Assuming that the (S) enantiomer elutes more slowly, rationalize this result on a reaction coordinate diagram.

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Textbook Question

In Chapter 13, we explain how to convert secondary alcohols into ketones using a mild oxidation reaction. When the following enantiomerically pure and optically active secondary alcohol is submitted to these reaction conditions, the product is optically inactive. Explain this observation.

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Textbook Question

A compound with two chiral centers that is meso will always have opposite absolute configurations at the two chiral centers. That is, a meso compound will never be (R,R) or (S,S); instead, it will be (R,S). Explain why this must be true.

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Textbook Question

In contrast to Assessment 6.63, the following compounds should be easily separable using standard methods. Why?

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Textbook Question

In Chapter 12, we introduce the S_N2 reaction, a nucleophilic substitution reaction that proceeds with inversion. Confirm that inversion has occurred in each of the following examples by determining the absolute configuration of the chiral center in the reactants and products.

(a)

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