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 43

Chapter 5, Problem 43

How might you separate enantiomers of 2-phenylpropionic acid?

Verified step by step guidance

Step 1: Recognize that enantiomers are non-superimposable mirror images of each other, and they have identical physical properties in achiral environments. To separate them, you need to create a diastereomeric pair, which has different physical properties.

Step 2: React the enantiomers of 2-phenylpropionic acid with a chiral resolving agent, such as a chiral amine (e.g., (R)- or (S)-phenylethylamine). This reaction forms diastereomeric salts.

Step 3: Since diastereomers have different solubilities, recrystallize the diastereomeric salts in an appropriate solvent to separate them based on their differing solubility properties.

Step 4: After separation, hydrolyze the diastereomeric salts to regenerate the original enantiomers of 2-phenylpropionic acid.

Step 5: Confirm the separation of enantiomers using techniques such as polarimetry or chiral chromatography to verify their optical activity and purity.

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Key Concepts

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

Enantiomers

Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. They have identical physical properties except for their interaction with polarized light and reactions in chiral environments. Understanding enantiomers is crucial for separating them, as their distinct spatial arrangements lead to different chemical behaviors.

Chirality

Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image, often due to the presence of a chiral center, typically a carbon atom bonded to four different substituents. This concept is fundamental in organic chemistry, especially in the context of drug design and synthesis, as the different enantiomers can have vastly different biological activities.

Resolution of Enantiomers

Resolution of enantiomers is the process of separating a racemic mixture into its individual enantiomers. Techniques such as chiral chromatography, enzymatic resolution, or the use of chiral auxiliaries are commonly employed. Understanding these methods is essential for practical applications in pharmaceuticals and organic synthesis, where the specific enantiomer may be required for desired activity.

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Related Practice

Textbook Question

Convert the line-angle drawings into Fischer projections.

(a)

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Textbook Question

We discuss the reaction of Grignard reagents (organomagnesium compounds) to ketones in Chapter 17. Mechanistically, the reaction proceeds by the nucleophilic addition of a methyl carbanion to the electrophilic carbon of the carbonyl, breaking the C―Oπ bond, resulting in an alkoxide intermediate that is subsequently protonated to produce the 3° alcohol.

(b) Why, when the substrate is modified slightly, does the reaction result in an excess of one stereoisomer?

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Textbook Question

Convert the line-angle drawings into Fischer projections.

(b)

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Textbook Question

Figure 6.52 <IMAGE> shows the lipase-catalyzed kinetic resolution of secondary alcohols. Show a reaction coordinate diagram that rationalizes the results obtained.

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Textbook Question

Convert the line-angle drawings into Fischer projections.

(c)

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Textbook Question

(a) Why does this reaction produce a racemic mixture of 3° alcohols?

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