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 35a

Chapter 5, Problem 35a

For the molecules shown,
(i) count the number of stereocenters present and
(ii) draw all possible stereoisomers.
(iii) Identify the relationships between stereoisomers as enantiomers or diastereomers.
(a)

Verified step by step guidance

Step 1: Identify stereocenters in the molecule. A stereocenter is typically a carbon atom bonded to four different groups. Examine each carbon atom in the molecule and determine if it meets this criterion.

Step 2: Count the total number of stereocenters. Once all stereocenters are identified, count them to determine the total number present in the molecule.

Step 3: Calculate the maximum number of stereoisomers. Use the formula 2^n, where 'n' is the number of stereocenters, to determine the maximum number of possible stereoisomers for the molecule.

Step 4: Draw all possible stereoisomers. For each stereocenter, assign configurations (R or S) systematically to generate all combinations of stereoisomers. Ensure that you represent each stereoisomer with its correct 3D structure (e.g., using wedge and dash bonds).

Step 5: Identify relationships between stereoisomers. Compare the stereoisomers to determine if they are enantiomers (non-superimposable mirror images) or diastereomers (stereoisomers that are not mirror images). Use the configurations (R/S) to help classify these relationships.

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Key Concepts

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

Stereocenters

A stereocenter, or chiral center, is a carbon atom that is bonded to four different substituents, leading to non-superimposable mirror images known as enantiomers. The presence of stereocenters in a molecule is crucial for determining its stereochemistry and the number of possible stereoisomers. Each stereocenter can exist in two configurations, contributing to the overall stereoisomer count.

Stereoisomers

Stereoisomers are compounds that have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. They can be classified into two main types: enantiomers, which are non-superimposable mirror images, and diastereomers, which are not mirror images of each other. The number of stereoisomers for a molecule can be calculated using the formula 2^n, where n is the number of stereocenters.

Enantiomers and Diastereomers

Enantiomers are pairs of stereoisomers that are mirror images of each other, possessing opposite configurations at all stereocenters. In contrast, diastereomers differ in configuration at one or more stereocenters but not all, making them non-mirror images. Understanding these relationships is essential for predicting the behavior of molecules in chemical reactions and their interactions in biological systems.

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Using chiral centers to predict types of stereoisomers.

Related Practice

Textbook Question

For the molecules shown,

(i) count the number of stereocenters present and

(ii) draw all possible stereoisomers.

(iii) Identify the relationships between stereoisomers as enantiomers or diastereomers.

(b)

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Textbook Question

Imagine a sample that is enriched in the R enantiomer. If the % ee of the sample is 83%,

(a) what percent of the mixture is racemic?

(b) What is the ratio of R to S?

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Textbook Question

How many stereoisomers are possible for each of the following molecules?

(a)

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Textbook Question

(S)-Propranolol, a drug used for the treatment of anxiety, has a specific rotation of -25.5° . Attempting to prepare it in enantiopure form, a chemist produced a compound that gave a specific rotation of -18.3° . What is the ratio of (S)- to (R)-propranolol produced by the chemist?

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Textbook Question

(R)-Selegiline, a monoamine oxidase (MAO) inactivator, was approved by the FDA in 1989 for the treatment of Parkinson's disease. In pure form, it has a specific rotation, [α]²⁰_D = - 11.0°. What is the expected specific rotation of a mixture containing 64% S and 36% R?

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Textbook Question

For the molecules shown,

(i) count the number of stereocenters present and

(ii) draw all possible stereoisomers.

(iii) Identify the relationships between stereoisomers as enantiomers or diastereomers.

(c)

919

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