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 34

Chapter 5, Problem 34

(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?

Verified step by step guidance

Step 1: Recall that specific rotation ([α]) is a property of chiral compounds and is dependent on the enantiomeric composition of the mixture. A racemic mixture (50% R and 50% S) has a specific rotation of 0° because the rotations of the two enantiomers cancel each other out.

Step 2: Identify the specific rotation of the pure (R)-enantiomer, which is given as [α]²⁰D = -11.0°. The (S)-enantiomer will have the opposite specific rotation, [α]²⁰D = +11.0°, because enantiomers rotate plane-polarized light in equal magnitude but opposite directions.

Step 3: Determine the enantiomeric excess (ee) of the mixture. The mixture contains 64% S and 36% R. The enantiomeric excess is calculated using the formula: ee = |%S - %R|. Substituting the values: ee = |64 - 36| = 28%.

Step 4: Calculate the specific rotation of the mixture using the formula: [α]mixture = ee × [α]pure enantiomer. Since the mixture is enriched in the S-enantiomer, the specific rotation will be positive. Substitute the values: [α]mixture = 0.28 × (+11.0°).

Step 5: Combine the contributions of both enantiomers to the specific rotation of the mixture. The remaining 72% of the mixture (not part of the enantiomeric excess) is racemic and contributes 0° to the specific rotation. Therefore, the final specific rotation of the mixture is determined by the enantiomeric excess contribution.

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Key Concepts

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

Specific Rotation

Specific rotation is a property of chiral compounds that quantifies the degree to which they rotate plane-polarized light. It is defined as the observed rotation of light at a specific wavelength and temperature, normalized by the concentration of the solution and the path length of the light. This property is crucial for distinguishing between enantiomers and understanding their optical activity.

Enantiomers

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, often differing in their interaction with polarized light. In the case of (R)-Selegiline and its (S) counterpart, they exhibit different specific rotations due to their chiral nature. Understanding the relationship between enantiomers is essential for predicting the optical activity of mixtures.

Weighted Average of Specific Rotation

The specific rotation of a mixture of enantiomers can be calculated using a weighted average based on the proportions of each enantiomer present. This involves multiplying the specific rotation of each enantiomer by its respective fraction in the mixture and summing these values. This concept is vital for determining the expected specific rotation of a racemic or non-racemic mixture.

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Guided course

05:43

Specific rotation vs. observed rotation.

Related Practice

Textbook Question

Using a 1.0-cm polarimeter cell and a solution prepared by dissolving 2.54 g of glucose in 1 L of H₂O, the specific rotation of d-glucose was calculated to be +52.7° at 20 °C using the sodium D line as the source of light. What was the observed rotation of the solution?

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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.

(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

(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

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)

views

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.

(a)

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(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]²⁰D = - 11.0°. What is the expected specific rotation of a mixture containing 64% S and 36% R?

Verified video answer for a similar problem:

Key Concepts

Specific Rotation

Enantiomers

Weighted Average of Specific Rotation

(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?