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Ch.6 - Alkyl Halides; Nucleophilic Substitution
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.6 - Alkyl Halides; Nucleophilic SubstitutionProblem 48a
Chapter 6, Problem 48a

A solution of pure (S)-2-iodobutane ([α] = +15.90°) in acetone is allowed to react with radioactive iodide, 131I–, until 1.0% of the iodobutane contains radioactive iodine. The specific rotation of this recovered iodobutane is found to be +15.58°.
a. Determine the percentages of (R)- and (S)-2-iodobutane in the product mixture.

Verified step by step guidance
1
Understand the problem: The reaction involves a substitution process where radioactive iodide (131I-) replaces the iodine in (S)-2-iodobutane. This substitution can lead to the formation of both (S)- and (R)-2-iodobutane due to racemization. The goal is to determine the percentages of (R)- and (S)-2-iodobutane in the final product mixture based on the specific rotation data.
Recall the formula for specific rotation: \( [\alpha] = \frac{\alpha_{\text{observed}}}{c \cdot l} \), where \( [\alpha] \) is the specific rotation, \( \alpha_{\text{observed}} \) is the observed rotation, \( c \) is the concentration, and \( l \) is the path length. Since the concentration and path length are constant, the observed specific rotation can be directly compared to determine the enantiomeric composition.
Define the enantiomeric excess (ee): \( \text{ee} = \frac{[\alpha]_{\text{mixture}}}{[\alpha]_{\text{pure}}} \times 100 \). Here, \( [\alpha]_{\text{mixture}} = +15.58° \) and \( [\alpha]_{\text{pure}} = +15.90° \). Calculate the enantiomeric excess to determine the excess percentage of the (S)-enantiomer over the (R)-enantiomer.
Relate enantiomeric excess to the percentages of (S)- and (R)-enantiomers: \( \text{ee} = \%S - \%R \). Since the total percentage of both enantiomers must equal 100%, \( \%S + \%R = 100 \). Use these two equations to solve for \%S and \%R.
Account for the radioactive substitution: The problem states that 1.0% of the iodobutane contains radioactive iodine. This means that the calculated percentages of (S)- and (R)-enantiomers should be adjusted to reflect the fact that 99.0% of the iodobutane remains non-radioactive. Use this information to finalize the percentages of (S)- and (R)-2-iodobutane in the product mixture.

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Key Concepts

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

Optical Activity and Specific Rotation

Optical activity refers to the ability of chiral compounds to rotate plane-polarized light. The specific rotation is a quantitative measure of this property, defined as the angle of rotation per unit concentration and path length. In this question, the specific rotation values of (S)-2-iodobutane and the product mixture are crucial for determining the composition of the enantiomers present.
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Specific rotation vs. observed rotation.

Enantiomers and Racemic Mixtures

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, such as (R)- and (S)-2-iodobutane. A racemic mixture contains equal amounts of both enantiomers, resulting in no net optical activity. The question involves calculating the percentages of each enantiomer in the product mixture after a reaction with radioactive iodide, which affects their specific rotations.
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Calculating EE, percent of each enantiomer, and sketching mixture

Radioactive Isotope Tracing

Radioactive isotope tracing is a technique used to track the incorporation of a radioactive atom into a molecule. In this scenario, the introduction of radioactive iodide (131I-) allows for the determination of the distribution of iodine in the resulting iodobutane. By analyzing the specific rotation and the percentage of radioactive iodine, one can infer the relative amounts of (R)- and (S)-2-iodobutane in the final product.
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Related Practice
Textbook Question

Optically active 2-bromobutane undergoes racemization on treatment with a solution of KBr. Propose a mechanism for this racemization.

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Textbook Question

Using cyclohexane as one of your starting materials, show how you would synthesize the following compounds.

(c)

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Textbook Question

A solution of pure (S)-2-iodobutane ([α] = +15.90°) in acetone is allowed to react with radioactive iodide, 131I–, until 1.0% of the iodobutane contains radioactive iodine. The specific rotation of this recovered iodobutane is found to be +15.58°.

b. What does this result suggest about the mechanism of the reaction of 2-iodobutane with iodide ion?

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Textbook Question

Using cyclohexane as one of your starting materials, show how you would synthesize the following compounds.

(e)

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Textbook Question

a. Optically active 2-bromobutane undergoes racemization on treatment with a solution of KBr. Give a mechanism for this racemization.

b. In contrast, optically active butan-2-ol does not racemize on treatment with a solution of KOH. Explain why a ­reaction like that in part (a) does not occur.

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Textbook Question

Strawberry growers have used large quantities of methyl bromide (b.p. 4 °C) to sterilize the soil before planting their crops. Like some of the freons, methyl bromide can diffuse up into the stratosphere, where it damages the protective ozone layer. Agricultural chemists have suggested using methyl iodide (b.p. 43 °C) as a replacement for methyl bromide. Why is methyl iodide likely to be more toxic to agricultural pests (and people) than methyl bromide? Why is methyl iodide less likely to reach the stratosphere than methyl bromide?

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