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Ch.6 - Alkyl Halides; Nucleophilic Substitution
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728당신이 사용하는 게 아니라요?교과서 변경
모든 교과서Wade 9th EditionCh.6 - Alkyl Halides; Nucleophilic Substitution문제 48a
6장, 문제 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.

검증된 단계별 안내
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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이 영상 해법은 위 문제에 도움이 된다고 튜터들이 추천한 것입니다.
영상 길이:
5m

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

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.
추천 영상:
05:43
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.
추천 영상:
03:08
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.
추천 영상:
03:06
Understanding the hydrogen isotopes.
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교과서 질문

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교과서 질문

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°.

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