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Ch.11 - Reactions of Alcohols
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728당신이 사용하는 게 아니라요?교과서 변경
모든 교과서Wade 9th EditionCh.11 - Reactions of Alcohols문제 35
11장, 문제 35

A student wanted to use the Williamson ether synthesis to make (R)-2-ethoxybutane. He remembered that the Williamson synthesis involves an SN2 displacement, which takes place with inversion of configuration. He ordered a bottle of (S)-butan-2-ol for his chiral starting material. He also remembered that the SN2 goes best on primary halides and tosylates, so he made ethyl tosylate and sodium (S)-but-2-oxide. After warming these reagents together, he obtained an excellent yield of 2-ethoxybutane.
a. What enantiomer of 2-ethoxybutane did he obtain? Explain how this enantiomer results from the SN2 reaction of ethyl tosylate with sodium (S)-but-2-oxide.
b. What would have been the best synthesis of (R)-2-ethoxybutane?
c. How can this student convert the rest of his bottle of (S)-butan-2-ol to (R)-2-ethoxybutane?

검증된 단계별 안내
1
Step 1: Understand the Williamson ether synthesis mechanism. This reaction involves an SN2 nucleophilic substitution, where the nucleophile attacks the electrophile, leading to inversion of configuration at the carbon center undergoing substitution. This inversion is crucial for determining the stereochemistry of the product.
Step 2: Analyze part (a). The student used sodium (S)-but-2-oxide as the nucleophile and ethyl tosylate as the electrophile. In the SN2 reaction, the nucleophile attacks the electrophilic carbon of ethyl tosylate, resulting in inversion of configuration. Since the starting material was (S)-but-2-oxide, the inversion leads to the formation of (R)-2-ethoxybutane.
Step 3: Address part (b). To synthesize (R)-2-ethoxybutane directly, the student should start with (R)-butan-2-ol instead of (S)-butan-2-ol. This would ensure that the nucleophile (R)-but-2-oxide undergoes inversion during the SN2 reaction, yielding (R)-2-ethoxybutane.
Step 4: Discuss part (c). To convert the remaining (S)-butan-2-ol to (R)-2-ethoxybutane, the student can first convert (S)-butan-2-ol to (S)-but-2-oxide using a strong base like sodium hydride. Then, perform the Williamson ether synthesis with ethyl tosylate, which will invert the configuration via SN2, producing (R)-2-ethoxybutane.
Step 5: Summarize the stereochemical implications. The key to understanding this problem is recognizing that the SN2 mechanism always results in inversion of configuration at the carbon center undergoing substitution. This inversion is why the stereochemistry of the starting material directly impacts the stereochemistry of the product.

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영상 길이:
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주요 개념

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

SN2 Reaction Mechanism

The SN2 (substitution nucleophilic bimolecular) reaction is a fundamental mechanism in organic chemistry where a nucleophile attacks an electrophile, resulting in the displacement of a leaving group. This reaction occurs in a single concerted step, leading to inversion of configuration at the chiral center. The reaction is favored by primary substrates due to less steric hindrance, making it crucial for synthesizing chiral compounds.
추천 영상:
08:33
Drawing the SN2 Mechanism

Chirality and Enantiomers

Chirality refers to the geometric property of a molecule that makes it non-superimposable on its mirror image, leading to the existence of enantiomers—two molecules that are mirror images of each other. In the context of the question, (R)- and (S)-2-ethoxybutane are enantiomers, and the specific configuration of the starting material influences the configuration of the product formed through the SN2 reaction.
추천 영상:
05:10
What is chirality?

Williamson Ether Synthesis

The Williamson ether synthesis is a method for creating ethers through the reaction of an alkoxide ion with a primary alkyl halide or tosylate. This reaction typically proceeds via an SN2 mechanism, allowing for the formation of ethers with specific stereochemistry. Understanding this synthesis is essential for determining the correct enantiomer produced when starting with chiral alcohols, as the stereochemical outcome is directly influenced by the configuration of the starting materials.
추천 영상:
03:50
The Mechanism of Williamson Ether Synthesis.
관련 실천
교과서 질문

To practice working through the early parts of a multistep synthesis, devise syntheses of

(b) 3-ethylpentan-2-one from compounds containing no more than three carbon atoms.

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교과서 질문

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(a) pentan-3-one from alcohols containing no more than three carbon atoms.

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교과서 질문

Use resonance forms of the conjugate bases to explain why methanesulfonic acid (CH3SO3H, pKa = –2.6) is a much stronger acid than acetic acid (CH3COOH, pKa = 4.8).

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교과서 질문

A good Williamson synthesis of ethyl methyl ether would be

What is wrong with the following proposed synthesis of ethyl methyl ether? First, ethanol is treated with acid to protonate the hydroxy group (making it a good leaving group), and then sodium methoxide is added to displace water.

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교과서 질문

(a) Show how ethanol and cyclohexanol may be used to synthesize cyclohexyl ethyl ether (tosylation followed by the Williamson ether synthesis).

(b) Why can't we synthesize this product simply by mixing the two alcohols, adding some sulfuric acid, and heating?

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교과서 질문

Phenols (pKa ≈ 10) are more acidic than other alcohols, so they are easily deprotonated by sodium hydroxide or potassium hydroxide. The anions of phenols (phenoxide ions) can be used in the Williamson ether synthesis, especially with very reactive alkylating reagents such as dimethyl sulfate. Using phenol, dimethyl sulfate, and other necessary reagents, show how you would synthesize methyl phenyl ether.

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