Textbook Question
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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Ch.11 - Reactions of Alcohols
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 11, Problem 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?
Verified step by step guidance1
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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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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.
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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.
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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.
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Related Practice
Textbook Question
To practice working through the early parts of a multistep synthesis, devise syntheses of
(a) pentan-3-one from alcohols containing no more than three carbon atoms.
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Textbook Question
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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Textbook Question
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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Textbook Question
(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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Textbook Question
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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