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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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 34
(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?
Verified step by step guidance1
Step 1: Begin with the tosylation of ethanol. Tosylation involves reacting ethanol with p-toluenesulfonyl chloride (TsCl) in the presence of a base like pyridine. This converts ethanol into ethyl tosylate, a good leaving group. The reaction can be represented as: CH₃CH₂OH + TsCl → CH₃CH₂OTs.
Step 2: Prepare cyclohexanol for the Williamson ether synthesis. Deprotonate cyclohexanol using a strong base such as sodium hydride (NaH) or potassium tert-butoxide (KOtBu). This forms the cyclohexoxide ion (C₆H₁₁O⁻), which is a strong nucleophile.
Step 3: Perform the Williamson ether synthesis. React the cyclohexoxide ion (C₆H₁₁O⁻) with ethyl tosylate (CH₃CH₂OTs). The nucleophilic cyclohexoxide ion attacks the electrophilic carbon in ethyl tosylate, displacing the tosyl group and forming cyclohexyl ethyl ether (C₆H₁₁OCH₂CH₃).
Step 4: Address part (b) of the problem. Mixing ethanol and cyclohexanol with sulfuric acid and heating would not yield cyclohexyl ethyl ether efficiently. This is because sulfuric acid promotes dehydration of alcohols, leading to the formation of alkenes rather than ethers. Additionally, the reaction lacks the necessary nucleophilic and electrophilic components for ether synthesis.
Step 5: Conclude that the tosylation followed by Williamson ether synthesis is a more controlled and effective method for synthesizing cyclohexyl ethyl ether, as it avoids side reactions like dehydration and ensures the formation of the desired ether product.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Tosylation
Tosylation is a chemical reaction that converts an alcohol into a tosylate, which is a better leaving group for nucleophilic substitution reactions. In this process, the hydroxyl group of the alcohol is replaced by a tosyl group (–OTs), making the molecule more reactive. This step is crucial for the synthesis of ethers, as it prepares the alcohol for subsequent reactions, such as the Williamson ether synthesis.
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Williamson Ether Synthesis
The Williamson ether synthesis is a method for creating ethers through the nucleophilic substitution of an alkoxide ion with a primary alkyl halide. This reaction typically involves the deprotonation of an alcohol to form an alkoxide, which then attacks the electrophilic carbon of the alkyl halide. This process is essential for synthesizing cyclohexyl ethyl ether from the tosylate formed in the previous step.
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Acid-Catalyzed Dehydration
Acid-catalyzed dehydration of alcohols involves the protonation of the alcohol's hydroxyl group, leading to the formation of a more reactive carbocation. While this method can facilitate the formation of ethers, it often results in the formation of alkenes or other side products due to elimination reactions. Therefore, simply mixing ethanol and cyclohexanol with sulfuric acid and heating is not effective for synthesizing cyclohexyl ethyl ether, as it does not favor the desired ether formation.
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Related Practice
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 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?
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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
Show the alcohol and the acid chloride that combine to make the following esters.
(c)
(d)
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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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