Textbook Question
Two unknowns, X and Y, both having the molecular formula C4H8O, give the following results with four chemical tests. Propose structures for X and Y consistent with this information.
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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 60
The Williamson ether synthesis involves the displacement of an alkyl halide or tosylate by an alkoxide ion. Would the synthesis shown be possible by making a tosylate and displacing it? If so, show the sequence of reactions. If not, explain why not and show an alternative synthesis that would be more likely to work.
Verified step by step guidance1
Step 1: Analyze the starting material and the product. The starting material is an alcohol (-OH group) attached to a bicyclic structure, and the product is an ether (-OCH₃ group) attached to the same bicyclic structure. The Williamson ether synthesis involves converting the alcohol into a good leaving group (e.g., tosylate) and then performing an SN2 reaction with an alkoxide ion.
Step 2: Consider the feasibility of the SN2 reaction. The bicyclic structure in the starting material is highly sterically hindered, which makes an SN2 reaction difficult. In an SN2 reaction, the nucleophile must approach the electrophilic carbon directly from the opposite side of the leaving group, but steric hindrance in this structure would block such an approach.
Step 3: Propose an alternative synthesis. Since the SN2 reaction is unlikely to succeed due to steric hindrance, an alternative method such as the use of an alkyl halide and silver oxide (Ag₂O) to form the ether might be more effective. This method avoids the steric hindrance issues associated with the SN2 mechanism.
Step 4: Outline the alternative synthesis. First, convert the alcohol into an alkyl halide using a reagent like thionyl chloride (SOCl₂) or phosphorus tribromide (PBr₃). Then, react the alkyl halide with methanol (CH₃OH) in the presence of silver oxide (Ag₂O) to form the desired ether.
Step 5: Summarize the reasoning. The tosylate approach is not feasible due to steric hindrance preventing the SN2 displacement. The alternative synthesis using an alkyl halide and silver oxide is more likely to succeed in forming the ether product.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Williamson Ether Synthesis
The Williamson ether synthesis is a method for creating ethers through the nucleophilic substitution of an alkyl halide or tosylate by an alkoxide ion. This reaction typically requires a strong nucleophile and is favored by primary alkyl halides to minimize steric hindrance, which can lead to elimination reactions instead of substitution.
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The Mechanism of Williamson Ether Synthesis.
Tosylate Formation
Tosylates are derivatives of alcohols formed by reacting an alcohol with tosyl chloride, making the alcohol a better leaving group. This transformation is crucial in the Williamson ether synthesis as it allows for the use of secondary or tertiary alcohols, which would otherwise be less reactive in nucleophilic substitution due to steric hindrance.
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Nucleophilic Substitution Mechanisms
Nucleophilic substitution can occur via two main mechanisms: SN1 and SN2. The SN2 mechanism involves a single concerted step where the nucleophile attacks the electrophile, displacing the leaving group. In contrast, the SN1 mechanism involves the formation of a carbocation intermediate, making it more favorable for tertiary substrates but less so for primary ones due to steric hindrance.
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Related Practice
Textbook Question
Chromic acid oxidation of an alcohol (Section 11-2A) occurs in two steps: formation of the chromate ester, followed by an elimination of H+ and chromium. Which step do you expect to be rate-limiting? Careful kinetic studies have shown that Compound A undergoes chromic acid oxidation over 10 times as fast as Compound B. Explain this large difference in rates.
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Textbook Question
(b) Under the same conditions, an optically active sample of trans-2-bromocyclopentanol reacts with concentrated aqueous HBr to give an optically inactive product, (racemic) trans-1,2-dibromocyclopentane. Propose a mechanism to show how this reaction goes with apparently complete retention of configuration, yet with racemization. (Hint: Draw out the mechanism of the reaction of cyclopentene with Br2 in water to give the starting material, trans-2- bromocyclopentanol. Consider how parts of this mechanism might be involved in the reaction with HBr.)
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Textbook Question
(a) The reaction of butan-2-ol with concentrated aqueous HBr goes with partial racemization, giving more inversion than retention of configuration. Propose a mechanism that accounts for racemization with excess inversion.
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Textbook Question
Show how you would synthesize the following compound. As starting materials, you may use any alcohols containing five or fewer carbon atoms and any necessary solvents and inorganic reagents.
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Textbook Question
The following pseudo-syntheses (guaranteed not to work) exemplify a common conceptual error.
(a) What is the conceptual error implicit in these syntheses?
(b) Propose syntheses that are more likely to succeed.
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