Textbook Question
Explain why bimolecular condensation is a poor method for making unsymmetrical ethers such as ethyl methyl ether.
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Ch. 14 - Ethers, Epoxides, and Thioethers
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 14, Problem 14a
Propose a mechanism for the following reaction.
Verified step by step guidance1
Step 1: Protonation of the oxygen atom in tetrahydrofuran (THF) occurs when it reacts with HBr. The oxygen atom in THF is nucleophilic and can accept a proton from HBr, forming a positively charged oxonium ion.
Step 2: The oxonium ion undergoes cleavage of the C-O bond, leading to the formation of a carbocation intermediate. This step is facilitated by the excess HBr, which provides a highly acidic environment.
Step 3: The bromide ion (Br⁻), generated from the dissociation of HBr, acts as a nucleophile and attacks the carbocation, forming a brominated intermediate. This step results in the addition of one bromine atom to the molecule.
Step 4: The process repeats as excess HBr continues to protonate the remaining ether oxygen, leading to further cleavage and formation of another carbocation. The bromide ion attacks this carbocation, resulting in the addition of a second bromine atom.
Step 5: The final product, 1,4-dibromobutane, is formed after both bromine atoms are added to the molecule. The reaction proceeds under acidic conditions with excess HBr to ensure complete conversion of THF into the dibrominated product.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrophilic Addition
Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, resulting in the formation of a more saturated product. In the case of tetrahydrofuran reacting with HBr, the electrophile (H+) adds to the oxygen atom, leading to the formation of a more reactive intermediate that can further react with Br- to yield the final product.
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1,2 vs 1,4 Addition
Ring Opening Mechanism
The ring opening mechanism involves the breaking of a cyclic structure to form a linear or acyclic product. In this reaction, tetrahydrofuran, a five-membered cyclic ether, undergoes ring opening upon protonation by HBr, allowing the formation of a linear dibrominated product. This process is crucial for understanding how cyclic ethers can be transformed into linear compounds.
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Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to occur at one location over another in a molecule. In the reaction of tetrahydrofuran with excess HBr, regioselectivity is observed as the bromine atoms add to the 1 and 4 positions of the resulting linear structure, leading to the formation of 1,4-dibromobutane. Understanding regioselectivity is essential for predicting the outcome of reactions involving multiple reactive sites.
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Related Practice
Textbook Question
Which of the following ethers can be formed in good yield by condensation of the corresponding alcohols? For those that cannot be formed by condensation, suggest an alternative method that will work.
(a) dibutyl ether
(b) ethyl n-propyl ether
(c) di-sec-butyl ether
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Textbook Question
Predict the products of the following reactions. An excess of acid is available in each case.
(d)
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Textbook Question
Predict the products of the following reactions. An excess of acid is available in each case.
(c) anisole (methoxybenzene) + HBr
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Textbook Question
Propose a mechanism for the acid-catalyzed condensation of n-propyl alcohol to n-propyl ether, as shown above. When the temperature is allowed to rise too high, propene is formed. Propose a mechanism for the formation of propene, and explain why it is favored at higher temperatures.
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Textbook Question
Predict the products of the following reactions. An excess of acid is available in each case.
(a) ethoxycyclohexane + HBr
(b) tetrahydropyran + HI
(c) anisole (methoxybenzene) + HBr
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