Textbook Question
Predict the products of the sulfuric acid-catalyzed dehydration of the following alcohols. When more than one product is expected, label the major and minor products.
(d) 1-isopropylcyclohexanol
(e) 2-methylcyclohexanol
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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 25
Explain why the acid-catalyzed condensation is a poor method for the synthesis of an unsymmetrical ether such as ethyl methyl ether, CH3CH2-O-CH3.
Verified step by step guidance1
Step 1: Understand the acid-catalyzed condensation reaction. This reaction typically involves alcohols reacting in the presence of an acid catalyst, such as H2SO4, to form ethers. The mechanism proceeds through the protonation of one alcohol molecule, followed by nucleophilic attack by another alcohol molecule.
Step 2: Recognize the challenge in synthesizing unsymmetrical ethers like ethyl methyl ether (CH3CH2-O-CH3). In acid-catalyzed condensation, both alcohols (ethanol and methanol in this case) are present in the reaction mixture, and the reaction does not favor selective interaction between specific alcohol molecules.
Step 3: Consider the competing reactions. When both ethanol and methanol are present, the acid catalyst can protonate either alcohol, leading to the formation of multiple products. For example, ethanol can react with itself to form diethyl ether, methanol can react with itself to form dimethyl ether, and ethanol can react with methanol to form ethyl methyl ether. This lack of selectivity results in a mixture of products.
Step 4: Analyze the steric and electronic factors. The acid-catalyzed condensation does not provide a mechanism to control which alcohol acts as the nucleophile and which acts as the electrophile. This randomness further contributes to the poor yield of the desired unsymmetrical ether.
Step 5: Conclude why this method is inefficient. Due to the lack of selectivity and the formation of multiple byproducts, acid-catalyzed condensation is a poor method for synthesizing unsymmetrical ethers like ethyl methyl ether. Alternative methods, such as Williamson ether synthesis, are preferred for such cases because they allow for better control and selectivity.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid-Catalyzed Condensation
Acid-catalyzed condensation involves the reaction of alcohols in the presence of an acid catalyst to form ethers. This process typically favors the formation of symmetrical ethers due to the equal reactivity of the alcohols involved. When unsymmetrical ethers are targeted, the reaction can lead to a mixture of products, complicating the synthesis and reducing yield.
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Acid Catalyzed
Unsymmetrical Ethers
Unsymmetrical ethers are compounds where the two alkyl groups attached to the oxygen atom are different, such as ethyl methyl ether. The synthesis of unsymmetrical ethers is challenging because the reactivity of the alcohols can lead to preferential formation of symmetrical products, making it difficult to control the outcome of the reaction.
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Selectivity in Organic Reactions
Selectivity refers to the ability of a reaction to favor the formation of one product over others. In the case of acid-catalyzed condensation, the lack of selectivity can result in a mixture of symmetrical and unsymmetrical ethers. This is particularly problematic for unsymmetrical ethers, as the desired product may be formed in low yield alongside unwanted byproducts.
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Related Practice
Textbook Question
Some alcohols undergo rearrangement or other unwanted side reactions when they dehydrate in acid. Alcohols may be dehydrated under mildly basic conditions using phosphorus oxy-chloride (POCl3) in pyridine. The alcohol reacts with phosphorus oxychloride much like it reacts with tosyl chloride (Section 11-5), displacing a chloride ion from phosphorus to give an alkyl dichlorophosphate ester. The dichlorophosphate group is an outstanding leaving group. Pyridine reacts as a base with the dichlorophosphate ester to give an E2 elimination. Propose a mechanism for the dehydration of cyclohexanol by POCl3 in pyridine.
Textbook Question
Propose a mechanism for each reaction.
(a)
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Textbook Question
Contrast the mechanisms of the two preceding reactions, the dehydration and condensation of ethanol.
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Textbook Question
When the following substituted cycloheptanol undergoes dehydration, one of the minor products has undergone a ring contraction. Propose a mechanism to show how this ring contraction occurs.
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Textbook Question
Propose a mechanism for each reaction.
(c)
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