Textbook Question
Predict the dehydrohalogenation product(s) that result when the following alkyl halides are heated in alcoholic KOH. When more than one product is formed, predict the major and minor products.
(f)
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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
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 7, Problem 57a,b
Show how you would prepare cyclopentene from each compound.
a. cyclopentanol
b. cyclopentyl bromide
Verified step by step guidance1
Step 1: For part (a), cyclopentanol, recognize that the conversion of an alcohol to an alkene involves an elimination reaction. Specifically, this is typically achieved through an acid-catalyzed dehydration reaction.
Step 2: Write the reaction mechanism for the dehydration of cyclopentanol. In the presence of a strong acid like H₂SO₄ or H₃PO₄, the hydroxyl group (-OH) is protonated to form a good leaving group (H₂O).
Step 3: After the water molecule leaves, a carbocation intermediate is formed. Since cyclopentanol is a secondary alcohol, the carbocation will be relatively stable, and no rearrangement is needed.
Step 4: A β-hydrogen (a hydrogen atom on the carbon adjacent to the carbocation) is removed by a base (often the conjugate base of the acid used, such as HSO₄⁻), leading to the formation of a double bond and yielding cyclopentene.
Step 5: For part (b), cyclopentyl bromide, recognize that the conversion of an alkyl halide to an alkene involves an elimination reaction, specifically an E2 mechanism. Treat cyclopentyl bromide with a strong base, such as NaOH or KOH, to abstract a β-hydrogen, leading to the formation of cyclopentene and the elimination of HBr.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Dehydration of Alcohols
Dehydration of alcohols is a common method to synthesize alkenes, where water is removed from the alcohol molecule. In the case of cyclopentanol, heating it with an acid catalyst can facilitate the elimination of a water molecule, resulting in the formation of cyclopentene. This reaction typically follows an E1 or E2 mechanism, depending on the conditions and the structure of the alcohol.
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01:23
General Reaction of Dehydration with POCl3
Nucleophilic Substitution
Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile replaces a leaving group in a molecule. For cyclopentyl bromide, a nucleophile can attack the carbon bonded to the bromine, leading to the formation of cyclopentene through an elimination reaction. This process often involves the conversion of the bromide to an alkene via an E2 mechanism, especially in the presence of a strong base.
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01:47Nucleophiles and Electrophiles can react in Substitution Reactions.
Elimination Reactions
Elimination reactions are processes where two atoms or groups are removed from a molecule, resulting in the formation of a double bond. In the context of preparing cyclopentene, both dehydration of cyclopentanol and the reaction of cyclopentyl bromide with a base involve elimination. Understanding the conditions that favor elimination over substitution is crucial for successfully synthesizing alkenes from these precursors.
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00:40Recognizing Elimination Reactions.
Related Practice
Textbook Question
Show how you would convert (in one or two steps) 1-phenylpropane to the three products shown below. In each case, explain what unwanted reactions might produce undesirable impurities in the product.
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Textbook Question
Show how you would prepare cyclopentene from each compound.
c. cyclopentane (not by dehydrogenation)
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Textbook Question
Using cyclohexane as your starting material, show how you would synthesize each of the following compounds. (Once you have shown how to synthesize a compound, you may use it as the starting material in any later parts of this problem.)
a. bromocyclohexane
b. cyclohexene
c. ethoxycyclohexane
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Textbook Question
Show how you would convert (in one or two steps) 1-phenylpropane to the three products shown below. In each case, explain what unwanted reactions might produce undesirable impurities in the product.
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Textbook Question
Using cyclohexane as your starting material, show how you would synthesize each of the following compounds. (Once you have shown how to synthesize a compound, you may use it as the starting material in any later parts of this problem.)
d. 3-bromocyclohex-1-ene
e. cyclohexa-1,3-diene
f. cyclohexanol
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