Textbook Question
When 1-chlorocyclohexene reacts with HBr, the major product is 1-bromo-1-chlorocyclohexane. Propose a mechanism for this reaction, and explain why your proposed intermediate is more stable than the other possible intermediate
Ch.8 - Reactions of Alkenes
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 8, Problem 3b
Predict the major products of the following reactions, and propose mechanisms to support your predictions.
(b) 
HINT: Remember to write out complete structures, including all bonds and charges, when writing a mechanism or determining the course of a reaction.
Verified step by step guidance1
Step 1: Recognize the reaction conditions. The presence of HBr and di-tert-butyl peroxide indicates that this is a free radical addition reaction. Di-tert-butyl peroxide serves as a radical initiator, which promotes anti-Markovnikov addition of HBr to the alkene.
Step 2: Initiation step: Di-tert-butyl peroxide decomposes under heat to form two tert-butoxy radicals. This decomposition is represented as: (CH₃)₃COOC(CH₃)₃ → 2(CH₃)₃CO•.
Step 3: Propagation step 1: The tert-butoxy radical abstracts a hydrogen atom from HBr, generating a bromine radical (Br•). This step is represented as: (CH₃)₃CO• + HBr → (CH₃)₃COH + Br•.
Step 4: Propagation step 2: The bromine radical reacts with 1-phenylpropene, attacking the less substituted carbon of the double bond (anti-Markovnikov addition). This forms a new carbon radical at the more substituted carbon. The intermediate radical is stabilized by resonance with the phenyl group.
Step 5: Termination step: The carbon radical formed in the previous step reacts with another HBr molecule, abstracting a hydrogen atom to form the final product. The major product is 1-bromo-2-phenylpropane, resulting from anti-Markovnikov addition.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrophilic Addition Reactions
Electrophilic addition reactions involve the addition of an electrophile to a nucleophile, typically across a double bond. In the case of alkenes, such as 1-phenylpropene, the π bond acts as a nucleophile, reacting with electrophiles like HBr. Understanding this mechanism is crucial for predicting the products formed during the reaction.
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Features of Addition Mechanisms.
Radical Mechanisms
Radical mechanisms involve the formation of free radicals, which are highly reactive species with unpaired electrons. The presence of di-tert-butyl peroxide in the reaction suggests a radical pathway, where the peroxide decomposes to generate radicals that can initiate the addition of HBr to the alkene. Recognizing this mechanism is essential for predicting the major products.
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Markovnikov's Rule
Markovnikov's Rule states that in the addition of HX to an alkene, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This rule helps predict the regioselectivity of the reaction products, particularly in the case of asymmetric alkenes like 1-phenylpropene, guiding the formation of the more stable carbocation intermediate.
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Related Practice
Textbook Question
Predict the major products of the following reactions, and propose mechanisms to support your predictions.
b. 2-methylpropene + HCl
1
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Textbook Question
When 1 mole of buta-1,3-diene reacts with 1 mole of HBr, both 3-bromobut-1-ene and 1-bromobut-2-ene are formed. Propose a mechanism to account for this mixture of products.
Textbook Question
Show how you would accomplish the following synthetic conversions.
(a) but−1−ene → 1−bromobutane
(b) but−1−ene → 2−bromobutane
5
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Textbook Question
Show how you would accomplish the following synthetic conversions.
(c) 2−methylcyclohexanol → 1−bromo−1−methylcyclohexane
2
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Textbook Question
Show how you would accomplish the following synthetic conversions.
(d) 2−methylbutan-2-ol → 2-bromo-3-methylbutane
1
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