Textbook Question
Propose a mechanism for the bromination of ethylbenzene shown below.
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Ch. 17 - Reactions of Aromatic Compounds
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 17, Problem 36
Predict the major products when the following compounds are irradiated by light and treated with (1) 1 equivalent of Br2 and (2) excess Br2.
(a) isopropylbenzene
(b) 
Verified step by step guidance1
Step 1: Analyze the reaction conditions. The problem specifies irradiation by light and treatment with bromine (Br2). This indicates a radical bromination reaction, which typically occurs at the most stable hydrogen-bearing carbon in the molecule.
Step 2: For (a) isopropylbenzene, identify the most stable radical that can form. The benzylic position (the carbon directly attached to the benzene ring) is the most reactive due to resonance stabilization of the benzylic radical. Bromination will occur at this position.
Step 3: For (b), the provided structure is indane (a bicyclic compound with a benzene ring fused to a cyclopentane ring). Identify the most stable radical that can form. The benzylic position (the CH2 group attached to the benzene ring) is the most reactive due to resonance stabilization of the benzylic radical. Bromination will occur at this position.
Step 4: When treated with 1 equivalent of Br2, only one bromine atom will be added to the most reactive position (benzylic position) in both compounds. This results in monosubstitution.
Step 5: When treated with excess Br2, further bromination can occur. For isopropylbenzene, additional bromination may occur at the other hydrogens on the benzylic carbon. For indane, additional bromination may occur at the remaining hydrogens on the benzylic CH2 group. The extent of bromination depends on the amount of Br2 and reaction conditions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Free Radical Halogenation
Free radical halogenation is a reaction mechanism where halogens (like Br2) react with alkanes or alkyl groups in the presence of light or heat, leading to the formation of free radicals. This process involves three steps: initiation, propagation, and termination. The light energy generates halogen radicals, which then abstract hydrogen atoms from the organic substrate, resulting in the formation of alkyl halides.
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Selectivity in Halogenation
Selectivity in halogenation refers to the preference of halogen radicals to abstract hydrogen atoms from certain positions in a molecule, influenced by factors like sterics and stability of the resulting radicals. For example, in isopropylbenzene, the more stable tertiary radical is favored over secondary or primary radicals, leading to specific products. This selectivity is crucial for predicting the major products of the reaction.
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Excess Halogen Effects
When excess halogen is used in a reaction, it can lead to further halogenation of the product formed. In the case of isopropylbenzene treated with excess Br2, not only will the initial halogenation occur, but the resulting alkyl halide can undergo additional halogenation, potentially leading to polyhalogenated products. Understanding this concept is essential for predicting the final products in reactions involving halogens.
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Related Practice
Textbook Question
(c) Based on what you know about the relative stabilities of alkyl radicals and benzylic radicals, predict the product of addition of HBr to 1-phenylpropene in the presence of a free-radical initiator.
(d) Propose a mechanism for this reaction.
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Textbook Question
(a) Based on what you know about the relative stabilities of alkyl cations and benzylic cations, predict the product of addition of HBr to 1-phenylpropene.
(b) Propose a mechanism for this reaction.
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Textbook Question
Predict the major products of the following reactions.
(d) p-xylene + Na (liquid NH3, CH3CH2OH)
(e)
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Textbook Question
Predict the major products of treating the following compounds with hot, concentrated potassium permanganate, followed by acidification with dilute HCl.
(a) isopropylbenzene
(b) p-xylene
(c)
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Textbook Question
Propose a mechanism for the reaction of benzyl bromide with ethanol to give benzyl ethyl ether.