Textbook Question
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)
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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 34
Propose a mechanism for the bromination of ethylbenzene shown below.
Verified step by step guidance1
Step 1: Recognize that the reaction involves the bromination of ethylbenzene under photochemical conditions (hv). This indicates a free radical mechanism rather than an electrophilic aromatic substitution.
Step 2: Initiation: The bromine molecule (Br₂) absorbs light energy (hv), causing homolytic cleavage of the Br-Br bond to generate two bromine radicals. This can be represented as:
Step 3: Propagation (Step 1): A bromine radical abstracts a hydrogen atom from the benzylic position of ethylbenzene, forming a benzylic radical. The benzylic radical is stabilized due to resonance with the aromatic ring. This step can be represented as:
Step 4: Propagation (Step 2): The benzylic radical reacts with another bromine molecule (Br₂), forming the brominated product (1-bromoethylbenzene) and regenerating a bromine radical. This step can be represented as:
Step 5: Termination: The reaction can terminate when two radicals combine to form a stable molecule, such as two bromine radicals forming Br₂ or a benzylic radical combining with a bromine radical to form the product. However, termination steps are less common under controlled conditions with limited bromine.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrophilic Aromatic Substitution
Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of ethylbenzene, the bromine (Br2) acts as the electrophile, and the reaction typically involves the formation of a sigma complex, where the aromaticity is temporarily lost before the hydrogen is replaced by bromine.
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Radical Mechanism
The bromination of ethylbenzene under light (hv) suggests a radical mechanism, specifically a free radical halogenation. This process involves the generation of bromine radicals from Br2, which then abstract hydrogen atoms from ethylbenzene, leading to the formation of a brominated product. The reaction proceeds through initiation, propagation, and termination steps characteristic of radical reactions.
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Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In the bromination of ethylbenzene, the presence of the ethyl group, which is an electron-donating group, influences the regioselectivity, favoring substitution at the benzylic position (the carbon adjacent to the aromatic ring) over other positions, leading to the formation of 1-bromoethylbenzene.
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Related Practice
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.
Textbook Question
Predict the major products of the following reactions.
(c) o-xylene + H2 (1000 psi, 100 °C, Rh catalyst)
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