Textbook Question
(b) Explain why m-xylene undergoes nitration 100 times faster than p-xylene.
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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 7
Propose a mechanism for the bromination of ethoxybenzene to give o- and p-bromoethoxybenzene.
Verified step by step guidance1
Step 1: Begin by identifying the substrate, ethoxybenzene, which is an aromatic compound with an ethoxy (-OCH2CH3) group attached to the benzene ring. The ethoxy group is an electron-donating group, activating the benzene ring and directing electrophilic substitution to the ortho and para positions.
Step 2: Recognize that bromination is an electrophilic aromatic substitution reaction. The bromine molecule (Br2) reacts with a Lewis acid catalyst, such as FeBr3, to generate the electrophile, bromonium ion (Br+). This step can be represented as:
Step 3: The bromonium ion (Br+) acts as the electrophile and attacks the activated benzene ring of ethoxybenzene. Due to the electron-donating nature of the ethoxy group, the ortho and para positions are more electron-rich and thus more reactive toward the electrophile. This results in the formation of a sigma complex (arenium ion) at either the ortho or para position.
Step 4: The sigma complex undergoes deprotonation at the site of substitution to restore aromaticity. A base, such as the FeBr4- anion formed earlier, abstracts the proton from the substituted carbon, regenerating the aromatic ring and yielding o-bromoethoxybenzene or p-bromoethoxybenzene.
Step 5: Conclude by noting that the reaction typically produces a mixture of ortho and para isomers, with the para isomer often being the major product due to steric hindrance at the ortho position. The final products are o-bromoethoxybenzene and p-bromoethoxybenzene.
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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 mechanism in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of bromination, bromine acts as the electrophile, and the presence of activating groups, like the ethoxy group in ethoxybenzene, influences the position of substitution, favoring ortho and para products.
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EAS Review
Resonance and Activating Groups
Resonance refers to the delocalization of electrons in a molecule, which stabilizes the structure. The ethoxy group is an electron-donating group that increases the electron density on the aromatic ring, making it more reactive towards electrophiles. This resonance effect explains why bromination occurs predominantly at the ortho and para positions relative to the ethoxy group.
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Bromination Mechanism
The bromination mechanism involves the generation of a bromonium ion from bromine, which then reacts with the aromatic compound. The first step is the formation of the electrophile, followed by the attack of the aromatic ring, leading to the formation of a sigma complex. Finally, deprotonation restores aromaticity, yielding the bromoethoxybenzene products.
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Related Practice
Textbook Question
Styrene (vinylbenzene) undergoes electrophilic aromatic substitution much faster than benzene, and the products are found to be primarily ortho- and para-substituted styrenes. Use resonance forms of the intermediates to explain these results.
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Textbook Question
p-Xylene undergoes nitration much faster than benzene. Use resonance forms of the sigma complex to explain this accelerated rate
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Textbook Question
In an aqueous solution containing sodium bicarbonate, aniline reacts quickly with bromine to give 2,4,6-tribromoaniline. Nitration of aniline requires very strong conditions, however, and the yields (mostly m-nitroaniline) are poor.
c. Although nitration of aniline is slow and gives mostly meta substitution, nitration of acetanilide (PhNHCOCH3) goes quickly and gives mostly para substitution. Use resonance forms to explain this difference in reactivity.
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Textbook Question
When bromine is added to two beakers, one containing phenyl isopropyl ether and the other containing cyclohexene, the bromine color in both beakers disappears. What observation could you make while performing this test that would allow you to distinguish the alkene from the aryl ether?
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Textbook Question
Predict the mononitration products of the following compounds.
a. o-nitrotoluene
b. m-chlorotoluene
c. o-bromobenzoic acid
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