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 6
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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Identify the structure of styrene (vinylbenzene). Styrene consists of a benzene ring with an attached vinyl group (-CH=CH2). The vinyl group is an electron-donating group due to resonance effects, which increases the electron density in the benzene ring.
Explain the effect of the vinyl group on the reactivity of the benzene ring. The electron-donating resonance effect of the vinyl group stabilizes the benzene ring, making it more reactive toward electrophilic aromatic substitution compared to benzene itself.
Draw the resonance structures of the intermediate carbocation formed during electrophilic aromatic substitution. Focus on the ortho and para positions relative to the vinyl group. Show how the positive charge can be delocalized into the vinyl group, providing additional stabilization.
Compare the stability of the intermediates for ortho, meta, and para substitution. The ortho and para intermediates are more stabilized due to resonance interactions with the vinyl group, while the meta intermediate does not benefit from this stabilization.
Conclude that the increased reactivity of styrene and the preference for ortho- and para-substitution are due to the resonance stabilization provided by the vinyl group. This explains why electrophilic aromatic substitution occurs faster and predominantly at these positions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrophilic Aromatic Substitution (EAS)
Electrophilic Aromatic Substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. This process involves the formation of a sigma complex (arenium ion), which is stabilized by resonance. The reactivity and orientation of the substitution depend on the substituents already present on the aromatic ring, influencing the stability of the intermediates formed during the reaction.
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EAS Review
Resonance Stabilization
Resonance stabilization refers to the delocalization of electrons across multiple structures, which enhances the stability of a molecule. In the case of styrene, the positive charge in the sigma complex can be distributed over the aromatic ring through resonance, particularly favoring the ortho and para positions due to the electron-donating effect of the vinyl group. This stabilization makes the formation of these products more favorable compared to meta substitution.
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03:43The radical stability trend.
Ortho- and Para-Directing Effects
Ortho- and para-directing effects are observed in electrophilic aromatic substitution reactions where certain substituents influence the position of new substituents on the aromatic ring. Electron-donating groups, like the vinyl group in styrene, increase electron density at the ortho and para positions, making these sites more reactive towards electrophiles. This results in a higher proportion of ortho- and para-substituted products compared to meta-substituted products.
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Related Practice
Textbook Question
Propose a mechanism for the aluminum chloride–catalyzed reaction of benzene with chlorine.
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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
Propose a mechanism for the bromination of ethoxybenzene to give o- and p-bromoethoxybenzene.
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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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