Textbook Question
Propose a mechanism that shows why p-chlorotoluene reacts with sodium hydroxide at 350 °C to give a mixture of p-cresol and m-cresol.
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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 23c,d
Propose mechanisms and show the expected products of the following reactions.
(c) p-nitrobromobenzene + methylamine (CH3–NH2)
(d) 2,4-dinitrochlorobenzene + excess hydrazine (H2N–NH2)
Verified step by step guidance1
Step 1: Recognize the type of reaction occurring in each case. Both reactions involve nucleophilic aromatic substitution (NAS), which is common for aromatic compounds with electron-withdrawing groups (like nitro groups) and halogens (like bromine or chlorine). These electron-withdrawing groups activate the aromatic ring for substitution.
Step 2: For reaction (c), identify the nucleophile and electrophile. Methylamine (CH3-NH2) acts as the nucleophile, and p-nitrobromobenzene is the electrophile. The nitro group (-NO2) on the benzene ring is an electron-withdrawing group that stabilizes the intermediate formed during substitution. The bromine atom is the leaving group.
Step 3: Propose the mechanism for reaction (c). The nucleophile (CH3-NH2) attacks the carbon atom of the benzene ring that is bonded to the bromine atom. This forms a Meisenheimer complex (a negatively charged intermediate stabilized by resonance due to the nitro group). Bromine is then eliminated, resulting in the substitution product: p-nitro-N-methylbenzene.
Step 4: For reaction (d), identify the nucleophile and electrophile. Hydrazine (H2N-NH2) acts as the nucleophile, and 2,4-dinitrochlorobenzene is the electrophile. The nitro groups at the 2- and 4-positions strongly activate the benzene ring for substitution, and chlorine is the leaving group.
Step 5: Propose the mechanism for reaction (d). The nucleophile (H2N-NH2) attacks the carbon atom of the benzene ring bonded to the chlorine atom, forming a Meisenheimer complex stabilized by resonance due to the nitro groups. Chlorine is eliminated, resulting in the substitution product: 2,4-dinitrophenylhydrazine. Since excess hydrazine is used, the reaction proceeds efficiently.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nucleophilic Aromatic Substitution
Nucleophilic aromatic substitution (NAS) is a reaction where a nucleophile replaces a leaving group on an aromatic ring. This process typically occurs in aromatic compounds that have electron-withdrawing groups, which stabilize the negative charge that develops during the reaction. In the case of p-nitrobromobenzene reacting with methylamine, the amino group acts as a nucleophile, attacking the carbon bonded to the bromine, leading to the substitution.
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Nucleophiles and Electrophiles can react in Substitution Reactions.
Hydrazine Reactivity
Hydrazine (H2N-NH2) is a strong nucleophile and can react with electrophilic aromatic compounds, particularly those with electron-withdrawing groups like 2,4-dinitrochlorobenzene. The reaction involves the nucleophilic attack of hydrazine on the carbon atom bonded to the chlorine, resulting in the formation of a hydrazone. This reaction is significant in organic synthesis, especially for the preparation of hydrazones from carbonyl compounds.
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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 context of the reactions provided, the presence of strong electron-withdrawing groups like nitro groups enhances the reactivity of the aromatic ring towards nucleophiles. Understanding EAS is crucial for predicting the products of reactions involving substituted aromatic compounds.
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Related Practice
Textbook Question
Propose mechanisms and show the expected products of the following reactions.
(a) 2,4-dinitrochlorobenzene + sodium methoxide (NaOCH3)
(b) 2,4-dimethylchlorobenzene + sodium hydroxide, 350 °C
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Textbook Question
What organocuprate reagent would you use for the following substitutions?
(a)
(b)
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Textbook Question
What products would you expect from the following reactions?
(a)
(b)
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Textbook Question
The highly reactive triple bond of benzyne is a powerful dienophile. Predict the product of the Diels–Alder reaction of benzyne (from chlorobenzene and NaOH, heated) with cyclopentadiene.
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Textbook Question
Show how you would use the Friedel–Crafts acylation, Clemmensen reduction, and/or Gatterman–Koch synthesis to prepare the following compounds:
h.
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