Textbook Question
(a) Propose a mechanism for the reaction of 2-bromopyridine with sodium amide to give 2-aminopyridine.
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Ch. 19 - Amines
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 19, Problem 12
Propose a mechanism for the sulfonation of pyridine, and point out why sulfonation occurs at the 3-position.
Verified step by step guidance1
Step 1: Recognize that sulfonation of pyridine involves electrophilic aromatic substitution. The electrophile in this reaction is the sulfur trioxide (SO₃) molecule, which is generated from fuming sulfuric acid (H₂SO₄). The reaction occurs under high temperature (230 °C) to overcome pyridine's reduced reactivity due to the electron-withdrawing nitrogen atom.
Step 2: Analyze the electronic structure of pyridine. The nitrogen atom in pyridine is electronegative and withdraws electron density from the ring, making the ring less reactive toward electrophiles. Additionally, the nitrogen's lone pair is not delocalized into the aromatic system, further reducing the ring's electron density.
Step 3: Consider the regioselectivity of the sulfonation. Electrophilic substitution on pyridine typically occurs at the 3-position because the 2- and 4-positions are destabilized by the positive charge that would result on the nitrogen atom in the intermediate resonance structures. Substitution at the 3-position avoids this destabilization.
Step 4: Propose the mechanism. First, the sulfur trioxide (SO₃) electrophile attacks the 3-position of pyridine, forming a sigma complex (arenium ion). This intermediate is stabilized by resonance, avoiding structures that place a positive charge on the nitrogen atom.
Step 5: Complete the reaction. The sigma complex undergoes deprotonation to restore aromaticity, resulting in the formation of 3-sulfonated pyridine. The sulfonic acid group (-SO₃H) is attached at the 3-position, as shown in the product structure.
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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 mechanism in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of pyridine, the nitrogen atom's lone pair can influence the reactivity and orientation of the substitution, making it crucial to understand how electrophiles interact with the aromatic system.
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EAS Review
Pyridine Structure and Reactivity
Pyridine is a six-membered aromatic ring containing a nitrogen atom, which affects its electronic properties. The nitrogen atom is electron-withdrawing, which alters the electron density on the ring and influences the position where electrophilic substitution occurs. Understanding the electronic effects of the nitrogen is essential for predicting the site of sulfonation.
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Regioselectivity in Sulfonation
Regioselectivity refers to the preference of a chemical reaction to occur at one site over another in a molecule. In the sulfonation of pyridine, the 3-position is favored due to the resonance stabilization of the intermediate cation formed during the reaction. This concept is vital for explaining why sulfonation predominantly occurs at this position rather than others.
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Related Practice
Textbook Question
The proton and 13C NMR spectra of a compound of formula C4H11N are shown here. Determine the structure of this amine, and give peak assignments for all of the protons in the structure.
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Textbook Question
We have considered nucleophilic aromatic substitution of pyridine at the 2-position and 3-position but not at the 4-position. Complete the three possible cases by showing the mechanism for the reaction of methoxide ion with 4-chloropyridine. Show how the intermediate is stabilized by delocalization of the charge onto the nitrogen atom.
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Textbook Question
(b) When 3-bromopyridine is used in this reaction, stronger reaction conditions are required and a mixture of 3-aminopyridine and 4-aminopyridine results. Propose a mechanism to explain this curious result.
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Textbook Question
Propose a mechanism for nitration of pyridine at the 4-position, and show why this orientation is not observed.
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Textbook Question
Rank each set of compounds in order of increasing basicity.
(d) pyrrole, imidazole, 3-nitropyrrole
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