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 11
Propose a mechanism for nitration of pyridine at the 4-position, and show why this orientation is not observed.
Verified step by step guidance1
Step 1: Understand the nitration reaction. Nitration typically involves the introduction of a nitro group (-NO₂) into an aromatic ring. This is achieved using a nitrating mixture of concentrated H₂SO₄ and HNO₃, which generates the nitronium ion (NO₂⁺), the electrophile in this reaction.
Step 2: Analyze the electronic structure of pyridine. Pyridine is an aromatic heterocycle with a nitrogen atom in the ring. The nitrogen atom is sp² hybridized and has a lone pair of electrons, which are not part of the aromatic π-system. The nitrogen atom is electron-withdrawing due to its electronegativity, making the ring less reactive toward electrophilic aromatic substitution (EAS) compared to benzene.
Step 3: Propose the mechanism for nitration at the 4-position. In EAS, the nitronium ion (NO₂⁺) attacks the aromatic ring to form a sigma complex (arenium ion). For nitration at the 4-position (para to the nitrogen), the intermediate sigma complex would have a positive charge delocalized onto the nitrogen atom. This is highly destabilized because the nitrogen is already electron-deficient due to its electronegativity.
Step 4: Explain why nitration at the 4-position is not observed. The destabilization of the sigma complex due to the positive charge on the nitrogen makes the 4-position unfavorable for electrophilic attack. Instead, nitration typically occurs at the 3-position (meta to the nitrogen), where the intermediate sigma complex avoids placing a positive charge on the nitrogen atom, leading to a more stable intermediate.
Step 5: Conclude the reasoning. The electronic effects of the nitrogen atom in pyridine direct electrophilic substitution to the 3-position rather than the 4-position. This is due to the destabilization of the intermediate sigma complex when the electrophile attacks the 4-position, making this orientation energetically unfavorable.
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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 pyridine, the nitrogen atom in the ring influences the reactivity and orientation of substitution due to its electronegativity and lone pair of electrons, which can stabilize the positive charge in the intermediate formed during the reaction.
Nitration Mechanism
Nitration involves the introduction of a nitro group (-NO2) into an aromatic compound, typically using a mixture of concentrated nitric acid and sulfuric acid. The mechanism begins with the generation of the nitronium ion (NO2+), which acts as the electrophile. In pyridine, the position of substitution is influenced by the electron-withdrawing nature of the nitrogen, which directs the electrophile to the less hindered positions on the ring.
Ortho/Para vs. Meta Directing Effects
In aromatic substitution, substituents can be classified as ortho/para or meta directors based on their electronic effects. Pyridine's nitrogen atom is an electron-withdrawing group, making it a meta director. This means that nitration predominantly occurs at the 3-position rather than the 4-position, as the electron density at the 4-position is reduced, making it less favorable for electrophilic attack.
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
Rank each set of compounds in order of increasing basicity.
(c) aniline, pyrrole, pyridine, piperidine
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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
Propose a mechanism for the sulfonation of pyridine, and point out why sulfonation occurs at the 3-position.
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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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