Textbook Question
Identify A–J:
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Ch. 18 - Reactions of Benzene and Substituted Benzenes
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
Chapter 19, Problem 87c
The pKa values of a few ortho-, meta-, and para-substituted benzoic acids are shown below:
The relative pKa values depend on the substituent. For chloro-substituted benzoic acids, the ortho isomer is the most acidic and the para isomer is the least acidic; for nitro-substituted benzoic acids, the ortho isomer is the most acidic and the meta isomer is the least acidic; and for amino-substituted benzoic acids, the meta isomer is the most acidic and the ortho isomer is the least acidic. Explain these relative acidities.
c. NH2: meta > para > ortho
Verified step by step guidance1
Step 1: Understand the relationship between pKa and acidity. A lower pKa value indicates a stronger acid, meaning the compound is more likely to donate a proton. Analyze the substituent effects on the benzoic acid derivatives provided in the image.
Step 2: Examine the amino-substituted benzoic acids (NH2 group). The pKa values are as follows: ortho (4.95), meta (4.73), and para (4.89). The meta isomer is the most acidic (lowest pKa), followed by para, and then ortho.
Step 3: Consider the electronic effects of the NH2 group. The amino group is an electron-donating group due to its lone pair of electrons, which can participate in resonance. This electron donation stabilizes the carboxylate anion formed after deprotonation, but the extent of stabilization depends on the position of the substituent.
Step 4: For the ortho isomer, steric hindrance between the NH2 group and the carboxylic acid group reduces the effectiveness of resonance stabilization. Additionally, intramolecular hydrogen bonding may occur, which can reduce the acidity of the carboxylic acid group.
Step 5: For the meta and para isomers, steric hindrance is minimized. The meta position does not allow direct resonance interaction between the NH2 group and the carboxylic acid group, but inductive effects dominate, making it slightly more acidic than the para isomer. The para position allows resonance interaction, but the electron-donating effect of NH2 reduces the overall acidity compared to the meta isomer.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acidity and pKa
Acidity in organic chemistry refers to the ability of a compound to donate a proton (H+). The strength of an acid is often quantified by its pKa value, which is the negative logarithm of the acid dissociation constant (Ka). A lower pKa value indicates a stronger acid, meaning it more readily donates protons. Understanding the relationship between pKa and acidity is crucial for analyzing the relative acidities of substituted benzoic acids.
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Identifying pKa values
Substituent Effects
The presence of different substituents on a benzene ring can significantly influence the acidity of benzoic acids. Electron-withdrawing groups (EWGs), such as nitro (NO2) and chloro (Cl), stabilize the negative charge on the conjugate base, enhancing acidity. Conversely, electron-donating groups (EDGs), like amino (NH2), destabilize the conjugate base, reducing acidity. The position of these substituents (ortho, meta, para) also affects their influence on acidity due to steric and electronic interactions.
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Ortho, Meta, and Para Positioning
The positioning of substituents on the benzene ring—ortho (adjacent), meta (one carbon apart), and para (opposite)—affects the acid strength of benzoic acids. For example, ortho-substituted acids may experience steric hindrance or intramolecular interactions that can either stabilize or destabilize the conjugate base. This positioning leads to variations in acidity, as seen in the provided data, where ortho, meta, and para isomers exhibit different pKa values based on the nature of the substituent.
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Related Practice
Textbook Question
Identify A–J:
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Textbook Question
Propose a mechanism for each of the following reactions:
b.
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Textbook Question
When heated with chromic acid, compound A forms benzoic acid. Identify compound A from its 1H NMR spectrum.
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Textbook Question
The pKa values of a few ortho-, meta-, and para-substituted benzoic acids are shown below:
The relative pKa values depend on the substituent. For chloro-substituted benzoic acids, the ortho isomer is the most acidic and the para isomer is the least acidic; for nitro-substituted benzoic acids, the ortho isomer is the most acidic and the meta isomer is the least acidic; and for amino-substituted benzoic acids, the meta isomer is the most acidic and the ortho isomer is the least acidic.
Explain these relative acidities.
a. Cl: ortho > meta > para
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Textbook Question
Propose a mechanism for each of the following reactions:
a.
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