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 87a
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
Verified step by step guidance1
Step 1: Analyze the substituent effect of chlorine (Cl) on the acidity of benzoic acid. Chlorine is an electron-withdrawing group due to its electronegativity and ability to participate in inductive effects. This increases the acidity of the benzoic acid by stabilizing the negative charge on the conjugate base (benzoate ion).
Step 2: Consider the ortho position. In the ortho isomer, the chlorine is closest to the carboxylic acid group. This proximity allows for stronger inductive effects, which stabilize the conjugate base more effectively, making the ortho isomer the most acidic (lowest pKa).
Step 3: Examine the meta position. In the meta isomer, the chlorine is farther from the carboxylic acid group compared to the ortho position. The inductive effect is weaker due to the increased distance, resulting in less stabilization of the conjugate base. Thus, the meta isomer is less acidic than the ortho isomer.
Step 4: Evaluate the para position. In the para isomer, the chlorine is the farthest from the carboxylic acid group. The inductive effect is significantly reduced due to the distance, leading to the least stabilization of the conjugate base. Therefore, the para isomer is the least acidic (highest pKa).
Step 5: Summarize the trend: The relative acidity of chloro-substituted benzoic acids follows the order ortho > meta > para, which is consistent with the strength of the inductive effect exerted by the chlorine substituent based on its position relative to the carboxylic acid group.
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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 is often measured by the pKa value, which indicates the strength of an acid. A lower pKa value corresponds to a stronger acid, meaning it more readily donates protons (H+). The pKa values of benzoic acids vary based on the substituents attached to the benzene ring, influencing the stability of the conjugate base formed after deprotonation.
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07:45
Identifying pKa values
Substituent Effects
The position and nature of substituents on a benzene ring significantly affect acidity. Electron-withdrawing groups (EWGs) like nitro or chloro increase acidity by stabilizing the negative charge on the conjugate base through resonance or inductive effects. Conversely, electron-donating groups (EDGs) like amino groups decrease acidity by destabilizing the conjugate base, leading to variations in pKa values among ortho, meta, and para isomers.
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Resonance and Inductive Effects
Resonance and inductive effects are key concepts in understanding the relative acidities of substituted benzoic acids. Resonance involves the delocalization of electrons across the molecule, which can stabilize or destabilize the conjugate base. Inductive effects refer to the electron-withdrawing or donating influence of substituents through sigma bonds, impacting the overall acidity. The combination of these effects explains why ortho, meta, and para isomers exhibit different acidities.
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Related Practice
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.
c. NH2: meta > para > ortho
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Textbook Question
Identify A–J:
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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
Propose a mechanism for each of the following reactions:
a.
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