Textbook Question
Draw the enol tautomers for each of the following compounds. For compounds that have more than one enol tautomer, indicate the one that is more stable.
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Ch. 17 - Reactions at the Alpha-Carbon
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 18, Problem 6
Explain why 92% of 2,4-pentanedione exists as the enol tautomer in hexane but only 15% of this compound exists as the enol tautomer in water.
Verified step by step guidance1
Understand the concept of keto-enol tautomerism: Keto-enol tautomerism is a chemical equilibrium between a ketone (or aldehyde) and its corresponding enol form. The equilibrium depends on the stability of each tautomer, which is influenced by the solvent and other factors.
Analyze the structure of 2,4-pentanedione: This compound has two carbonyl groups (C=O) separated by a single methylene group (-CH2-). The enol form involves the migration of a hydrogen atom and the formation of a double bond between the alpha-carbon and one of the carbonyl carbons, along with the formation of an -OH group.
Consider the role of the solvent hexane: Hexane is a nonpolar solvent. In nonpolar environments, the enol form of 2,4-pentanedione is stabilized by intramolecular hydrogen bonding between the hydroxyl (-OH) group and the adjacent carbonyl oxygen. This intramolecular hydrogen bonding significantly increases the stability of the enol tautomer in hexane.
Consider the role of the solvent water: Water is a polar solvent and can form hydrogen bonds with the keto form of 2,4-pentanedione. The keto form is more polar than the enol form, so it is better stabilized by the polar environment of water. Additionally, the intramolecular hydrogen bonding that stabilizes the enol form in hexane is less significant in water due to competition with water molecules for hydrogen bonding.
Conclude the reasoning: The high percentage of the enol tautomer in hexane (92%) is due to the stabilization provided by intramolecular hydrogen bonding in the nonpolar solvent. In contrast, the lower percentage of the enol tautomer in water (15%) is due to the polar solvent stabilizing the keto form more effectively through hydrogen bonding and solvation effects.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Tautomerism
Tautomerism is a chemical phenomenon where two or more structural isomers, known as tautomers, readily interconvert. In the case of 2,4-pentanedione, the keto form and the enol form are tautomers. The stability of these forms can vary significantly depending on the solvent and environmental conditions, influencing the equilibrium between them.
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Tautomerization Mechanisms
Solvent Effects on Tautomerism
The solvent plays a crucial role in determining the stability of tautomers. In nonpolar solvents like hexane, the enol form of 2,4-pentanedione is favored due to reduced hydrogen bonding and steric interactions, allowing for a higher percentage of the enol tautomer. Conversely, in polar solvents like water, the keto form is stabilized through hydrogen bonding, leading to a lower percentage of the enol form.
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05:11Tautomerization Mechanisms
Hydrogen Bonding
Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to an electronegative atom interacts with another electronegative atom. In water, the ability of the solvent to form extensive hydrogen bonds stabilizes the keto form of 2,4-pentanedione, thus reducing the proportion of the enol tautomer compared to nonpolar solvents where such interactions are minimal.
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00:48The definition of hydrogenation.
Related Practice
Textbook Question
Draw the enol tautomers for each of the following compounds. For compounds that have more than one enol tautomer, indicate the one that is more stable.
d.
e.
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Textbook Question
A ketone undergoes acid-catalyzed bromination, acid-catalyzed chlorination, racemization, and acid-catalyzed deuterium exchange at the ⍺-carbon. All of these reactions have similar rate constants.What does this tell you about the mechanisms of these reactions?
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Textbook Question
Explain why the -hydrogen of an N,N-disubstituted amide is less acidic (pKa = 30) than the -hydrogen of an ester (pKa = 25).
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Textbook Question
Rank the compounds in each of the following groups from strongest acid to weakest acid:
c.
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Textbook Question
Rank the compounds in each of the following groups from strongest acid to weakest acid:
a.
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