Textbook Question
Rank each group of compounds in order of increasing heat of hydrogenation.
(b)
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Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
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
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Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 2
Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 2Chapter 15, Problem 2
In a strongly acidic solution, cyclohexa-1,4-diene tautomerizes to cyclohexa-1,3-diene. Propose a mechanism for this rearrangement, and explain why it is energetically favorable.
Verified step by step guidance1
Step 1: Begin by understanding the concept of tautomerization. Tautomerization is a chemical reaction that results in the structural rearrangement of a compound, typically involving the migration of a hydrogen atom and a switch of a single bond and adjacent double bond. In this case, cyclohexa-1,4-diene is converting to cyclohexa-1,3-diene.
Step 2: Consider the role of the acidic solution. In a strongly acidic environment, protons (H⁺ ions) are abundant. These protons can facilitate the rearrangement by protonating the double bond, making it more susceptible to rearrangement.
Step 3: Propose the initial step of the mechanism. The protonation of cyclohexa-1,4-diene occurs at one of the double bonds, forming a carbocation intermediate. This is a key step as it sets the stage for the rearrangement.
Step 4: Describe the rearrangement process. The carbocation formed is stabilized by the migration of a hydrogen atom from the adjacent carbon, leading to the formation of a new double bond. This migration results in the shift from a 1,4-diene to a 1,3-diene structure.
Step 5: Explain the energetic favorability. The formation of cyclohexa-1,3-diene is energetically favorable due to increased conjugation. Conjugated dienes are more stable than isolated dienes because the delocalization of electrons across the conjugated system lowers the overall energy of the molecule.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Tautomerization
Tautomerization is a chemical process where a compound undergoes structural rearrangement, typically involving the migration of a hydrogen atom and a switch of a single bond and adjacent double bond. In the context of cyclohexa-1,4-diene, tautomerization involves the shift of double bonds to form cyclohexa-1,3-diene, which is facilitated by acidic conditions.
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Tautomerization Mechanisms
Acid Catalysis
Acid catalysis involves the use of an acid to increase the rate of a chemical reaction. In this mechanism, the acid donates a proton to the cyclohexa-1,4-diene, stabilizing the intermediate and facilitating the rearrangement of bonds. The presence of a strong acid helps in the protonation of the diene, making the tautomerization process more favorable.
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Energetic Favorability
Energetic favorability refers to the tendency of a chemical reaction to proceed because it results in a lower energy state. Cyclohexa-1,3-diene is more stable than cyclohexa-1,4-diene due to the conjugation of double bonds, which lowers the overall energy of the molecule. This increased stability makes the tautomerization energetically favorable in acidic conditions.
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Related Practice
Textbook Question
When 3-bromo-1-methylcyclohexene undergoes solvolysis in hot ethanol, two products are formed. Propose a mechanism that accounts for both of these products.
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Textbook Question
The central carbon atom of an allene is a member of two double bonds, and it has an interesting orbital arrangement that holds the two ends of the molecule at right angles to each other.
a. Draw an orbital diagram of allene, showing why the two ends are perpendicular.
b. Draw the two enantiomers of penta-2,3-diene. A model may be helpful.
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Textbook Question
Draw another resonance form for each of the substituted allylic cations shown in the preceding figure, showing how the positive charge is shared by another carbon atom. In each case, state whether your second resonance form is a more important or less important resonance contributor than the first structure. (Which structure places the positive charge on the more-substituted carbon atom?)
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Textbook Question
Rank each group of compounds in order of increasing heat of hydrogenation.
(a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene
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