Textbook Question
Sketch the pi molecular orbitals of hexa-1,3,5-triene.
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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 36d
Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 36dChapter 15, Problem 36d
Show that the [6 + 2] cyclization of hexa-1,3,5-triene with maleic anhydride is thermally forbidden but photochemically allowed.
Verified step by step guidance1
Step 1: Understand the reaction type. The [6 + 2] cyclization involves a pericyclic reaction where hexa-1,3,5-triene (a conjugated triene) reacts with maleic anhydride (a dienophile) to form a cyclic product. This reaction can proceed thermally or photochemically, depending on the electronic configuration of the reactants.
Step 2: Apply the Woodward-Hoffmann rules for pericyclic reactions. These rules state that the symmetry of molecular orbitals determines whether a reaction is thermally allowed or forbidden. For a [6 + 2] cyclization, analyze the symmetry of the π molecular orbitals in hexa-1,3,5-triene and maleic anhydride.
Step 3: Analyze the thermal reaction. In the ground state (thermal conditions), the π molecular orbitals of hexa-1,3,5-triene and maleic anhydride must interact constructively. However, the [6 + 2] cyclization requires an antarafacial interaction (opposite faces of the π system), which is symmetry-forbidden under thermal conditions.
Step 4: Analyze the photochemical reaction. Under photochemical conditions, light excites the π electrons of hexa-1,3,5-triene to a higher energy state. This changes the symmetry of the molecular orbitals, allowing a suprafacial interaction (same face of the π system) with maleic anhydride. Thus, the reaction becomes symmetry-allowed photochemically.
Step 5: Conclude the analysis. Based on the Woodward-Hoffmann rules, the [6 + 2] cyclization of hexa-1,3,5-triene with maleic anhydride is thermally forbidden due to symmetry constraints but photochemically allowed due to the altered orbital symmetry in the excited state.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Cycloaddition Reactions
Cycloaddition reactions involve the formation of a cyclic compound from two or more reactants. In the case of [6 + 2] cyclization, a six-membered and a two-membered component combine to form an eight-membered ring. Understanding the mechanism and stereochemistry of these reactions is crucial for predicting their feasibility under different conditions.
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Thermal vs. Photochemical Reactions
Thermal reactions occur at elevated temperatures and typically involve the breaking and forming of bonds through heat energy. In contrast, photochemical reactions are driven by light energy, allowing certain reactions to proceed that would be otherwise forbidden thermally due to energy barriers. This distinction is key in understanding why the cyclization of hexa-1,3,5-triene with maleic anhydride is thermally forbidden but photochemically allowed.
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Orbital Symmetry and Woodward-Hoffmann Rules
The Woodward-Hoffmann rules provide a framework for predicting the outcomes of pericyclic reactions based on the symmetry of molecular orbitals. In the case of the [6 + 2] cyclization, the conservation of orbital symmetry dictates that the reaction cannot proceed under thermal conditions due to symmetry mismatch, while it can proceed under photochemical conditions where the symmetry can be altered by light absorption.
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Related Practice
Textbook Question
Show the electronic configuration of the ground state of hexa-1,3,5-triene.
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Textbook Question
The pentadienyl radical, H2C=CH–CH=CH–CH2•, has its unpaired electron delocalized over three carbon atoms.
b. How many MOs are there in the molecular orbital picture of the pentadienyl radical?
c. How many nodes are there in the lowest-energy MO of the pentadienyl system? How many in the highest-energy MO?
d. Draw the MOs of the pentadienyl system in order of increasing energy
Textbook Question
The pentadienyl radical, H2C=CH–CH=CH–CH2•, has its unpaired electron delocalized over three carbon atoms.
a. Use resonance forms to show which three carbon atoms bear the unpaired electron.
Textbook Question
Show the Diels–Alder product that would actually result from heating hexa-1,3,5-triene with maleic anhydride.
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Textbook Question
Show what product would result from the [6 + 2] cycloaddition of hexa-1,3,5-triene with maleic anhydride.
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