Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Problem 19

Chapter 15, Problem 19

Show that the [4 + 2] Diels–Alder reaction is photochemically forbidden.

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Understand the Diels–Alder reaction: The [4 + 2] cycloaddition is a pericyclic reaction where a conjugated diene (4 π-electrons) reacts with a dienophile (2 π-electrons) to form a six-membered ring. It typically proceeds via a concerted mechanism.

Recall the Woodward–Hoffmann rules: These rules determine whether a pericyclic reaction is thermally or photochemically allowed based on the conservation of orbital symmetry. For the Diels–Alder reaction, the reaction is thermally allowed if it proceeds via a suprafacial interaction of the π-electrons.

Analyze the π-electron system under photochemical conditions: When the system is exposed to light, one of the π-electrons in the diene or dienophile is excited to a higher energy state. This changes the electronic configuration and alters the symmetry of the molecular orbitals.

Apply the Woodward–Hoffmann rules for photochemical conditions: Under photochemical excitation, the reaction involves an excited state where the symmetry of the molecular orbitals no longer supports a suprafacial interaction of the π-electrons. This makes the reaction photochemically forbidden.

Conclude based on orbital symmetry: The [4 + 2] Diels–Alder reaction is photochemically forbidden because the conservation of orbital symmetry is violated in the excited state, preventing the reaction from proceeding under photochemical conditions.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Diels–Alder Reaction

The Diels–Alder reaction is a cycloaddition reaction between a conjugated diene and a dienophile, forming a six-membered ring. It is a [4 + 2] cycloaddition, where four π electrons from the diene and two π electrons from the dienophile are involved. This reaction is widely used in organic synthesis due to its ability to create complex cyclic structures efficiently.

Orbital Symmetry

Orbital symmetry refers to the symmetry properties of molecular orbitals involved in a reaction. According to the Woodward-Hoffmann rules, reactions can be classified as allowed or forbidden based on the symmetry of the interacting orbitals. In the case of the Diels–Alder reaction, the symmetry of the diene and dienophile's orbitals must match for the reaction to proceed, which is crucial for understanding its photochemical behavior.

Photochemical Reactions

Photochemical reactions involve the absorption of light, leading to changes in the electronic state of molecules. These reactions can alter the symmetry and energy of molecular orbitals, potentially allowing or forbidding certain pathways. In the context of the Diels–Alder reaction, the photochemical conditions can disrupt the orbital symmetry required for the reaction to occur, rendering it photochemically forbidden.

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Related Practice

Textbook Question

Predict the products of the following Diels–Alder reactions.

(c)

(d)

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Textbook Question

We predicted that the products would have a 1,2- or 1,4-relationship of the proper substituents. Draw the charge-separated resonance forms of the reactants to support these predictions.

(a) (b)

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Textbook Question

Show that the [4 + 4] cycloaddition of two butadiene molecules to give cycloocta-1,5-diene is thermally forbidden but photochemically allowed.

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Textbook Question

There is a different, thermally allowed cycloaddition of two butadiene molecules. Show this reaction, and explain why it is thermally allowed. (Hint: Consider the dimerization of cyclopentadiene.)

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Textbook Question

One milligram of a compound of molecular weight 160 is dissolved in 10 mL of ethanol, and the solution is poured into a 1-cm UV cell. The UV spectrum is taken, and there is an absorption at λ_max = 247 nm. The maximum absorbance at 247 nm is 0.50. Calculate the value of e for this absorption.

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Textbook Question

Predict the products of the following Diels–Alder reactions.

(a)

(b)

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