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Ch. 28 - Pericyclic Reactions
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 28 - Pericyclic ReactionsProblem 12a(4)
Chapter 25, Problem 12a(4)

a. Name the kind of sigmatropic rearrangement that occurs in each of the following reactions.
4. Illustration of a sigmatropic rearrangement reaction with reactants and products labeled, featuring a heat symbol.

Verified step by step guidance
1
Step 1: Understand the concept of sigmatropic rearrangements. These are pericyclic reactions where a sigma bond adjacent to one or more π systems migrates to a new position, with the π system reorganizing itself. The reaction is classified as [m,n], where m and n represent the number of atoms in the two fragments involved in the rearrangement.
Step 2: Analyze the structure of the starting material and the product. Identify the sigma bond that is breaking and the new sigma bond that is forming. Count the number of atoms in the two fragments (m and n) to determine the type of rearrangement.
Step 3: Check for common types of sigmatropic rearrangements, such as the [1,3]-hydride shift, [1,5]-hydride shift, [3,3]-Claisen rearrangement, or [3,3]-Cope rearrangement. Match the observed changes in the molecule to one of these known rearrangements.
Step 4: Verify the reaction mechanism. Ensure that the rearrangement follows the rules of orbital symmetry and conservation of aromaticity (if applicable). This step confirms that the reaction is thermally allowed under the Woodward-Hoffmann rules.
Step 5: Assign the name of the sigmatropic rearrangement based on the analysis. For example, if the reaction involves a [3,3] shift, it could be a Claisen or Cope rearrangement depending on the functional groups involved.

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Key Concepts

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

Sigmatropic Rearrangement

Sigmatropic rearrangements are a class of pericyclic reactions where a sigma bond and a π bond are rearranged in a concerted manner. These reactions involve the migration of a substituent across a σ-bond, leading to the formation of new bonds and the reorganization of the molecular structure. Understanding the mechanism and the types of sigmatropic rearrangements is crucial for predicting the products of such reactions.
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Nomenclature of Sigmatropic Shifts

Types of Sigmatropic Rearrangements

Sigmatropic rearrangements are categorized based on the number of atoms involved in the migration. The most common types are [1,3]- and [1,5]-sigmatropic rearrangements, where the numbers indicate the positions of the migrating atoms. Each type has distinct characteristics and stereochemical outcomes, which are essential for identifying the specific rearrangement occurring in a given reaction.
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Woodward-Hoffmann Rules

The Woodward-Hoffmann rules provide a framework for predicting the stereochemical outcomes of pericyclic reactions, including sigmatropic rearrangements. These rules are based on the conservation of orbital symmetry and help determine whether a reaction will proceed under thermal or photochemical conditions. Familiarity with these rules is vital for understanding the feasibility and selectivity of sigmatropic rearrangements.
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Woodward-Fieser Rules Example 1
Related Practice
Textbook Question

Compare the reaction between 2,4,6-cycloheptatrienone and cyclopentadiene to the reaction between 2,4,6-cycloheptatrienone and ethene. Why does 2,4,6-cycloheptatrienone use two π\(\pi\) electrons in one reaction and four π\(\pi\) electrons in the other?

a.

b.

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Textbook Question

Explain why maleic anhydride reacts rapidly with 1,3-butadiene but does not react at all with ethene under thermal conditions.

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Textbook Question

b. Using arrows, show the electron rearrangement that takes place in each reaction.

1.

2.

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Textbook Question

a. Name the kind of sigmatropic rearrangement that occurs in each of the following reactions.

3.

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Textbook Question

Will a concerted reaction take place between 1,3-butadiene and 2-cyclohexenone in the presence of ultraviolet light?

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Textbook Question

b. Using arrows, show the electron rearrangement that takes place in each reaction.

4.

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