Ch. 28 - Pericyclic Reactions

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 28 - Pericyclic Reactions

Problem 18b

Chapter 25, Problem 18b

b. Will thermal 1,5-migrations of carbon occur with retention or inversion of configuration?

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Understand the concept of thermal 1,5-migrations: These are pericyclic reactions where a substituent migrates from one position to another within a molecule, typically involving a cyclic transition state. The reaction is governed by orbital symmetry rules (Woodward-Hoffmann rules).

Determine the stereochemical outcome: The stereochemistry of the migrating group depends on whether the reaction proceeds via a suprafacial or antarafacial pathway. Suprafacial migration occurs on the same face of the molecule, while antarafacial migration occurs on opposite faces.

Apply the Woodward-Hoffmann rules: For thermal pericyclic reactions, the reaction pathway is determined by the number of electrons involved. In a 1,5-migration, the reaction typically involves a [3,3]-sigmatropic rearrangement, which is suprafacial for thermal conditions.

Analyze the configuration of the migrating group: In a suprafacial pathway, the migrating group retains its original stereochemical configuration because it moves on the same face of the molecule during the rearrangement.

Conclude the stereochemical outcome: Based on the suprafacial nature of the thermal 1,5-migration, the reaction will occur with retention of configuration for the migrating carbon group.

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Key Concepts

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

1,5-Migration in Organic Chemistry

1,5-Migration refers to the rearrangement of a substituent from one carbon atom to another that is five positions away in a molecule. This process often occurs in the context of rearrangements involving carbocations or radical intermediates. Understanding the mechanism of 1,5-migrations is crucial for predicting the outcome of reactions involving these types of intermediates.

Retention vs. Inversion of Configuration

Retention and inversion of configuration describe how the spatial arrangement of atoms around a chiral center changes during a reaction. Retention means that the configuration remains the same, while inversion indicates that it has changed to the opposite configuration. The outcome depends on the mechanism of the reaction, particularly whether it proceeds through a planar intermediate or involves a direct bond-breaking and bond-forming process.

Thermal vs. Photochemical Reactions

Thermal reactions occur at elevated temperatures and typically involve the breaking and forming of bonds through heat energy, while photochemical reactions are driven by light energy. The conditions under which a reaction occurs can significantly influence the mechanism and stereochemical outcomes, including whether a migration leads to retention or inversion of configuration. Understanding these differences is essential for predicting reaction pathways in organic chemistry.

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Related Practice

Textbook Question

a. Will thermal 1,3-migrations of carbon occur with retention or inversion of configuration?

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Textbook Question

Explain why [1,3] sigmatropic migrations of hydrogen cannot occur under thermal conditions, but [1,3] sigmatropic migrations of carbon can.

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Textbook Question

Chorismate mutase is an enzyme that promotes a pericyclic reaction by forcing the substrate to assume the conformation needed for the reaction. The product of the pericyclic reaction is prephenate that is subsequently converted into the amino acids phenylalanine and tyrosine. What kind of a pericyclic reaction does chorismate mutase catalyze?

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Textbook Question

Explain why the hydrogen and the methyl substituent are trans to one another after photochemical ring closure of provitamin D₃ to form 7-dehydrocholesterol.

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Textbook Question

Show how 5-methyl-1,3-cyclopentadiene rearranges to form 1-methyl-1,3-cyclopentadiene and 2-methyl-1,3-cyclopentadiene.

Textbook Question

Does the [1,7] sigmatropic rearrangement that converts provitamin D₃ to vitamin D₃ involve suprafacial or antarafacial rearrangement?

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