Textbook Question
b. Using arrows, show the electron rearrangement that takes place in each reaction.
4.
16. Conjugated Systems
Sigmatropic Rearrangement
1:35 minutesProblem 18b
Textbook Question
b. Will thermal 1,5-migrations of carbon occur with retention or inversion of configuration?
Verified step by step guidance1
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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1mKey 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.
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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.
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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, 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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