Molecular Rearrangments

Course Overview

  • CHM 505-A - 2024/25

  • Focus on Molecular Rearrangements

Rearrangement Reactions

  • Definition: A rearrangement reaction is a type of organic reaction in which the carbon skeleton of a molecule is rearranged to form a structural isomer.

  • Substituent movement: Often involves a substituent (R) moving from one atom to another within the same molecule.

    • Example:

      • Starting from 1-butene, with the aid of an acid catalyst:

        • CH3CH2-C=C-H

        • Rearranges to 2-butene: CH3-CH=C-H

Types of Rearrangement Mechanisms

  • Free-Radical Rearrangements:

    • Involves the migrating group (W) moving with one electron.

    • Migration Origin (A): Atom where the group originates.

    • Migration Terminus (B): Atom where the group is transferred.

  • Classifications:

    • Nucleophilic 1,2-shifts are more common than electrophilic or free-radical shifts.

    • Cyclic transition states can complicate categorization of reactions.

Transition States in Rearrangements

  • Nucleophilic Migration: Electric overlap occurs as a nucleophile attacks.

  • Free-Radical Migration: One electron interacts with one vacant orbital; results in higher energy transition states due to electron repulsion.

  • Electrophilic Migration: Two electrons interact with two electron pairs; also leads to higher energy states.

Intramolecular vs Intermolecular Rearrangements

  • Intramolecular Rearrangement: The rearrangement occurs within the same molecule, retaining molecular integrity.

  • Intermolecular Rearrangement: The group detaches from one molecule and may transfer to another molecule.

  • Crossover Experiments: Used to determine the nature of the rearrangement by mixing reactants and analyzing products.

Nucleophilic Rearrangement Mechanisms

  1. Open Sextet Formation: The migration terminus must have an open sextet.

  2. Creation of Carbocation or Nitrene: Formation of these intermediates is crucial for the rearrangement.

  3. Simultaneous Steps: Often, the migration occurs alongside other reactions, creating stable intermediates without full transformations.

Examples of Rearrangement Reactions

  • Curtius Rearrangement:

    • Thermal decomposition of carboxylic azides yielding isocyanates.

    • Hydrolysis can lead to amines.

  • Hofmann Bromamide Reaction: Conversion of primary amides to primary amines.

  • Lossen Rearrangement: Hydroxamate esters converting to isocyanates, which can yield further products.

Migratory Aptitudes

  • Definition: A measure of how easily a group will undergo a rearrangement.

  • Order of Migratory Aptitudes: Aryl, H-vinyl (alkenyl) > t-Bu > cyclopropyl > 2° alkyl > 1° alkyl.

Mechanistic Details on Pinacol Rearrangement

  • Rearrangement through protonation, loss of water, and formation of carbocation, followed by migration to stabilize the carbocation.

  • Regioselectivity: Determined by the stability of the carbocation formed, influenced by substituent arrangements.

Overall Summary

  • Molecular rearrangements are fundamental reactions in organic chemistry, characterized by structural isomer formation, mechanistic pathways, and intricate migratory patterns influenced by carbocation stability and electron interactions.