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🥼organic chemistry review

Carbocation Rearrangements

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

Carbocation rearrangements are a type of reaction in organic chemistry where a carbocation (a positively charged carbon atom) undergoes a structural change to form a more stable carbocation. This process is particularly relevant in the context of preparing alkyl halides from alcohols.

5 Must Know Facts For Your Next Test

  1. Carbocation rearrangements can occur during the conversion of alcohols to alkyl halides, where a carbocation intermediate is formed.
  2. The driving force for carbocation rearrangements is the formation of a more stable carbocation, which is typically achieved by the migration of a hydrogen or alkyl group.
  3. Rearrangements can involve the formation of a more substituted carbocation, which is generally more stable due to increased electron delocalization.
  4. Hydride shifts and alkyl group migrations are common types of carbocation rearrangements that can occur during the preparation of alkyl halides from alcohols.
  5. The specific rearrangement pathway depends on the structure of the starting alcohol and the reaction conditions, such as the type of halide used and the presence of Lewis acids.

Review Questions

  • Explain how carbocation rearrangements can occur during the conversion of alcohols to alkyl halides.
    • During the conversion of alcohols to alkyl halides, a carbocation intermediate is often formed. This carbocation can undergo rearrangements to a more stable form through processes like hydride shifts or alkyl group migrations. The driving force for these rearrangements is the formation of a more substituted and therefore more stable carbocation. The specific rearrangement pathway depends on the structure of the starting alcohol and the reaction conditions, such as the type of halide used and the presence of Lewis acids.
  • Describe the role of carbocation stability in determining the outcome of carbocation rearrangements.
    • The stability of the carbocation intermediate is a key factor in determining the outcome of carbocation rearrangements. Generally, more substituted carbocations are more stable due to increased electron delocalization. During the conversion of alcohols to alkyl halides, the formation of a carbocation intermediate can lead to rearrangements that result in the generation of a more stable carbocation. The specific rearrangement pathway, such as hydride shifts or alkyl group migrations, depends on the structure of the starting alcohol and the reaction conditions, as these factors influence the relative stability of the possible carbocation intermediates.
  • Analyze how the choice of halide and the presence of Lewis acids can affect the occurrence and nature of carbocation rearrangements in the preparation of alkyl halides from alcohols.
    • The choice of halide and the presence of Lewis acids can significantly impact the occurrence and nature of carbocation rearrangements during the preparation of alkyl halides from alcohols. The type of halide used (e.g., chloride, bromide, iodide) can influence the ease of carbocation formation, with more reactive halides (like iodide) being more likely to facilitate rearrangements. Additionally, the presence of Lewis acids, such as boron trifluoride or aluminum chloride, can stabilize the carbocation intermediate and promote rearrangements by lowering the activation energy barrier. The specific rearrangement pathway, whether it involves hydride shifts, alkyl group migrations, or other processes, will depend on the interplay between the halide, the Lewis acid, and the structure of the starting alcohol, all of which can affect the relative stability of the possible carbocation intermediates.