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🥼organic chemistry review

Halogen Abstraction

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

Definition

Halogen abstraction is a type of radical substitution reaction in organic chemistry where a halogen atom (such as chlorine, bromine, or iodine) is selectively removed from an alkane or other organic compound by a radical species. This process is a key step in the radical halogenation of alkanes, leading to the formation of alkyl halides.

5 Must Know Facts For Your Next Test

  1. Halogen abstraction is the first step in the radical halogenation of alkanes, where a hydrogen atom is selectively removed to form an alkyl radical.
  2. The halogen abstraction step is typically initiated by the homolytic cleavage of a halogen-halogen bond (e.g., Cl-Cl) to generate reactive halogen radicals.
  3. The reactivity of the halogen radicals decreases in the order: F• > Cl• > Br• > I•, with fluorine being the most reactive and iodine being the least reactive.
  4. Halogen abstraction is a reversible process, and the equilibrium between the alkyl radical and the alkane can be shifted by the presence of other radical species or by changing the reaction conditions.
  5. The selectivity of the halogen abstraction step is influenced by factors such as the stability of the resulting alkyl radical and the availability of hydrogen atoms for abstraction.

Review Questions

  • Explain the role of halogen abstraction in the radical halogenation of alkanes.
    • Halogen abstraction is the first step in the radical halogenation of alkanes, where a hydrogen atom is selectively removed from the alkane by a reactive halogen radical (such as chlorine or bromine) to form an alkyl radical. This alkyl radical can then undergo further reactions, such as addition to another halogen molecule, to produce the desired alkyl halide product. The halogen abstraction step is crucial in initiating the radical chain reaction and determining the selectivity of the overall halogenation process.
  • Describe the factors that influence the reactivity and selectivity of the halogen abstraction step.
    • The reactivity of the halogen radicals involved in the halogen abstraction step decreases in the order: F• > Cl• > Br• > I•, with fluorine being the most reactive and iodine being the least reactive. The selectivity of the halogen abstraction is influenced by the stability of the resulting alkyl radical, with more stable radicals being preferentially formed. Additionally, the availability of hydrogen atoms for abstraction can also affect the selectivity, as the halogen radical will preferentially abstract hydrogen from the most accessible and reactive C-H bonds. Reaction conditions, such as temperature and the presence of other radical species, can also shift the equilibrium between the alkyl radical and the alkane, influencing the overall course of the reaction.
  • Evaluate the importance of the reversibility of the halogen abstraction step in the context of radical halogenation reactions.
    • The reversibility of the halogen abstraction step is a crucial aspect of radical halogenation reactions. The equilibrium between the alkyl radical and the alkane can be shifted by the presence of other radical species or by changing the reaction conditions, such as temperature or pressure. This reversibility allows for the optimization of the reaction, as the equilibrium can be manipulated to favor the desired products. Additionally, the reversibility of the halogen abstraction step enables the possibility of selective halogenation, as the reaction can be tuned to target specific C-H bonds within the alkane substrate. The ability to control the selectivity of the halogen abstraction is a key factor in the widespread use of radical halogenation reactions in organic synthesis.