10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation

Radical halogenation transforms alkanes into alkyl halides through a series of steps. This process involves the substitution of hydrogen atoms with halogen atoms, creating a mix of products due to varying reactivity of different hydrogen types and potential rearrangements.

The reactivity of hydrogens follows a specific order, with tertiary being the most reactive. Factors like bond strength, radical stability, and halogen type influence the outcome. Understanding these aspects helps predict and control the products of radical halogenation reactions.

Radical Halogenation of Alkanes

Process of radical halogenation

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• Converts alkanes to alkyl halides by substituting a hydrogen atom with a halogen atom (Cl, Br)
• Initiation step:
  • Homolytic cleavage of the diatomic halogen molecule ($X_2$) by heat or light generates two halogen radicals ($X \cdot$)
• Propagation steps:
  1. Hydrogen abstraction: Halogen radical abstracts a hydrogen from the alkane, forming an alkyl radical and HX
  2. Halogen abstraction: Alkyl radical reacts with another $X_2$ molecule, forming the alkyl halide product and regenerating the halogen radical
• Termination steps:
  • Combination: Two radicals combine to form a stable molecule
    • Alkyl radicals can combine with each other or with halogen radicals (ethane, 2-methylpropane)
  • Disproportionation: Two alkyl radicals react, with one abstracting a hydrogen from the other, forming an alkane and an alkene (propane and propene)

Mixtures in radical halogenation products

• Alkanes contain different types of hydrogens (primary, secondary, tertiary) with varying reactivity towards radical halogenation
• Alkyl radical formed during the reaction can undergo rearrangements
  • Hydrogen shifts or skeletal rearrangements lead to more stable radicals (1-methylpropyl radical to 2-methylpropyl radical)
• Multiple propagation steps can occur before termination allowing for the formation of various isomeric alkyl halides (1-chlorobutane, 2-chlorobutane)
• Termination steps involving different radicals can lead to a variety of byproducts
  • Combination of alkyl radicals forms higher molecular weight alkanes (hexane from propyl radicals)
  • Disproportionation of alkyl radicals forms alkanes and alkenes (butane and 2-butene from butyl radicals)

Reactivity of hydrogens in chlorination

• Reactivity order: Tertiary (3°) > Secondary (2°) > Primary (1°) based on the stability of the resulting alkyl radical
• Tertiary radicals are the most stable due to stabilization by hyperconjugation with three neighboring alkyl groups
• Secondary radicals are more stable than primary radicals with stabilization by hyperconjugation with two neighboring alkyl groups
• Primary radicals are the least stable, stabilized by hyperconjugation with only one neighboring alkyl group
• More stable radicals have lower activation energy required for formation leading to higher reaction rates and preferential formation of more substituted alkyl halides (2-chloro-2-methylbutane over 1-chloro-2-methylbutane)
• The relative stability of radicals affects the selectivity of the reaction, favoring the formation of more stable radical intermediates

Factors affecting radical halogenation

• Bond dissociation energy: The strength of the C-H bond being broken influences the ease of hydrogen abstraction
• Radical stability: More stable radicals form more readily, affecting the product distribution
• Selectivity: The preference for forming certain products based on radical stability and reaction conditions
• Halogen reactivity: Different halogens (Cl, Br) have varying reactivities, impacting the overall reaction rate and product distribution