Chapter 13 Free Radical Reactions

Free Radical Reactions in Organic Chemistry

  • Objectives: Predict outcomes of free-radical reactions

    • Halogenation of alkanes

    • Allylic substitution reactions

    • Halogenation of alkenes

How Radicals Form and React (13.1 & 13.2)

  • Radical Formation:

    • Homolysis: Symmetrical bond cleavage resulting in one electron remaining with each fragment.

    • Symmetrical bond formation: Each radical donates an electron to form a new bond.

Halogenation of Alkanes (13.3 - 13.6)

  • Radical Substitution Reaction:

    • Example: Chlorination of methane involves three steps:

      1. Initiation: UV light breaks Cl-Cl bond.

      2. Propagation: CH4 participates in a chain reaction.

      3. Termination: Combines two radicals, ending the chain reaction without forming new radicals.

Structure and Stability of Radicals

  • The geometric structure of alkyl radicals is predominantly trigonal planar (sp2).

  • More substituted radicals are generally more stable.

Energy Considerations in Free Radicals

  • Activation energies of chloride and bromide radicals differ:

    • Chloride: Lower activation energy, less selective.

    • Bromide: Higher activation energy, more selective.

  • Reflects differences in potential energy during reactions involving different degrees of substitution (1°, 2°).

Allylic Carbon Halogenation (13.10)

  • Allylic and Benzylic Radicals:

    • Stabilized by electron delocalization.

    • Notable selectivity in bromination at allylic C-H bonds.

Radical Addition to Double Bonds (13.13 & 13.14)

  • Radicals can remove electrons from double bonds, leading to new radicals.

  • Anti-Markovnikov Addition:

    • Notable in reactions like HBr + ROOR compared to standard HBr addition.

Radical Polymerization

  • Definition: Formation of large molecules (polymers) from smaller units (monomers, e.g., plastics).

  • Stages include:

    1. Initiation

    2. Propagation

    3. Termination: Combines two growing chains.

Substituted Ethylene Polymerization

  • Substituted ethylene can undergo polymerization resulting in alternating groups

  • More highly substituted secondary radicals are formed during the process.