Radical reactions involve highly reactive species with unpaired electrons. These reactions proceed through homolytic bond cleavage and often follow a chain reaction mechanism. Unlike polar reactions, they're less affected by steric hindrance and can be initiated by heat, light, or chemical initiators.

Radical reactions have three main stages: initiation, propagation, and termination. They're influenced by factors like radical stability and the presence of antioxidants. These reactions are crucial in various applications, including polymerization, halogenation of alkanes, combustion, and even some enzyme-catalyzed processes.

Radical Reactions

Characteristics of radical reactions

• Involve species with unpaired electrons called radicals or free radicals
  • Highly reactive and unstable
  • Electrically neutral, unlike ions which are charged
• Proceed through homolytic bond cleavage, where each atom gets one electron from the broken bond
  • Polar reactions involve heterolytic bond cleavage, where one atom gets both electrons
• Often have a chain reaction mechanism, consisting of initiation, propagation, and termination steps
  • Polar reactions typically do not involve chain reaction mechanisms
• Less sensitive to steric hindrance compared to polar reactions
  • Radicals are smaller than the corresponding carbenium ions
• Often exothermic and have lower activation energies than polar reactions
  • Due to the high reactivity of radicals and the lack of charge development in the transition state
• Can be initiated by heat, light, or chemical initiators (peroxides, azo compounds)
  • Polar reactions are typically initiated by acids, bases, or other polar reagents
• Influenced by bond dissociation energy, which affects the ease of radical formation

Stages of radical chain reactions

1. Initiation: Generation of radicals from non-radical species

   • Homolytic cleavage of a weak bond by heat, light, or a chemical initiator (Cl-Cl -> 2 Cl•)

2. Propagation: Radicals react with stable molecules to form new radicals, which continue the chain

   • Two types of propagation steps:
     • Hydrogen abstraction (R• + R'-H -> R-H + R'•)
     • Addition to multiple bonds (R• + C=C -> R-C-C•)

3. Termination: Destruction of radicals by combination or disproportionation

   • Combination: Two radicals combine to form a stable molecule (R• + R'• -> R-R')
   • Disproportionation: A hydrogen atom is transferred from one radical to another, forming two stable molecules (R-CH2-CH2• + R'• -> R-CH=CH2 + R'-H)

• Overall rate depends on the rates of initiation, propagation, and termination steps
  • Steady-state approximation: Concentration of radicals remains constant during the reaction, as the rates of initiation and termination are equal

Types of Radical Reactions

• Radical substitution: Replacement of an atom or group by a radical (e.g., halogenation of alkanes)
• Radical addition: Addition of a radical to a multiple bond (e.g., polymerization reactions)
• Radical rearrangement: Intramolecular reorganization of atoms within a radical species

Factors Affecting Radical Reactions

• Radical stability: More stable radicals are generally less reactive
  • Stability order: tertiary > secondary > primary > methyl
• Presence of antioxidants: Compounds that can inhibit radical reactions by scavenging free radicals

Applications of radical reactions

• Polymerization reactions: Production of polymers (polyethylene, polystyrene)
  • Initiated by radicals generated from peroxides or azo compounds
  • Propagation involves addition of radicals to the double bond of the monomer
• Halogenation of alkanes: Production of halogenated compounds (chloroform, carbon tetrachloride)
  • Initiated by light or heat, which cleaves the halogen-halogen bond homolytically
  • Propagation involves hydrogen abstraction from the alkane and addition of the alkyl radical to a halogen molecule
• Combustion reactions: Burning of hydrocarbons in engines and power plants
  • Initiated by heat or a spark, which generates radicals from the fuel molecules
  • Propagation involves a complex series of hydrogen abstraction and addition reactions with oxygen
• Lipid peroxidation: Oxidative degradation of lipids in cell membranes
  • Initiated by reactive oxygen species (ROS) like hydroxyl radicals (•OH)
  • Propagation involves hydrogen abstraction from the lipid and addition of oxygen to form peroxyl radicals (ROO•)
• Enzyme-catalyzed radical reactions: Certain enzymes use radicals as intermediates
  • Ribonucleotide reductase reduces ribonucleotides to deoxyribonucleotides
    • Generates a thiyl radical (RS•) from a cysteine residue, which abstracts a hydrogen atom from the ribonucleotide