Free-radical reaction

Free-radical reaction refers to a chemical reaction involving free radicals. Free radicals are atoms, molecules, or ions with unpaired electrons on an otherwise open shell configuration. These unpaired electrons make free radicals highly reactive towards other substances or even towards each other. Free-radical reactions are important in many biological and environmental processes, including oxidation reactions, polymerization, and in the aging process of cells.

Mechanism

The mechanism of free-radical reactions can be divided into three main steps: initiation, propagation, and termination.

Initiation

Initiation is the step where free radicals are generated. This can occur through various means, such as the homolytic cleavage of a bond under the influence of heat, light, or certain chemicals. For example, the homolytic cleavage of a chlorine molecule (\(Cl_2\)) under ultraviolet light generates two chlorine radicals (\(Cl\cdot\)).

Propagation

In the propagation step, the generated radicals react with stable molecules to produce new radicals. This step is a chain reaction, as the newly formed radicals can further react with other stable molecules, continuing the cycle. For instance, a chlorine radical can react with a methane molecule (\(CH_4\)) to produce a methyl radical (\(CH_3\cdot\)) and hydrochloric acid (\(HCl\)).

Termination

The termination step occurs when two radicals react with each other, effectively neutralizing their reactivity by forming a stable molecule. For example, two methyl radicals can combine to form ethane (\(C_2H_6\)).

Types of Free-radical Reactions

Free-radical reactions can be classified into several types, including:

• Addition reactions: Free radicals add to double or triple bonds in alkenes or alkynes to form larger molecules.
• Substitution reactions: A free radical replaces an atom or group of atoms in a molecule.
• Polymerization: Free radicals can initiate the polymerization of monomers into polymers.

Applications

Free-radical reactions have wide applications in organic chemistry, polymer science, and medicine. They are used in the synthesis of various organic compounds, the manufacturing of plastics, and in the understanding of biological processes such as DNA damage and repair mechanisms.

Health and Environmental Impact

While free-radical reactions are essential in many biological processes, excessive free radical production can lead to cellular damage and is associated with various diseases, including cancer and neurodegenerative diseases. Antioxidants are substances that can neutralize free radicals and are important in preventing cellular damage.

In the environment, free-radical reactions contribute to air pollution and ozone depletion. For example, free radicals produced by the action of sunlight on air pollutants can lead to the formation of smog.

See Also

• Antioxidant
• Oxidative stress
• Radical (chemistry)