PHD → Organic chemistry → Reaction mechanism ↓
Radical reactions
Introduction
Radical reactions play an important role in organic chemistry, being responsible for many processes in both synthetic and biological systems. These reactions involve radicals, which are highly reactive species with unpaired electrons and can lead to the formation of new bonds or the breaking of old bonds.
Understanding radicals
A radical is a molecule, atom, or ion that has an unpaired valence electron. Because of this unpaired electron, radicals are usually very reactive, trying to pair the lone electron with another to reach a more stable state.
Formation of radicals
Radicals can be generated in several ways, most commonly by thermal, photochemical, or chemical means.
Thermal methods
In thermal methods, heat causes homolytic cleavage, where the bond between two atoms is broken equally, and each atom retains one electron from the bond:
r−r' → r˙ + r'˙
Photochemical methods
Photochemical methods involve the absorption of light. When a molecule absorbs light, it may become excited, and the bonds may undergo homolytic cleavage:
R−R' → R˙ + R'˙ (on hv)
hv represents the energy of the light used.
Chemical methods
Some compounds, such as peroxides and organic halides, can react or decompose to form radicals. For example:
(ROOR) → 2RO˙
Propagation of radical reactions
Once initiated, radical reactions proceed through a series of propagation steps. These involve the formation of new radicals by reacting radicals with stable molecules:
r˙ + r'h → rh + r'˙
This is a basic example of a propagation step where a radical abstracts a hydrogen atom from a stable molecule.
Example of radical bromination
Termination phase
Radical reactions eventually terminate when two radicals combine to form a stable product, reducing the number of radicals:
r˙ + r'˙ → r−r'
This situation stops the radical chain reaction because the radicals are destroyed without producing new radicals.
Basic reactions in biological systems
Radical reactions are found in nature. For example, in biological systems, radicals are involved in processes such as the formation of biopolymers and DNA synthesis.
Factors affecting radical stability
The stability of radicals is important in determining the pathways and products of radical reactions. Factors such as hybridization, resonance, and the presence of electron-withdrawing or donating groups affect radical stability:
Echo
Resonance-stabilized radicals are more stable because the unpaired electrons are displaced on more than one atom.
Hybridization
sp 2 -hybridized carbon radicals are generally more stable than sp 3 -hybrid carbon radicals because they have greater s-character and lower p-orbital energy.
Applications of radical reactions
Radical reactions are used in many synthetic processes, such as polymerization, halogenation, and oxidative processes.
Polymerization
The process of building larger molecules (polymers) from smaller molecules (monomers) often involves radical mechanisms.
n (CH2=CH2) → [−CH2−CH2−]n (using radicals)
Halogenation
Radical halogenation is a radical reaction used to incorporate halogens into organic molecules.
RH + X2 → RX + HX
Conclusion
Radical reactions are a fundamental aspect of organic chemistry, providing pathways for building complex molecules. By understanding the nature, origin, and behavior of radicals, chemists can manipulate these reactions for a variety of applications ranging from synthetic chemistry to biological systems.