Free radicals are generated due to the homolytic bond cleavage, a type of bond breaking where each atom holding the two-electron covalent bond gets one electron. Such a homolytic bond cleavage requires an input of energy, either in the form of heat or light.
When the energy is obtained from heat sources, it is referred to as a thermal cleavage, and it is a photochemical cleavage when the energy source is light.
These two sources increase the energy of the atoms in the bonds that undergo rapid movement. The bond electrons excite and deexcite, rapidly destabilizing the bonds. Once the molecules attain sufficient energy equivalent to the bond dissociation energy, they undergo cleavage. The bond dissociation energy for most molecules with single bonds is between 200-500 kJ/mol.
If the bond is to be photolyzed, in that case, there is a relationship between the Energy (E) and the frequency of light expressed as, E=hυ. Since the energy of light is directly proportional to frequency and inversely proportional to wavelength (E=hc/λ), illuminating a molecule with laser emitting light of wider wavelengths can cause some molecules to break easily. The UV-visible wavelength range is between 200-800 nm. Molecules like Cl2 and H2O2 can easily cleave in this wavelength range, however; alkanes, water, and N2 do not get photolyzed easily.
Only a few radical species are required to start a chain reaction. Those few radicals also propagate the reaction until all the substrate molecules are consumed, and the reaction terminates.
Free Radicals, a crucial reactive intermediate in organic chemistry is covered in detail in CurlyArrows' Introductory Organic Chemistry Course. Preview the Book.
Related Reading- Free Radicals (Introduction, Structure, and Types)
