Free Radicals

All reactions involve the breaking and reforming of bonds.
Breaking bonds can be referred to as bond fission. The way bonds break can have an influence upon reactions.
Covalent bonds involve the sharing of electrons:
H:Cl
When the bonds break, the electrons are redistributed between the two atoms. There are two ways in which this can happen.

Homolytic Fission

In this type of fission, one of the shared electrons goes to each atom:
The dot shows that each Br atom has gained an electron from the shared pair.
The Atoms have no overall electric charge, because they have both regained their electron that they submitted to the shared pair.
An unpaired electron has a tendency to pair up again with another electron from another substance.
These highly reactive atoms (or groups of atoms) with unpaired electrons are called free radicals.
Another example is when the C-H bond in methane is broken:

In this type of fission, when the bond breaks, the electrons just go to one of the atoms.
This atom becomes negatively charged, because it has an extra electron.
The other atom becomes positively charged.
H:Cl H⁺ + :Cl^-
When the bonds holding HCl together are broken, both of the electrons go to the chlorine atom, making it a negative ion. The Hydrogen atom is short of one electron, so it becomes a positive ion.
Heterolytic fission is common in molecules with polar bonds.
Bromomethane contains a C-Br bond, which can break heterolytically. The Bromine atom is more electronegative so it takes both of the electrons.
The bromine atom becomes negatively charged; it becomes a nucleophile.
The CH₃ molecule becomes positively charged; it becomes an electrophile.
Polarity occurs when there is an electronegativity difference between two bonded atoms; bromine is more electronegative than carbon; it has a greater electron pulling power.
For many chemical reactions, the conditions in which they take place can determine how a bond breaks. For example, when the bromomethane is dissolved in a polar solvent, the C-Br bond breaks heterolytically; however in a non polar substance it breaks homolytically.

The key feature of a free radical is the unshared electron.
The unshared electron makes it very reactive.
Some free radicals are not as reactive as others, and can last long enough to behave as normal molecules. Nitrogen monoxide is an example.
The nitrogen atom only has seven electrons in its outer energy level, the odd unpaired electron makes it a free radical.
Di-oxygen is also a stable free radical, because of the two unpaired electrons.

Filled outer electron shells are more stable than unfilled ones.
Free radicals are so reactive because they tend to grab electrons from other atoms or molecules to fill their outer energy levels.
For example a chlorine free radical will grab an electron from a pair of hydrogen atoms, forming a new bond between a hydrogen and a chlorine atom and making a hydrogen free radical.
The shows the movement of one electron.
The hydrogen free radical is also very reactive, so this again can create another free radical, the process continues.
The reaction between the chlorine and hydrogen atoms is a typical photochemical free radical reaction; it is light that provides the energy for it to happen.
Free radical chain reactions occur in three stages:

A photodissociation reaction usually occurs to initiate the process, for example chlorine molecules are decomposed by light, forming two chlorine free radicals:
Cl₂ + hv 2Cl^.
Only two chlorine free radicals are formed, but these initiate the process.

Eventually two free radicals will collide with each other (not very common).
When this does happen, the radicals lose some of their energy when forming a new molecule. This destroys the radicals, and terminates the reaction.
H^. + H^. H₂
Cl^.+ Cl^. Cl₂
H^. + Cl^. HCl
The overall effect of the reaction is to combine hydrogen and chlorine:
H_2(g) + Cl_2(g) 2HCl_(aq)
Free radical reactions usually occur in the gas phase, are initiated by heat or light, occur very fast, and can result in a variety of products.