Radical (chemistry)

In chemistry, radicals (often refered to as free radicals) are atomic or molecular species with unpaired electrons or an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions. Radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes. "Radical" and "Free Radical" are frequently used interchangeably, however a radical may be trapped within a solvent cage or be otherwise bound. Historically, "Radical" was used to refer to a collection of atoms that remain unchanged over the course of a reaction, however this usage is, today, uncommon. The first organic free radical (the triphenylmethyl radical) was identified by Moses Gomberg in 1900.

The chemistry of radicals

Terminology

In chemistry free radicals take part in radical addition and radical substitution as reactive intermediates. Reactions involving free radicals are usually divided into three categories: initiation, propagation, and termination.

Related Topics:
Radical addition - Radical substitution - Reactive intermediate

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Initiation reactions are those which result in a net increase in the number of free radicals. They may involve the formation of free radicals from stable species as in Reaction 1 above or they may involve reactions of free radicals with stable species to form more free radicals.
• Propagation reactions are those reactions involving free radicals in which the total number of free radicals remains the same.
• Termination reactions are those reactions resulting in a net decrease in the number of free radicals. Typically two free radicals combine to form a more stable species, for example: 2Cl·→ Cl₂

Formation

The formation of radicals requires covalent bonds to be broken homolytically, a process that requires significant amounts of energy. For example, splitting H2 into 2H· has a ΔH° of +435 kJ/mol, and Cl2 into 2Cl· has a ΔH° of +243 kJ/mol. This is known as the homolytic bond dissociation energy, and is usually abbreviated as the symbol DH°. The bond energy between two covalently bonded atoms is affected by the structure of the molecule as a whole, not just the identity of the two atoms, and radicals requiring more energy to form are less stable than those requiring less energy. Homolytic bond cleavage most often happens between two atoms of similar electronegativity. In organic chemistry this is often the O-O bond in peroxide species or O-N bonds.

Related Topics:
Homolytically - Bond dissociation energy - Peroxide

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

However, propagation is a very exothermic reaction. Note that all species are electrically neutral although radical ions do exist.

Related Topics:
Exothermic reaction - Radical ion

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Persistence and stability

Long lived radicals can be placed into two categories

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Stable Radicals

:Radicals can be long lived if they occur in a conjugated π system, such as the radical derived from α-tocopherol (vitamin E)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Persistent Radicals

:Persistent radical compounds are those whose longevity is due to steric crowding around the radical center and makes it physically difficult for the radical to react with another molecule. Examples of these include Gomberg's radical (trimethylphenyl), Fremy's salt (Potassium nitrosodisulfonate, (KSO3)2NO·)and nitroxides, (general formula R2NO·) such as TEMPO.

Related Topics:
Fremy's salt - Nitroxide - TEMPO

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Combustion

Probably the most familiar free-radical reaction for most people is combustion. In order for combustion to occur the relatively strong O=O double bond must be broken to form oxygen free radicals. It is noteworthy that oxygen is actually a diradical with two unpaired electrons in the outer orbitals. Reactivity is limited because these electrons have parallel spins. However, this barrier is overcome by enzymes in the body (respiration) and by energy (heat). The flammability of a given material is strongly dependent on the concentration of free radicals that must be obtained before initiation and propagation reactions dominate leading to combustion of the material. Once the combustible material has been consumed, termination reactions again dominate and the flame dies out.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Polymerization

In addition to combustion, many polymerization reactions involve free radicals. As a result many plastics, enamels, and other polymers are formed through radical polymerization.

Related Topics:
Polymerization - Radical polymerization

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Recent advances in radical polymerization methods known as Living Radical Polymerization such as:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Reversible Addition-Fragmentation chain Transfer (RAFT)
• Atom Transfer Radical Polymerization (ATRP)
• Nitroxide Mediated Polymerization (NMP)

These methods produce polymers with a much narrower distribution of molecular weights.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Atmospheric radicals

In the upper atmosphere free radicals are produced through dissociation of the source molecules, particularly the normally unreactive chlorofluorocarbons by solar ultraviolet radiation or by reactions with other stratospheric constituents. These free radicals then react with ozone in a catalytic chain reaction which destroys the ozone, but regenerates the free radical, allowing it to participate in additional reactions. Such reactions are believed to be the primary cause of depletion of the ozone layer and this is why the use of chlorofluorocarbons as refridgerants has been restricted.

Related Topics:
Chlorofluorocarbon - Ultraviolet radiation - Ozone - Chain reaction - Ozone layer

~ ~ ~ ~ ~ ~ ~ ~ ~ ~