A free radical or radical is an atom or group of
atoms that has one or more unpaired electrons. The radical
typically has no positive or negative charge, as in ions
(e.g.salt, NaCl consists of positive
Na+ ions and negative
Cl- ions, but not of radicals).
However, despite its lack of charge, radicals generally are very
reactive, short-lived, and typically appear as reaction
intermediates rather than as end products.
Following are some Lewis Structures of free radicals:
H
.. .. ..
:Cl· :Br· H:C ·
.. .. ..
H
Each dot represents an electron in the outer electron shell of the
atom. Electrons are either unshared, or shared to form a chemical bond
(as in the third example). The Lewis structures show that the radical
has an incomplete octet, which means that the outer electron shell
(which determines the formation and strength of chemical
bonds) has less than 8 electrons. Most atoms strive for an electron
configuration with 8 electrons in the outer shell since it corresponds
with the configuration of the noble gases neon and argon. These
structures are most stable. Thus, since the free radicals are not
following the octet rule they are generally highly energetic and
reactive.
Usually, the paired electrons aren't written, and the radicals are
simply written as:
Cl· Br· H3C· or CH3·
Free radicals are formed by the dissociation of a chemical bond.
This process is called homolytic cleavage, because the two atoms that
are involved receive one electron from the shared pair.
hν
Cl-Cl -> Cl· + Cl·
hν
H-H -> H· + H·
H H H H
| | hν | |
H-C-C-H -> H-C· + ·C-H
| | | |
H H H H
Breaking these chemical bonds requires energy. This energy is
provided by heating the reaction mixture, or by
light/electromagnetic radiation (hν). The amount of energy
required is dependent on the nature of the chemical bond; strong
chemical bonds require a large amount of energy to break, but the
resulting free radicals will be short-lived and highly reactive. Weak
chemical bonds require a small energy input, and the resulting free
radicals are longer-lived but exhibit a lower reactivity.
Free radicals are often involved in chain reactions.
This type of reactions consists of three steps: an initiation, where
the free radical is formed, a propagation, where the free radicals
react to form the main product, and a termination, where free radicals
combine, and the reaction is stopped. For example, look at the
chlorination of methane:
Initiation:
hν
Cl-Cl -> Cl· + Cl·
Propagation:
H H
| |
H-C-H + Cl· -> ·C-H + HCl
| |
H H
H H
| |
·C-H + Cl-Cl -> Cl-C-H + Cl· (note that another Cl· radical is formed)
| |
H H
Termination:
H H H H
| | | |
H-C· + ·-C-H -> H-C-C-H
| | | |
H H H H
Free radical reaction mechanisms commonly occur in
polymerization reactions. Because the termination step is dependent on
the recombination of radical-pairs, the (average) polymer length is
dependent on the radical concentration in the initiation phase.
However, free radicals can also result in the degradation of
polymers, and rubbers. The free radicals attack the polymer
chain, and induce scission, or crosslinking. In some
cases, an antioxidant is added to prevent radical degradation. The
antioxidant is a compound that easily decomposes into low-reactive free
radicals that "trap" the harmful free radicals. Typical antioxidants for
plastics and rubbers are phenols.
Similarly to polymer degradation is the degradation of cell tissue in
the human body. In this case, oxygen, or peroxide radicals attack the
molecules of the cell tissue, and form long-lived free radicals that can
further degrade the surrounding tissue. Again, antioxidants (which can
be found in fruit and vegetables) neutralize the harmful free
radicals. | 
