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.

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