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Radicals - Introduction

Homolysis

In most organic molecules' reactions, there is either a formation or deconstruction of covalent bonds that occurs. The cleavage of a covalent bond may proceed in various ways.

On the one hand, the two bonding electrons may be allocated to only one of the combined atoms. As a result, two ions are then formed. This type of bond breaking is called heterolysis, or heterolytic cleavage.

Fig.1
Breaking of covalent bonds.

On the other hand, the two bonding electrons may be evenly allocated to each of the combined atoms. As a result, so-called radicals are formed. Radicals are atoms or molecules that contain at least one unpaired electron - that is, they have at least one single-occupied atom or molecular orbital. This type of bond breakage is called homolysis, or homolytic cleavage.

Fig.2

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Fig.3

In the illustration of reaction mechanisms, the relocation of single electrons is represented by an arrow with half of an arrowhead. The arrow always points in the direction that the electron is moving in.

The homolytic cleavage of a bond requires energy. In this case, heat or radiation may be applied. The amount of energy that is necessary in a homolytic cleavage is equal to that amount of energy that has been released during the formation of the bond. This energy is called dissociation energy. In radical reactions, radical formers are often used. A radical former is a compound that can easily be cleaved into radicals by only a small amount of energy. These radicals then generate the desired reagent radical by abstracting, for example, a hydrogen atom. Well-known radical formers are di-t-butyl peroxide, dibenzoyl peroxide, and azobisisobutyrylnitrile (AIBN).

Tab.1
Radical formers
Di-t-butyl peroxide
Fig.4
Dibenzoyl peroxide
Fig.5
Azobisisobutyrylnitrile (AIBN)
Fig.6

Radical side reactions may be prevented by applying a free-radical scavenger. Tetraethyllead, which increases the octane number of gasoline, is one well-known example of such a free-radical scavenger.

Exercises: Homolysis

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