Fission of a Covalent Bond: Homolytic and Heterolytic Fission; Free Radicals

A covalent bond can break, or 'fission', in two fundamentally different ways, and which pathway a given bond follows determines what kind of reactive species is produced.

In homolytic fission (homolysis), the shared bonding electron pair splits exactly in half, one electron going to each of the two fragments. Both resulting fragments are electrically neutral, but each now carries one unpaired electron; such a species is called a free radical, conventionally written with a single dot to show the unpaired electron (for example, $\text{Cl}^\bullet$). Free radicals are highly reactive and generally short-lived. Homolysis is typically brought about by supplying energy directly to the bond -- as heat or, very commonly, ultraviolet or visible light -- and is favoured for non-polar bonds in non-polar surroundings, where there is no polar solvent available to stabilise separated ions. The classic example is the very first (initiation) step of the free-radical halogenation of alkanes: $\text{Cl}_2$ absorbs light and splits homolytically into two chlorine free radicals, $\text{Cl}_2 \xrightarrow{h\nu} 2\text{Cl}^\bullet$, which then go on to abstract hydrogen atoms from the alkane and propagate the reaction chain. …