homolysis
Homolysis
Homolysis or homolytic fission (or homolytic cleavage) is the dissociation of a molecular bond (mainly non-polar bonds) by a process where each of the fragments (an atom or molecule) retains one of the originally bonded electrons. During homolytic fission of a neutral molecule with an even number of electrons, two free radicals will be generated. The energy involved in this process is called bond-dissociation energy.
Because of the relatively high energy required to break bonds in this manner, homolysis occurs primarily under certain circumstances:
- Light (e.g. ultraviolet radiation)
- Heat
Similarly, an arrow with a single barb (unlike the sign used in heterolysis) is used to show homolytic cleavage, that is the transfer of electron specifically.
Mechanism
In the above example, when \[\ce{Cl2}\] undergoes homolysis, an external energy source as ultraviolet light, heat provides the necessary activation energy to promote electrons from the bonding orbital to the antibonding orbital by weakening the \[\ce{Cl-Cl}\] bond by decreasing electron density in the bonding orbital and increasing electron density in the antibonding orbital. As the bond weakens, the \[\ce{Cl-Cl}\] bond lengthens and the bond dissociation energy is overcome, leading to the equal splitting of the bond due to the identical electronegativities.