14 4.1 Homolytic and Heterolytic Cleavage
Chapter 4.1 Learning Objectives
- Learn the fundamentals of homolytic and heterolytic cleavage.
4.1.1 Introduction
For the reactions we learned so far, bond breaking occurs in the way that one part of the bond takes both electrons (the electron pair) of the bond away. For example of SN1 reaction, the leaving group Br leaves with the electron pair to form Br– and carbocation intermediate.
This process is called heterolytic bond cleavage, the σ bond breaks heterolytically. As we have always been doing, an arrow with the double-barbs is used to show heterolytic cleavage, that is the transfer of electron pair specifically:
There is another type of bond breaking process, in which each part of the σ bond takes one electron away, as shown below:
This is called homolytic cleavage, or homolysis. The electron pair separate evenly to each part, and as a result both products contain a single electron. The species that contain one or more single electron is called radical (or free radical). Radicals are produced from homolytic cleavage. The arrow with sing-barb (like the shape of a fishhook) is used to show homolytic cleavage, that is single electron transfer specifically:
Homolysis occurs mainly for non-polar bonds, heat or light (delta is the symbol for heat; hv is used to show light) is needed to provide enough energy for initiating the process. For example:
Radical is another highly reactive reaction intermediate, because of the lack of octet.
Key Takeaways
- Chemical bonds can break homolytically or heterolytically.
- Homolytic cleavage of bonds leads to the formation of radicals, neutral reactive intermediates.
- Heterolytic cleavage of bonds leads to the formation of ions. Carbocations and carbanions are examples of reactive intermediates formed in such reactions.
