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:
Radical is another highly reactive reaction intermediate, because of the lack of octet. The substitution reaction we will learn in this chapter involves the radical intermediate.