Bond Fission:
Organic reactions usually involve making and breaking of covalent bonds. The fission of bonds can take place in two ways:
1) Homolytic fission:
In homolytic fission, the cleavage of covalent bond between two atoms takes place in such a way that each atom retains one electron of the shared pair. This is a symmetrical fission and leads to the formation of neutral species (atoms or groups of atoms) having unpaired electrons. These species are called free radicals. The free radicals are denoted by putting dot over the symbol of atom or group of atoms. The single electron movement is shown by “half headed” curved arrows (
). For example,
2) Heterolytic Fission:
Heterolytic fission is unsymmetrical so that one of the atoms takes both the electrons of the shared pair leaving none on the other. This results into two charged particles. One atom has a sextet of electrons and therefore, has a positive charge while other will have a valence octet with at least one lone pair and therefore, has a negative charge. The species that has a sextet at the carbon and is positively charged is called a carbocation (earlier called carbonium ion). The species that has a carbon atom with the shared pair of electrons and carrying a negative charge is called carbanion. For example,
