A covalent bond holding two atoms is made of two electrons. The bond can cleave or break in two ways - equally (homolytic fission) or unequally (heterolytic fission).
A heterolytic bond cleavage results in unequal bond-breaking where one atom in the bond retains both the bond electrons.
A double-headed curly arrow shows the shift of two bond electrons on one atom.
The atom retaining the electrons is the most electronegative atom of the two.
The cleavage creates two ions, one rich in electrons (the anion) and the other deficient in electrons (the cation).
The anion is denoted with a negative charge (B-), and the cation is denoted with a positive sign (A+).
Importance of Heterolytic Bond Cleavage
The heterolytic bond cleavage generates reactive species- nucleophiles (Nu-) and electrophiles (E+).
Examples of positively charged electrophiles that are formed by heterolytic bond cleavage are- the methyl (CH3+), primary (CH3-CH2+), secondary (CH3-CH+-CH3), tertiary ((CH3)3C+) carbocations, proton (H+), and halonium ion (X+), etc.
Carbocations are necessary substrates required in the first step of the SN1 reaction or as an intermediate in the alkene addition reaction and electrophilic aromatic substitution reaction.
Negatively charged nucleophiles are lone pair(s) containing electronegative atoms that are denoted with a negative charge. For example, Cl-, OH-, etc.
Post the cleavage, in addition to the original lone pair, the electronegative atom carries the excess bond electrons. They are assigned a negative charge and called anionic/charged nucleophiles.
Such heterolytic bond breakage is also responsible for creating negative charge centers on atoms that initially had no lone pair, such as carbon nucleophiles of R-, CN-, etc.
Read about homolytic bond cleavage.