Reaction mechanisms and bond cleavage

Every organic reaction begins with a bond breaking somewhere, and there are exactly two fundamentally different ways a covalent bond's shared electron pair can be divided between the two atoms that were bonded. In homolytic cleavage, the two bonding electrons split evenly, one going to each atom, producing two neutral species that each carry an unpaired electron — these are called radicals. Radicals are highly reactive because an unpaired electron is energetically unfavourable; radicals typically react further by seeking out another unpaired electron to pair with. Homolysis is favoured by conditions that supply energy without directing it toward a particular atom, such as heat or ultraviolet light, and by weak, nonpolar bonds.

In heterolytic cleavage, both bonding electrons go to just one of the two atoms, producing a cation (the atom that lost both electrons) and an anion (the atom that gained both). This is the pathway that dominates ionic organic chemistry, and it is favoured by polar bonds, where the electron density is already unevenly distributed toward one atom before the bond even breaks, and by polar solvents that can stabilise the resulting charged species.

The species produced by heterolytic cleavage divide into two functional roles that drive the overwhelming majority of ionic organic reaction mechanisms. A nucleophile ("nucleus-loving") is an electron-rich species — carrying a lone pair of electrons, a pi bond, or a negative charge — that seeks out and donates electron density to a region of positive or partial positive charge, forming a new bond in the process. An electrophile ("electron-loving") is the complementary electron-poor species — often carrying a partial or full positive charge, or an empty orbital — that accepts the electron pair a nucleophile donates. Almost every ionic organic mechanism, from nucleophilic substitution to electrophilic addition, can be described as a nucleophile attacking an electrophile, which is exactly why correctly identifying which reacting species is the nucleophile and which is the electrophile is the first and most essential step in predicting how an organic reaction will proceed.