homolysis and heterolysis

A covalent bond is two atoms sharing a pair of electrons. When that bond breaks, there is a fork in the road: the two electrons can split up fairly, one going to each atom, or they can stay together and both go to a single atom. These two ways of breaking a bond have names — homolysis (an even, fair split) and heterolysis (an uneven split where one atom takes both). Which path a bond takes shapes the entire reaction that follows.

In homolysis (homo- meaning same, equal), the bonding pair divides evenly, one electron to each fragment, and you draw it with two fishhook arrows. The products are radicals, each carrying a lonely unpaired electron — this is the breaking that starts radical chain reactions, usually triggered by heat or light on a weak, nonpolar bond. In heterolysis (hetero- meaning different), the pair stays whole and follows one atom, drawn with a single full-headed curved arrow. The atom that keeps both electrons becomes negative (an anion); the atom left without becomes positive (a cation, often a carbocation). Heterolysis is the breaking behind ionic, polar mechanisms.

Knowing which kind of cleavage to expect is half of reading a mechanism. Nonpolar bonds split homolytically into radicals when energized; polar bonds, especially those to a good leaving group, split heterolytically into ions. The choice tells you what intermediates to expect, what arrows to draw (fishhooks versus full), and what conditions favor the reaction — and it sits right at the boundary between the radical chemistry and the ionic chemistry that fill the rest of the course.