Beginning with acid-base reactions and continuing though the chapters on nucleophilic substitution, carbonyl addition, acyl transfer, carbanion intermediate, and electrophilic mechanisms, we have stud...Beginning with acid-base reactions and continuing though the chapters on nucleophilic substitution, carbonyl addition, acyl transfer, carbanion intermediate, and electrophilic mechanisms, we have studied reaction mechanisms in which both electrons in a covalent bond or lone pair move in the same direction. We will learn in this chapter how free radicals are often formed from homolytic cleavage, an event where the two electrons in a breaking covalent bond move in opposite directions.