Chapter 10: Radical Reactions

Radical reactions represent a fundamental class of organic transformations distinct from ionic mechanisms, characterized by the involvement of species containing unpaired electrons. This chapter examines the structural features and reactivity patterns of radicals, exploring how electronic effects such as hyperconjugation and resonance stabilization influence their chemical behavior and relative stability. The mechanistic framework of radical chemistry is built upon three sequential stages: initiation, in which radicals are generated typically through homolytic bond cleavage; propagation, where chain reactions proceed through stepwise radical transformations; and termination, the process by which radical chains are extinguished through radical-radical coupling or other quenching mechanisms. Practical applications of these mechanistic principles are illustrated through classic alkane halogenation reactions, particularly chlorination and bromination, which demonstrate how radical selectivity and regioselectivity can be predicted and controlled through understanding radical stability and transition state geometry. The chapter also addresses the role of radical inhibitors in suppressing unwanted radical pathways and contrasts the mechanistic and selectivity differences between radical and cationic or anionic alternatives. Specialized topics include allylic bromination using N-bromosuccinimide, a reaction that exploits the enhanced stability of allylic radicals to achieve selective functionalization at benzylic or allylic positions. Chain-growth radical polymerization is presented as a major industrial application, showing how controlled radical initiation can lead to formation of long polymer chains through successive monomer additions. The chapter provides detailed notation conventions for depicting radical mechanisms, enabling students to clearly represent electron movement and structural changes in systems with odd-electron species. Together, these concepts equip students with essential tools to analyze and predict the outcomes of radical transformations across diverse organic syntheses and materials chemistry applications.