Chapter 7: Alkyl Halides: Nucleophilic Substitution and Elimination Reactions

The bimolecular nucleophilic substitution mechanism, commonly abbreviated as SN2, proceeds through a single concerted step in which the nucleophile attacks the carbon bearing the halogen while the leaving group departs simultaneously, resulting in inversion of stereochemical configuration at the reaction center. In contrast, the unimolecular nucleophilic substitution mechanism, known as SN1, operates through a two-step process involving initial formation of a carbocation intermediate followed by nucleophile capture, which can lead to racemization of stereochemistry and sometimes rearrangement of the carbon skeleton when carbocation stability permits. The chapter systematically explores the factors that determine which mechanism predominates for a given substrate, including the nucleophilicity and basicity of the attacking species, the quality of the leaving group, the polarity and ionizing ability of the reaction solvent, and the structural features of the alkyl halide that influence carbocation stability. Primary alkyl halides generally undergo SN2 reactions efficiently due to minimal steric hindrance around the reactive center, while tertiary substrates favor SN1 pathways because they form more stable carbocations and present greater steric barriers to bimolecular attack. Secondary alkyl halides often display competing reactivity between both mechanisms. Through reaction coordinate diagrams and detailed mechanistic analysis, the chapter develops predictive frameworks enabling students to anticipate reaction pathways based on structural and environmental variables. This foundational knowledge of substitution mechanisms proves essential for understanding organic synthesis, as these transformations represent among the most frequently encountered bond-breaking and bond-forming processes in preparative chemistry.