Chapter 17: Elimination Reactions

Elimination reactions represent a fundamental class of organic transformations in which two substituents are removed from a molecule to form a new double or triple bond. This chapter examines the mechanisms, regioselectivity, and stereochemical outcomes that define elimination processes, distinguishing them from substitution reactions and explaining when each pathway predominates. The two primary mechanisms—E1 and E2—proceed through distinct intermediates and rate laws, with E2 reactions occurring in a single concerted step where a base abstracts a proton while the leaving group departs simultaneously, typically favoring anti-periplanar geometry for optimal orbital overlap. E1 mechanisms, by contrast, involve carbocation intermediates formed after initial bond breaking, followed by deprotonation to yield the alkene product. The chapter explores how reaction conditions such as temperature, solvent polarity, base strength, and substrate structure determine which mechanism operates. Zaitsev's rule predicts that elimination generally produces the more substituted alkene as the major product, though this regioselectivity can be overridden by using bulky, non-nucleophilic bases like potassium tert-butoxide in E2 reactions, leading to Hofmann elimination and formation of the less substituted alkene. Stereochemical considerations play a critical role, as E2 eliminations require specific geometric relationships between the departing hydrogen and leaving group, with anti-periplanar arrangements proving most favorable due to transition state orbital alignment. The chapter further distinguishes between elimination and substitution by analyzing competing pathways under various conditions, demonstrating how substrate structure, nucleophile basicity, and temperature influence product distribution. Practical applications including dehydration of alcohols, dehydrohalogenation of alkyl halides, and elimination from quaternary ammonium salts illustrate the synthetic utility of these reactions. By understanding elimination mechanisms and their governing principles, students develop the ability to predict reaction outcomes, design selective syntheses, and recognize elimination as a versatile strategy for constructing carbon-carbon unsaturation in organic synthesis.