Chapter 19: Electrophilic Addition to Alkenes

The discussion contrasts the reactivity patterns of different nucleophile types, distinguishing between hard nucleophiles that favor direct carbonyl attack and soft nucleophiles that preferentially add to β-carbons in conjugated systems. Students learn how the electronic character of a nucleophile—whether it is classified as hard or soft based on electronegativity and polarizability—determines the regioselectivity and outcome of addition reactions. The chapter introduces conjugate addition as a powerful synthetic tool where nucleophiles attack the β-position of α,β-unsaturated carbonyl compounds, allowing the developing negative charge to be stabilized through resonance delocalization across the conjugated π-system. Key nucleophile classes including enolate ions, organometallic reagents such as cuprates and Grignard reagents, amines, and thiols are examined in terms of their mechanistic preferences and synthetic utility. The chapter explains how Michael acceptors—compounds featuring extended conjugation such as enones, enoates, and nitroalkenes—serve as electrophilic partners in these transformations. Michael addition reactions, often facilitated by base catalysis, generate new carbon-carbon bonds and produce intermediates suitable for further synthetic elaboration. Multi-step strategies such as Robinson annulation, which combines Michael addition with intramolecular aldol condensation to construct cyclized ketone products, demonstrate how sequential conjugate additions can be integrated into larger synthetic sequences for natural product synthesis and ring-forming reactions. The Stork enamine reaction represents an alternative approach using masked enolate equivalents under mild conditions. Throughout the chapter, kinetic versus thermodynamic control is emphasized as a guiding principle for predicting reaction outcomes based on temperature, reagent choice, and reaction conditions, equipping students with predictive frameworks for designing efficient synthetic transformations.