Chapter 19: Alkenes, Alkynes, and Aromatics

The content establishes a foundational framework for organic chemistry by classifying functional groups according to the oxidation level of carbon atoms and organizing reactions into four major categories: addition, elimination, substitution, and rearrangement. Polar reactions, which involve electron-pair transfer from nucleophilic to electrophilic sites, represent the most common mechanistic pathway and are investigated through spectroscopic techniques including infrared and nuclear magnetic resonance analysis. Alkenes, containing carbon-carbon double bonds composed of sigma and pi components, display geometric isomerism due to restricted rotation around the double bond, with the E-Z nomenclature system applied to determine stereochemical configuration. These compounds characteristically undergo electrophilic addition reactions, whereby the electron-rich pi bond attacks electrophilic reagents following Markovnikov's principle, which predicts formation of the most stable carbocation intermediate. Additional alkene transformations include free radical polymerization, syn-selective hydrogenation, anti-selective halogenation, and oxidative cleavage reactions. Alkynes, possessing carbon-carbon triple bonds with one sigma and two pi components, exhibit acidic properties in terminal positions, enabling deprotonation to form acetylide anions that serve as strong carbon nucleophiles for bond formation. Alkyne hydration proceeds through enol intermediates that tautomerize to ketones, while selective reduction using Lindlar catalysts produces cis alkenes. Aromatic compounds, exemplified by benzene, demonstrate exceptional stability through completely delocalized pi-electron systems and prefer substitution over addition reactions to maintain aromaticity. Hückel's rule establishes the criterion for aromaticity as possession of 4n+2 pi electrons in cyclic, planar conjugated systems, extending to heterocyclic and ionic species. Electrophilic aromatic substitution mechanisms govern halogenation, nitration, sulfonation, and Friedel-Crafts transformations. The chapter integrates biological applications through discussion of pheromone synthesis in mountain pine beetle communication and explores carbon allotropes including graphene, graphite, nanotubes, and fullerenes, demonstrating how organic chemistry principles connect to ecosystem health and environmental sustainability.