Chapter 7: Delocalized Chemical Bonding

Resonance contributors represent individual electron configurations, while the resonance hybrid describes the actual electron distribution that none of the individual contributors fully captures alone. The chapter demonstrates how this electron delocalization generates resonance stabilization energy, a measurable thermodynamic advantage that makes delocalized systems significantly more stable than would be predicted by localized bonding models. A major focus examines aromaticity, the special stability arising from cyclic conjugation, which is governed by Hückel's rule—a predictive tool determining whether cyclic polyenes exhibit aromatic character. The molecular orbital perspective underpins the entire discussion, with detailed treatment of pi-systems and linear combinations of atomic orbitals to visualize how electron density distributes in conjugated molecules. The chapter analyzes important reactive intermediates including allylic cations, anions, and radicals, showing how delocalization distributes positive or negative charge across multiple atoms, thereby reducing the concentration of charge at any single site and enhancing stability. Antiaromaticity is introduced as a destabilizing phenomenon in cyclic systems with specific electron counts, contrasting sharply with aromatic stabilization. The treatment integrates structure-property relationships throughout, emphasizing how delocalization directly influences bond character, ionization energy, electron affinity, and chemical reactivity patterns. By connecting resonance concepts to molecular orbital theory and providing systematic analysis of diverse functional groups and intermediates, this chapter establishes the theoretical foundation for predicting reactivity in conjugated and aromatic systems.