Chapter 3: Determining Organic Structures

The distinction between Brønsted-Lowry and Lewis frameworks serves as the foundation, with the former emphasizing proton transfer mechanisms and the latter expanding the definition to encompass electron pair interactions more broadly. Understanding acid and base strength relies critically on the pKa scale, where lower values indicate stronger acids and higher values indicate stronger bases, allowing chemists to predict which direction an equilibrium will favor. The chapter systematically develops reasoning strategies for assessing acidity by examining how conjugate base stability determines acid strength. Resonance effects prove particularly important, as delocalization of negative charge across multiple atoms significantly stabilizes the conjugate base and thus increases parent acid strength. Electronegativity of atoms bearing charge, orbital hybridization differences between sp, sp², and sp³ carbons, and inductive effects from nearby electron-withdrawing or electron-donating groups all modulate acidity in predictable ways. The text explores these principles through diverse functional groups including phenols, amides, nitriles, alcohols, carboxylic acids, and sulfonic acids, demonstrating how structural features translate to measurable pKa values. Lewis acid-base theory expands beyond proton transfer to include electron pair acceptance by species lacking hydrogen, such as boron trifluoride or aluminum chloride, broadening applicability across reaction types. The chapter emphasizes that acid-base interactions frequently initiate or terminate organic reaction mechanisms, making proton transfer logic central to mechanistic reasoning throughout subsequent chapters. Context sensitivity of pKa values in aqueous versus non-aqueous solvents receives attention, establishing that while environment matters, pKa remains a reliable guide for predicting reactivity. Ultimately, the chapter cultivates systematic thinking about which atoms become protonated or deprotonated under given conditions and which equilibrium states are thermodynamically favored, reasoning that underpins all mechanistic organic chemistry.