Chapter 5: Organic Reactions

The core mechanistic insight centers on electron flow from nucleophiles, which are electron-rich species or those with available lone pairs and negative charges, toward electrophiles, which are electron-deficient sites capable of accepting electron density. The curly arrow notation is introduced as the primary visual language for representing electron pair movement during reactions, with the arrow tail originating from electron sources such as lone pairs, negative charges, or bonding electrons, and the arrow head pointing toward electron-deficient regions. The chapter explores various categories of nucleophiles ranging from neutral molecules containing lone pairs like amines and alcohols to anionic species such as hydroxide and cyanide ions, as well as pi systems in alkenes and aromatic compounds and sigma bonds in reducing agents. Electrophiles are systematically classified by their capacity to accommodate electron density through low-energy empty orbitals, including simple protons, carbonyl groups, and alkyl halides with accessible antibonding orbitals. Critical concepts such as frontier orbital theory, specifically the interaction between highest occupied and lowest unoccupied molecular orbitals, determine both nucleophilic strength and electrophilic susceptibility. The chapter presents systematic rules for mechanism construction including verification of orbital alignment, confirmation of charge balance, and accurate tracking of bond formation and cleavage. Multi-step pathways involving sequential proton transfers and intermediate formation, exemplified through reactions like cyanide addition to aldehydes and acid-catalyzed ether synthesis, demonstrate how complex transformations decompose into elementary electron transfer steps. Students emerge with the conceptual framework that the vast diversity of organic reactions represents variations on fundamental nucleophile-electrophile interaction patterns.