8ungherEbaReanrenammsucnoesatbobeiongothe 25 Important Concepts 1.Hoterocycloalkanes-named using cycloalkane nes exist in t 2 d 4 hete ned by 3 Aromatic rocycloalkanes1-Hetr24- The hete and ihande 25 Important Concepts natic substitution occurs readily ans2aaeshnsceestiohe 7 7 Quinine, strychnine and many hallucinogens also belong to the quinoline or isoquinoline families: 25 Important Concepts 1. Heterocycloalkanes – named using cycloalkane nomenclature • Nitrogen - aza • Oxygen - oxa • Sulfur - thia • Many common names exist in the literature 2. Strained Cyclic Heterocycloalkanes – strained 3 and 4 heterocycloalkanes are easily opened by nucleophiles. 3. Aromatic Heterocycloalkanes – 1-Hetero-2,4- cyclopentadienes are aromatic (6 π electrons). • The heteroatom is sp2 hybridized and contributes 2 electrons to the π system. The diene unit is electron￾rich and reactive in electrophilic aromatic substitutions. 25 Important Concepts 4. Pyridine – formed by replacing a –CH= unit in benzene with sp2 nitrogen • Azabenzenes are electron-poor • Electrophilic aromatic substitution of azabenzenes is sluggish. • Nucleophilic aromatic substitution occurs readily. • Chichibabin reaction • Organometallic substitutions next to the nitrogen • Displacement of halide ion in halopyridines by nucleophiles 5. Azanaphthalenes (benzopyridines) – Quinoline and isoquinoline contain an electron-poor pyridine ring (subject to nucleophilic attack) and an electron-rich benzene ring (subject to electrophilic aromatic substitution, usually at positions closest to the heterocyclic unit)