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heterocyclic compounds

heterocyclic compounds - Heterocyclic Chemistry This guide...

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Heterocyclic Chemistry This guide to the lecture material and content of Heterocyclic chemistry has been broken up into several broad categories which have then been arranged by page. This should NOT be used instead of lecture material, or relied solely upon for information, and I take no responsibility for any errors contained herein. 1. Contents and Nomenclature 2. Introduction to Heterocycles Aromatic and non-aromatic heterocycles Hückels rule, and some resonance forms 4. Nucleophilic substitution At the 2 and 3 positions Halogenation Friedel Crafts Acylation Vilsmeir and Mannich Reactions Selective substitution at the 3-position, and group direction 8. Metallation of Heterocycles Metallation with organolithium Reduction of Heterocycles Cycloaddition 9. Synthesis of Heterocycles Paal-Knorr Synthesis Feist Benary Synthesis Knorr Synthesis 11. Pyridine Chemistry Nuclophilic aromatic substitution (Chichibaben) Addition/elimination of halopyridines 3-substituted pyridines Alkyl pyridines Important Nomenclature before beginning N 1 2 3 4 5 6 X 1 2 3 4 5 The left diagram shows the nomenclature that you should properly familiarise yourself with before reading this document. In nitrogen containing rings, any reactions happening on the nitrogen will typically be called N-substituted.
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Heterocyclic compounds There are two classes of heterocyclic rings in chemistry. 1. Aromatic Rings X X 5-membered 6-membered H N O S pyrrole furan thiophene N N N pyridine pipyridine 2. Saturated Rings X X 5-membered 6-membered H N O S H N N H H N pyrrolidine tetrahydrof uran tetrahydrothiophene piperidine piperazine Aromatic rings in heterogenous compounds have to obey Hückels rules in the same way that homogenous aromatic compounds need to. It is also because of this reason that 5-membered rings such as pyrrole can be aromatic, despite having only 5 atoms contributing to the p-orbital. This is because the nitrogen atom donates its lone pair to the delocalised system. This additional electron causes it to become aromatic and obey Hückels rule of (4n+2) aromaticity.
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It is energetically favourable to hybridise one of the p-orbitals containing the lone pair, since it means it may interact with the conjugated ring system to make it aromatic, giving it energy of stabilization. This aromatic stability gives a lower energy conformation, which is favourable.
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