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Chapter28 - Chapter 28 Biomolecules Heterocycles and...

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Chapter 28 Biomolecules: Heterocycles and Nucleic Acids Based on McMurry’s Organic Chemistry, 6th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 2 Heterocycles Cyclic organic compounds are carbocycles or heterocycles Carbocycle rings contain only carbon atoms Heterocycle rings atoms in addition to carbon (N,S,O are common) Heterocycles include many important natural materials as well as pharmaceuticals
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 3 28.1 Five-Membered Unsaturated Heterocycles Pyrrole, furan, and thiophene are common five- membered unsaturated heterocycles Each has two double bonds and N, O, or S
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 4 Pyrrole Commercially from coal tar or by treatment of furan with ammonia over an alumina catalyst at 400°C.
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 5 Furan Made commercially by extrusion of CO from furfural, which is produced from sugars
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 6 Thiophene From coal tar or by cyclization of butane or butadiene with sulfur at 600°C
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 7 Unusual Reactivity Pyrrole is an amine but it is not basic Pyrrole, furan, and thiophene are conjugated dienes but they undergo electrophilic substitution (rather than addition)
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 8 28.2 Structures of Pyrrole, Furan, and Thiophene Pyrrole, furan, and thiophene are aromatic (Six π electrons in a cyclic conjugated system of overlapping p orbitals) In pyrrole π electrons come from C atoms and lone pair on sp 2 -N
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 9 Why Pyrrole is Not a Base The nitrogen lone pair is a part of the aromatic sextet, protonation on nitrogen destroys the aromaticity, giving its conjugate acid a very low pK a (0.4) The carbon atoms of pyrrole are more electron-rich and more nucleophilic than typical double-bond carbons (see comparison with cyclopentadiene)
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 10 28.3 Electrophilic Substitution Reactions of Pyrrole, Furan, and Thiophene The heterocycles are more reactive toward electrophiles than benzene
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 11 Position of Substitution Electrophilic substitution normally occurs at C2, the position next to the heteroatom, giving more stable intermediate
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 12 28.4 Pyridine, a Six-Membered Heterocycle Nitrogen-containing heterocyclic analog of benzene Lone pair of electrons on N not part occupies an sp2 orbital in the plane of the ring and is not involved in bonding (Figure 28.3).
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Based on McMurry, Organic Chemistry, Chapter 28, 6th e 13 Electronic structure of pyridine Pyridine is a stronger base than pyrrole but a weaker base than alkylamines The sp 2 -hybridized N holds the lone-pair electrons more tightly than the sp 3 -hybridized nitrogen in an alkylamine
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