9. Benzene and Derivatives

9. Benzene and Derivatives - 2010 Department of Chemistry...

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© 2010 Department of Chemistry, The University of Western Ontario 9.1 9. Benzene and Derivatives (text 9.1 – 9.6 and 9.8) A. Structure Benzene, C 6 H 6 , discovered by Michael Faraday in 1825, has 4 units of unsaturation. It is remarkably stable due to resonance. Due to resonance, all carbon-carbon bonds are equivalent. All carbon and hydrogen atoms are in the same plane due to sp 2 hybridization. The structure of benzene is best described by molecular orbital theory. The remaining p orbitals of the sp 2 -hybridized carbon atoms overlap sideways to form a molecular orbital that has a torus-shaped orbit on each side of the ring.
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9.2 The carbon-carbon “double bonds” of the Lewis structure are so stable that benzene and compounds containing benzene rings don’t undergo the typical alkene reactions. H 2 O H No Reaction Is there a way to estimate how much stability resonance imparts to a benzene ring? Yes, by measuring the heat ( H ) of hydrogenation reactions of benzenes and alkenes. H 2 addition to one C=C in cyclohexene: H = –120 kJ/mol. H 2 Ni H = –120 kJ/mol
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9.3 So, we expect benzene to be 3 × –120 = –360 kJ/mol, but the measured value is H = –208 kJ/mol. (Because of the stability of benzene, this reaction requires extreme conditions for hydrogenation to occur.) 3 H 2 Ni H = –209 kJ/mol i.e. Benzene is 360 – 209 = 151 kJ/mol more stable than a cyclohexane ring containing 3 C=C is expected to be. This increased stability due to resonance is called resonance energy .
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9.4 B. Aromaticity Aromatic is the term used to describe cyclic compounds that contain double bonds but do not undergo the typical reactions of alkenes. Resonance stabilization is a central feature of aromaticity. For a compound to be aromatic, it must: o Have a 2p orbital on each ring atom o Be planar or nearly planar o Have 2, 6, 10, 14, 18, etc. electrons Rings with heteroatoms (atoms other than C or H) can be aromatic. The heteroatoms are sp 2 -hybridized so that a lone pair can be placed in the remaining p orbital, which forms a molecular orbital with the p orbitals of the other atoms. (See “How To 9.1” in the text.) O furan N pyrrole H
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9.5 Heterocyclic aromatic compounds are very common in bio- organic molecules,
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This note was uploaded on 05/01/2011 for the course CHEMISTRY Chemistry taught by Professor Dr.felixlee during the Fall '11 term at UWO.

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9. Benzene and Derivatives - 2010 Department of Chemistry...

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