lecture 21.1

lecture 21.1 - 21-21-1Organic Organic...

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Unformatted text preview: 21-21-1Organic Organic ChemistryChemistryWilliam H. BrownWilliam H. BrownChristopher S. FooteChristopher S. FooteBrent L. IversonBrent L. Iverson21-21-2Benzene andBenzene andand the Concept ofand the Concept ofAromaticityAromaticityChapter 21Chapter 2121-21-3Benzene - Kekul Benzene - Kekul The first structure for benzene was proposed by August Kekul in 1872This structure, however, did not account for the unusual chemical reactivity of benzeneCHCHCHCHCHCHCCCCCCHHHHHH21-21-4Benzene - Resonance ModelBenzene - Resonance ModelThe concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzenes structurethe carbon skeleton is a regular hexagonall C-C-C and H-C-C bond angles 120sp2-sp22-1 109 pm120120120139 pmCCCCCCHHHHHH21-21-5Benzene - The Resonance ModelBenzene - The Resonance ModelThe pi system of benzene(a) the carbon framework with the six 2porbitals(b) overlap of the parallel 2porbitals forms one torus above the plane of the ring and another below itthis orbital represents the lowest-lying pi-bonding molecular orbital21-21-6Benzene - Resonance Benzene - Resonance We often represent benzene as a hybrid of two equivalent Kekul structureseach makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in betweenBenzene as a hybrid of two equivalentcontributing structures21-...
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lecture 21.1 - 21-21-1Organic Organic...

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