Week 6 Slides - Organic Chemistry II UC-Berkeley Extension...

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Organic Chemistry II UC-Berkeley Extension x36B Fall 2011 October 17, 2011 Week 6: Benzene and the Concept of Aromaticity: structure of benzene, resonance stabilization, p-orbital overlap, rules of aromaticity, heterocyclic aromatics, aromatic ions, nomenclature, acidity of phenols, Alkyl-aryl ether synthesis, Kolbe carboxylation, quinones, benzylic oxidation and halogenation, benzyl ethers.
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Week 6 Benzene - Kekulé 2 In 1872, August Kekulé proposed the following structure for benzene. This structure, however, did not account for the unusual chemical reactivity of benzene. C H C H C H C H C H C H C C C C C C H H H H H H
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Week 6 Benzene - Resonance 3 We often represent benzene as a hybrid of two equivalent Kekulé structures. Each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between. Benzene as a hybrid of two equivalent contributing structures
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Week 6 Benzene - Resonance Model 4 The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure. The carbon skeleton is a planar regular hexagon. All C-C-C and H-C-C bond angles 120°. sp 2 -sp 2 σ σ 2 -1s 109 pm 120° 120° 120° 139 pm C C C C C C H H H H H H
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Week 6 The Pi System of Benzene 5 (a) The carbon framework with the six 2 p orbitals. (b) Overlap of the parallel 2 p orbitals forms one torus above the plane of the ring and another below it this orbital represents the lowest-energy pi-bonding molecular orbital.
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Week 6 Orbitals of the pi System of Benzene 6
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Week 6 Benzene-Molecular Orbital Model 7 The molecular orbital representation of the pi bonding in benzene.
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Week 6 Benzene - Resonance 8
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Week 6 Benzene- Resonance Energy 9
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Week 6 Concept of Aromaticity 10 The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations. To be aromatic, a compound must Be cyclic . Have one p orbital on each atom of the ring. Be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring. Have a closed loop of (4 n + 2) pi electrons in the cyclic arrangement of p orbitals.
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Week 6 Antiaromatic Hydrocarbons 11
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Week 6 Cyclooctatetraene 12 Cyclooctatetraene, with 8 pi electrons is not aromatic; it shows reactions typical of alkenes. X-ray studies show that the most stable conformation is a nonplanar “tub” conformation. Although overlap of 2
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This note was uploaded on 01/22/2012 for the course CHEM II taught by Professor G during the Fall '11 term at University of California, Berkeley.

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Week 6 Slides - Organic Chemistry II UC-Berkeley Extension...

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