310-28 - CH 310 N LECTURE 28 Textbook Assignment: Chap 21...

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1 CH 310 N LECTURE 28 Textbook Assignment: Chap 21 Benzene & Aromaticity Homework (for credit): POW 9 posted (Due Apr 4) Today’s Topics: Benzene & the Concept of Aromaticity Notice & Announcements: XM 2 Papers in Undergrad office 2 Benzene & Benzene & Aromaticity Aromaticity Chapter 21
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3 Benzene Benzene - Kekul Kekul é • The first structure for benzene was proposed by August Kekulé in 1872: • This structure, however, did not account for the unusual chemical reactivity of benzene C C C C C C H H H H H H C C C C C C H H H H H H 4 • Benzene is represented 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 Benzene Benzene - Resonance Model Resonance Model
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5 Benzene Benzene • 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 regular hexagon all C-C-C and H-C-C bond angles 120° sp 2 - 2 σ σ 2 -1 s 109 pm 120° 139 pm C C C C CC H H H H H H 6 The π system of benzene: 1. the carbon framework with the six 2 p orbitals 2. overlap of the parallel 2 p orbitals forms one torus above the plane of the ring and another below it 3. this orbital represents the lowest-lying pi-bonding molecular orbital Benzene Benzene – Molecular Molecular Orbitals Orbitals
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7 y y orbitals in phase: bonding MO orbitals out of phase: antibonding MO Ethylene Ethylene – Molecular Molecular Orbitals Orbitals 8 The molecular orbital representation of the π bonding in benzene: Benzene Benzene – Molecular Molecular Orbitals Orbitals
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9 Resonance energy: Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical
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310-28 - CH 310 N LECTURE 28 Textbook Assignment: Chap 21...

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