Chapter 22-1

Chapter 22-1 - CHEM 262 Wei You Chapter 22. Reaction of...

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1 CHEM 262 Wei You Chapter 21. Benzene and the Concept of Aromaticity
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2 Structure 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 http://en.wikipedia.org/wiki/Friedrich_August_Kekul%C3%A9_von_Stradonitz Problem : did not account for the unusual chemical reactivity of benzene
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140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 146 pm 134 pm All C—C bond distances = 140 pm 3
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4 sp 2 - sp 2 σ σ sp 2 -1 s 109 pm 120° 120° 120° 139 pm C C C C C C H H H H H H The Resonance Model
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Benzene - Stability 5
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Concept of Aromaticity 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 empty 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 fit the Huckel Rule of having (4 n + 2) pi electrons in the ring 6
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Frost Circles Frost circle: a graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds inscribe a polygon of the same number of sides as the ring in question such that one of the vertices is at the bottom of the ring Molecular Orbitals that are: below the horizontal line through the center of the ring are bonding MOs on the horizontal line are nonbonding MOs above the horizontal line are antibonding MOs 7
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Frost Circles Draw Frost circles for cyclobutadiene and benzene; are they aromatic? 8
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Cyclobutadiene 9
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Antiaromaticity An antiaromatic substance is destabilized by cyclic conjugation. Note the heats of hydrogenation (kJ/mol) below: 120 208 heat of hydrogenation of benzene is 152 kJ/mol less than 3 times the heat of hydrogenation of cyclohexene 97 410 heat of hydrogenation of cyclooctatetraene is more than 4 times the heat of hydrogenation of cyclooctene 10
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- has alternating long (146 pm) and short (133 pm) bonds - not planar - not aromatic – reacts as a regular alkene
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Chapter 22-1 - CHEM 262 Wei You Chapter 22. Reaction of...

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