Ch_322b_14.03

Ch_322b_14.03 - levels an open shell system nonaromatic...

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Huckel's Rule: The (4n + 2) ! -Electron Rule for Predicting Aromaticity : In 1931, the German physicist Erich Huckel carried out molecular orbital calculations on planar carbocycles in which each carbon has a p-orbital. His calculations showed that monocyclic systems containing (4n + 2) ! -electrons , where n = 0, 1, 2, 3. .., have closed shells of ! -electrons like benzene. These systems have a single lowest bonding ! -molecular orbital followed by pairs of bonding ! -MOs. The Huckel Formula Prediction These closed shell ! -electron systems are predicted to be "aromatic" by the Huckel rule. Total Number of ! -Electrons 2 6 10 14 n = 0 1 2 3 bonding MOs
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A Second Prediction of Aromaticity A second way to evaluate aromatic systems is to inscribe a regular polygon inside a circle with one corner of the polygon at the bottom of the circle. Wherever a corner touches the circle, there is an energy level. Examples Benzene hexagon predicted energy levels a closed shell system "aromatic" Cyclooctatetraene octagon 8 ! predicted energy
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Unformatted text preview: levels an open shell system nonaromatic Properties of Cyclooctatetraene In contrast to benzene, this nonaromatic compound is very reactive. It is a yellow liquid, bp 152 o C, that is stable at low temperatures, but polymerizes upon heating. It is slowly oxidized by air, and easily undergoes catalytic hydrogenation. Cyclooctatetraene is not planar, as required for an aromatic compound. It has a tub shape. 1.34 A o 1.48 A o The different bond lengths indicate localized bonds. Quiz 14.03 Use the polygon inside a circle to predict which of the the following species is aromatic or non-aromatic. H + H-H-solution Determine the number of ! electrons in each system. 4 2 4 6 Inscribe the polygon inside a circle and find the levels of the molecular orbitals. Then fill them, starting with the lowest energy MO. Determine if the electrons all lie in MOs that are stabilized. number of electrons 4 2 4 6 non-aromatic aromatic non-aromatic aromatic...
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_14.03 - levels an open shell system nonaromatic...

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