2 - Delocalization and Conjugation in Polyalkenes...

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Delocalization and Conjugation in Polyalkenes Conjugation: polyalkenes in which two double bonds are separated by just one single bond are “conjugated” polyalkenes. • conjugated double bonds have different properties than isolated double bonds. • electrons can “delocalize” in conjugated π -systems. 1,3,5-hexatriene β -carotene • the p-orbitals of the π -bonds must be coplanar for the alkenes to be conjugated: • delocalization in conjugated π -systems lowers the energy difference between the HOMO and LUMO.
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Delocalization and Conjugation in Polyalkenes Delocalization in conjugated π -systems lowers the energy difference between the HOMO and LUMO. – highly delocalized π -systems can absorb lower energy (higher wavelength) light alkene ! max (nm) 165 217 268 364
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Benzene is Special Benzene (C 6 H 6 ) was discovered in 1825 • all of the C-C bond lengths are identical (1.40 Å) • the heat of hydrogenation is substantially lower than that expected from a triene • the structure was Frst proposed by Kekule in 1865 benzene is an “aromatic” compound
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Hückel’s Rule Hückel’s Rule: planar, fully conjugated, monocyclic systems with (4n+2) π electrons have a closed shell of electrons all in bonding orbitals and are exceptionally stable. These systems are “ aromatic .” other examples of aromatic compounds: analogous systems with 4n π electrons are “ anti-aromatic cyclopentadienyl anion cycloheptatrienyl cation
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Drawing Resonance Structures for a discussion of resonance, see RC Ch 6 sec. 5 Resonance structures show the delocalization of electrons • bonds, unshared electron pairs, carbocations, or single electrons can be moved using the curved arrow notation without moving any atoms • arrows should originate at a source of electron density (i.e. a bond or a lone pair) examples:
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This note was uploaded on 11/10/2011 for the course CH 1b taught by Professor Natelewis during the Winter '09 term at Caltech.

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2 - Delocalization and Conjugation in Polyalkenes...

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