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Ch13B_new - Spring2011 CHEM224 O O NH O m aleimid e O H H O...

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Spring2011 CHEM224 Akira Kawamura Chapter 13B: Conjugated Unsaturated Systems A. Overview Today’s main goals: (1) Structures and spectroscopic properties of conjugated systems, (2) The Diels-Alder reaction Why important? (1) Conjugated unsaturated bonds have unique chemical and spectroscopic properties, (2) The Diels-Alder reaction is a powerful reaction which permits the formation of two C-C bonds simultaneously. Key points: Molecular orbitals of 1,3-butadiene (13.7) – See the problem below. p-Atomic orbitals (p-AOs) calc. butadiene four sp2 carbons = four p-orbitals four MOs π -Molecular orbitals ( π -MOs) HOMO LUMO e n e r g y 0 nodes 1 2 3 UV/vis absorption (13.9) Compounds w/ conjugated double/triple bonds (C=C, C=O, etc.) UV/vis absorption corresponds to the electron transition from HOMO to LUMO: ~200-600 nm Longer the conjugation, narrower the HOMO/LUMO gap, which means the absorption of longer wavelength (lower energy). Electrophilic attack on conjugated dienes: 1,4-addition (13.10) HBr Br + 1,2-adduct Br 1,4-adduct
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