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Conjugated Dienes and U.V. Spectroscopy
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Some Dienes
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Some Conjugated Molecules
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Prevents DNA from Unraveling
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and for Ethylene
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Orbital Depiction of Ethylene
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Orbital Depiction
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H hydrogenation
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Carbocation is Resonance Stabilized
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Carbocation Stability
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Conjugate Addition of HBr
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1,2- vs. 1,4-Addition
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Kinetic and Thermodynamic Products
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Kinetic vs. Thermodynamic Control
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2 Different 1,4- Products Form
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Thermodynamic Products
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Addition to 1,3,5-Hexatriene
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At 0 o C 1,2-addition is favored
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At 40 o C, 1,4-addition is preferred
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Form more stable carbocation
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Allylic Bromination With NBS
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Free Radical Mechanism
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Mechanism involves the gradual generation of Br 2
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Resonance stabilization leads to possibly several products
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Diels-Alder Reaction a Pericyclic Reaction
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Dienophiles Must be Electron Deficient
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Stereochemistry
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Bicyclic Adducts endo and exo
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Endo Addition
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Endo Rule
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Identify the Diene and Dienophile that gives the Adducts
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Two Phosphatase Inhibitors
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A Radiosensitizer
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Synthesis of LS-5
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1,3-Butadiene and Ethylene Molecular Orbitals
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Proper Orbital Overlap
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Overlap Must be Constructive
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Unsymmetrical Diels-Alder Reactions
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Lewis Acid catalysis in a Diels Alder Reaction
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2 o Intermediate is Preferred
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Stereochemistry in endo Addition
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Electrocyclic Reaction in Anti-Tumor Agent
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Diradical Removes H atoms from Deoxyribose in Cancer Cell
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[2+2] Cycloaddion is Forbidden
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Photochemical [2+2] Cycloaddition Need to Irradiate
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Suprafacial – Allowed
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Antarafacial - Forbidden
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[4+2] is Suprafacial, hence Allowed
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Cycloaddition Reactions can be Planned Based on the Number of Pairs of e - ’s that Move
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Formation of Cyclobutanes
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A [4+6] Cycloaddition (odd # pairs of e - ’s move) Thermally Allowed
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Retro Diels-Alder, followed by Diels-Alder
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Intramolecular photochemical [2+2] cycloaddion
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Ultraviolet Spectroscopy The absorption of a 171-nm photon excites an electron from the p bonding MO of ethylene to the p* antibonding MO.
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Conjugated Dienes Absorb Energy in UV Region
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UV Spectrum of Isoprene max = 222 nm
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Conjugation lowers the energy differences between the HOMO and LUMO energy levels, and so conjugated dienes absorb at longer wavelengths than isolated dienes, and trienes absorb at longer wavelengths than dienes, etc. Alkyl group substitution on double bonds also causes absorption to occur at longer wavelengths.
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HOMO-LUMO Gap lessens with increased conjugation
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Woodward-Fieser Rules for Predicting max Double bond extending conjugation 30 nm Exocyclic double bond5 nm Alkyl group5 nm Cl, Br5 nm OH, OR6 nm SH, SR30 nm NH 2, NHR, NR 2 60 nm
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Predict max in U.V. Spectrum
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Determination of max
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