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Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy. Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003, Prentice Hall. Definitions.

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Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

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  1. Organic Chemistry, 5th EditionL. G. Wade, Jr. Chapter 15Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003,Prentice Hall

  2. Definitions • Conjugated double bonds are separated by one single bond. Example: 1,3-pentadiene. • Isolated double bonds are separated by two or more single bonds. 1,4-pentadiene. • Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. => Chaper 15

  3. Resonance Energy • Heat of hydrogenation for trans-1,3-pentadiene is less than expected. • H for 1-pentene is 30.0 kcal/mol and for trans-2-pentene is 27.4 kcal/mol, so expect 57.4 kcal for trans-1,3-pentadiene. • Actual H is 53.7 kcal, so the conjugated diene is more stable. • Difference, (57.4 – 53.7) 3.7 kcal/mol, is the resonance energy. => Chaper 15

  4. Relative Stabilities twice 1-pentene more substituted => Chaper 15

  5. => Structure of 1,3-Butadiene • Most stable conformation is planar. • Single bond is shorter than 1.54 Å. • Electrons are delocalized over molecule. Chaper 15

  6. Constructing Molecular Orbitals • Pi molecular orbitals are the sideways overlap of p orbitals. • p orbitals have 2 lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charge. • When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed. • When + and - lobes overlap, waves cancel out and a node forms; antibonding MO. => Chaper 15

  7. 1 MO for 1,3-Butadiene • Lowest energy. • All bonding interactions. • Electrons are delocalized over four nuclei. => Chaper 15

  8. 2 MO for 1,3-Butadiene • 2 bonding interactions • 1 antibonding interaction • A bonding MO => Chaper 15

  9. 3* MO for 1,3-Butadiene • Antibonding MO • Empty at ground state • Two nodes => Chaper 15

  10. 4* MO for 1,3-Butadiene • All antibonding interactions. • Highest energy. • Vacant at ground state. => Chaper 15

  11. MO Energy Diagram The average energy of electrons is lower in the conjugated compound. => Chaper 15

  12. Conformations of 1,3-Butadiene • s-trans conformer is more stable than the s-cis by 2.3 kcal. • Easily interconvert at room temperature. => Chaper 15

  13. Allylic Cations • Carbon adjacent to C=C is allylic. • Allylic cation is stabilized by resonance. • Stability of 1 allylic  2 carbocation. • Stability of 2 allylic  3 carbocation. => Chaper 15

  14. 1,2- and 1,4-Additionto Conjugated Dienes • Electrophilic addition to the double bond produces the most stable intermediate. • For conjugated dienes, the intermediate is a resonance stabilized allylic cation. • Nucleophile adds to either carbon 2 or 4, both of which have the delocalized positive charge. => Chaper 15

  15. => Addition of HBr Chaper 15

  16. Major product at -80C => Kinetic vs. Thermodynamic Control Major product at 40C Chaper 15

  17. Allylic Radicals • Stabilized by resonance. • Radical stabilities: 1 < 2 < 3 < 1 allylic. • Substitution at the allylic position competes with addition to double bond. • To encourage substitution, use a low concentration of reagent with light, heat, or peroxides to initiate free radical formation. => Chaper 15

  18. + HBr + Br  => Allylic Bromination Chaper 15

  19. Bromination Using NBS • N-Bromosuccinimide (NBS) provides a low, constant concentration of Br2. • NBS reacts with the HBr by-product to produce Br2 and prevent HBr addition. => Chaper 15

  20. MO’s for the Allylic System => Chaper 15

  21. SN2 Reactions of Allylic Halides and Tosylates => Chaper 15

  22. => Diels-Alder Reaction • Otto Diels, Kurt Alder; Nobel prize, 1950 • Produces cyclohexene ring • Diene + alkene or alkyne with electron-withdrawing group (dienophile) Chaper 15

  23. diene dienophile Diels-Alder adduct => Examples of Diels-Alder Reactions Chaper 15

  24. Stereochemical Requirements • Diene must be in s-cis conformation. • Diene’s C1 and C4 p orbitals must overlap with dienophile’s p orbitals to form new sigma bonds. • Both sigma bonds are on same face of the diene: syn stereochemistry. => Chaper 15

  25. Concerted Mechanism => Chaper 15

  26. Endo Rule The p orbitals of the electron-withdrawing groups on the dienophile have a secondary overlap with the p orbitals of C2 and C3 in the diene. => Chaper 15

  27. => Regiospecificity The 6-membered ring product of the Diels-Alder reaction will have electron-donating and electron-withdrawing groups 1,2 or 1,4 but not 1,3. Chaper 15

  28. Symmetry-Allowed Reaction • Diene contributes electrons from its highest energy occupied orbital (HOMO). • Dienophile receives electrons in its lowest energy unoccupied orbital (LUMO). => Chaper 15

  29. “Forbidden” Cycloaddition [2 + 2] cycloaddition of two ethylenes to form cyclobutene has anti-bonding overlap of HOMO and LUMO => Chaper 15

  30. Photochemical Induction Absorption of correct energy photon will promote an electron to an energy level that was previously unoccupied. => Chaper 15

  31. [2 + 2] Cycloaddition Photochemically allowed, but thermally forbidden. => Chaper 15

  32. Ultraviolet Spectroscopy • 200-400 nm photons excite electrons from a  bonding orbital to a * antibonding orbital. • Conjugated dienes have MO’s that are closer in energy. • A compound that has a longer chain of conjugated double bonds absorbs light at a longer wavelength. => Chaper 15

  33.   * for ethylene and butadiene => Chaper 15

  34. Obtaining a UV Spectrum • The spectrometer measures the intensity of a reference beam through solvent only (Ir) and the intensity of a beam through a solution of the sample (Is). • Absorbance is the log of the ratio • Graph is absorbance vs. wavelength. => Chaper 15

  35. The UV Spectrum • Usually shows broad peaks. • Read max from the graph. • Absorbance, A, follows Beer’s Law: A = cl where  is the molar absorptivity, c is the sample concentration in moles per liter, and l is the length of the light path in centimeters. Chaper 15

  36. UV Spectrum of Isoprene => Chaper 15

  37. Sample UV Absorptions => Chaper 15

  38. Woodward-Fieser Rules => Chaper 15

  39. End of Chapter 15 Chaper 15

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