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Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Chaper 15. 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. Example: 1,4-pentadiene.Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene.

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Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

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    1. Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

    2. Chaper 15 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. Example: 1,4-pentadiene. Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. =>

    3. Chaper 15 3 Stabilities of Dienes Heat of hydrogenation for 1,4-pentadiene is –252 kJ/mol, about twice that of 1-pentene. ?H for 1-pentene is -126 kJ/mol and for trans-2-pentene is -116 kJ/mol, so expect -242 kJ/mol for trans-1,3-pentadiene. Actual ?H is -225 kJ/mol, so the conjugated diene is more stable. Difference, -242 kJ (-225 kJ) = 17 kJ/mol, is the resonance energy. =>

    4. Chaper 15 4 Relative Stabilities

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

    6. 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. =>

    7. Chaper 15 7 Ethylene Pi MO’s The combination of two p orbitals must give two molecular orbitals. Constructive overlap is a bonding MO. Destructive overlap is an anti-bonding MO. =>

    8. Chaper 15 8 ?1 MO for 1,3-Butadiene Lowest energy. All bonding interactions. Electrons are delocalized over four nuclei. =>

    9. Chaper 15 9 ?2 MO for 1,3-Butadiene 2 bonding interactions. 1 antibonding interaction. A bonding MO. =>

    10. Chaper 15 10 ?3* MO for 1,3-Butadiene Antibonding MO. Empty at ground state. Two nodes. =>

    11. Chaper 15 11 ?4* MO for 1,3-Butadiene All antibonding interactions. Highest energy. Vacant at ground state. =>

    12. Chaper 15 12 MO Energy Diagram

    13. Chaper 15 13 Conformations of 1,3-Butadiene s-trans conformer is more stable than the s-cis by 12 kJ/mol (2.8 kcal/mol). Easily interconvert at room temperature.

    14. Chaper 15 14 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.

    15. Chaper 15 15 1,2- and 1,4-Addition to 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. =>

    16. Chaper 15 16 Addition of HBr

    17. Chaper 15 17 Kinetic vs. Thermodynamic Control

    18. Chaper 15 18 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. =>

    19. Chaper 15 19 Allylic Bromination

    20. Chaper 15 20 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.

    21. Chaper 15 21 MO’s for the Allylic System

    22. Chaper 15 22 SN2 Reactions of Allylic Halides and Tosylates

    23. Chaper 15 23 Diels-Alder Reaction Otto Diels, Kurt Alder; Nobel prize, 1950 Produces cyclohexene ring Diene + alkene or alkyne with electron-withdrawing group (dienophile)

    24. Chaper 15 24 Examples of Diels-Alder Reactions

    25. Chaper 15 25 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. =>

    26. Chaper 15 26 Concerted Mechanism

    27. Chaper 15 27 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.

    28. Chaper 15 28 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.

    29. Chaper 15 29 Pericyclic Reactions Diels-Alder reaction is example. Woodward and Hoffmann predicted reaction products using their theory of conservation of orbital symmetry. MO’s must overlap constructively to stabilize the transition state. =>

    30. Chaper 15 30 Symmetry-Allowed Reaction Diene contributes electrons from its highest energy occupied orbital (HOMO). Dienophile receives electrons in its lowest energy unoccupied orbital (LUMO).

    31. Chaper 15 31 “Forbidden” Cycloaddition [2 + 2] cycloaddition of two ethylenes to form cyclobutene has anti-bonding overlap of HOMO and LUMO.

    32. Chaper 15 32 Photochemical Induction Absorption of correct energy photon will promote an electron to an energy level that was previously unoccupied.

    33. Chaper 15 33 [2 + 2] Cycloaddition Photochemically allowed, but thermally forbidden.

    34. Chaper 15 34 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. =>

    35. Chaper 15 35 ? ? ?* for ethylene and butadiene

    36. Chaper 15 36 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

    37. Chaper 15 37 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.

    38. Chaper 15 38 UV Spectrum of Isoprene

    39. Chaper 15 39 Sample UV Absorptions

    40. Chaper 15 40 Woodward-Fieser Rules

    41. Chaper 15 41 End of Chapter 15

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