1. Organic Chemistry, 5th 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

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 =>
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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 =>
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4. Relative Stabilities
twice 1-pentene
more substituted
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5. Structure of 1,3-Butadiene
Most stable conformation is planar.
Single bond is shorter than 1.54 Å.
Electrons are delocalized over molecule.
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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. =>
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7. 1 MO for 1,3-Butadiene Lowest energy. All bonding interactions.
Electrons are delocalized over four nuclei =>
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8. 2 MO for 1,3-Butadiene 2 bonding interactions
1 antibonding interaction
A bonding MO =>
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9. 3* MO for 1,3-Butadiene Antibonding MO Empty at ground state
Two nodes =>
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10. 4* MO for 1,3-Butadiene All antibonding interactions. Highest energy.
Vacant at ground state =>
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11. MO Energy Diagram The average energy of electrons is lower in the
conjugated
compound.
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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.
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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.
=>
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14. 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 =>
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15. Addition of HBr
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16. Kinetic vs. Thermodynamic Control
Major product at 40C
Major product at -80C
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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 =>
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18. Allylic Bromination
+ HBr
+ Br 
=>
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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.
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20. MO’s for the Allylic System
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21. SN2 Reactions of Allylic Halides and Tosylates
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22. Diels-Alder Reaction Otto Diels, Kurt Alder; Nobel prize, 1950
Produces cyclohexene ring
Diene + alkene or alkyne with electron-withdrawing group (dienophile)
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23. Examples of Diels-Alder Reactions
diene
dienophile
Diels-Alder adduct
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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 =>
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25. Concerted Mechanism
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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.
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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.
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28. Symmetry-Allowed Reaction
Diene contributes electrons from its highest energy occupied orbital (HOMO).
Dienophile receives electrons in its lowest energy unoccupied orbital (LUMO).
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29. “Forbidden” Cycloaddition
[2 + 2] cycloaddition of two ethylenes to form cyclobutene has anti-bonding overlap of HOMO and LUMO
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30. Photochemical Induction
Absorption of correct energy photon will promote an electron to an energy level that was previously unoccupied.
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31. [2 + 2] Cycloaddition
Photochemically allowed, but thermally forbidden.
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32. Ultraviolet Spectroscopy
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 =>
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33.   * for ethylene and butadiene
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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 =>
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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.
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36. UV Spectrum of Isoprene
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37. Sample UV Absorptions
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38. Woodward-Fieser Rules
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39. End of Chapter 15
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