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Extended Pi Systems Linear Multiple Conjugated p -bonds 1,3,5-Hexadiene H 2 C=CH—CH=CH—CH=CH 2 PowerPoint Presentation
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Extended Pi Systems Linear Multiple Conjugated p -bonds 1,3,5-Hexadiene H 2 C=CH—CH=CH—CH=CH 2 Thermodynamically stable because of p - p interactions and resonance Kinetically reactive Low Ea for electrophilic additions Carbocation is highly delocalized. b -carotene.

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Presentation Transcript
slide1
Extended Pi Systems
    • Linear Multiple Conjugated p-bonds
      • 1,3,5-Hexadiene H2C=CH—CH=CH—CH=CH2
      • Thermodynamically stable because of p-p interactions and resonance
      • Kinetically reactive
        • Low Ea for electrophilic additions
        • Carbocation is highly delocalized
slide2

b-carotene

      • Highly conjugated molecules are often highly reactive
  • Cyclic Extended p systems are unreactive
    • The simplest cyclic conjugated system is benzene
    • Benzene is very unreactive because it has 2 resonance forms without going to the radical, cation, or anion
    • Inert enough to use as a solvent for other organic reactions
    • Benzene chemistry is the subject of Chapter 15
slide3

20%

  • Diels Alder Cycloaddition Reaction
    • Dienes and Alkenes react to give cyclohexadienes
      • Cycloadditions are the last new major category of reaction we will learn
      • The reaction works best with e- rich dienes and

e- poor alkenes (dienophiles)

      • Electron-Poor Alkenes
        • Substitute the alkene with e- withdrawing (electronegative) groups
        • Induction = removing e- density through s-bonds with electronegative groups (halides, haloalkyl groups)
        • Electron withdrawing groups can also work through resonance

Nitriles

Carbonyls

slide5
Diene’s substituted with electron donating groups (alkyl groups) are

electron rich

    • Sample Reactions
  • The concerted Diels-Alder Reaction Mechanism

1. Concerted mechanisms happen all in one step (like SN2)

slide6
Diels-Alder Mechanism:
    • T.S. stabilized like benzene
    • 3 weak p-bonds broken, 1 weak p-bond and 2 strong s-bonds formed
slide7
Diels-Alder Reactions are Stereospecific
    • Stereochemistry at the dienophile is retained
    • Stereochemistry of diene is retained
  • The Endo Rule
slide8
Exo Addition puts two ester groups near the bridgehead CH2
  • Endo Addition puts two ester groups away from the bridgehead CH2
      • Endo Cycloaddition is preferred in making a bicyclic
      • Attraction of the p-systems of the diene and dienophile explains
      • Diels-Alder Stereochemistry:
slide9
Electrocyclic Reactions = ring from a single p-system
    • Heat or Light can drive the formation of a ring from a p-system
      • Pericyclic Reaction = reaction with a cyclic transition state
        • Diels-Alder Reactions
        • Electrocyclic Reactions
      • Cyclization is preferred for trienes (DH = -14.5 kcal/mol)
      • Ring cleavage is preferred for dienes (DH = -9.7 kcal/mol)
      • Addition of heat (D) forces the reaction to the most stable product (thermodynmic control)
      • Addition of light (hv) forces the reaction to the least stable product (kinetic control)
slide10
Electrocyclic Reactions are Concerted and Stereospecific
    • Concerted Mechanisms
    • Stereospecificity
      • Cyclobutene thermal ring opening is conrotatory
        • sp3—sp3s-bonds rehybridize to sp2 + p orbital for double bond
        • New p-orbitals must rotate to become planar with p-bond
        • Cylcobutene with heat rotates same direction (clockwise)
slide11
Line-structure depictions

b) Butadiene light activated ring closing is disrotatory

trans, trans

cis, trans

slide12
Hexatriene thermal ring closing is disrotatory
  • Cyclohexadiene light activated ring opening is conrotatory
  • Summary:
slide13
Polymerization of Conjugated Dienes
      • 1,2-Polymerization
      • 1,4-Polymerization
      • Products are still unsaturated
        • Cross-linked polymers polymerized the unsaturation
        • Increased Elasticity: cross-links cause polymer to snap back after deformation
slide14
UV-Visible Spectroscopy
    • UV-Visible Light
      • UV l = 200-400 nm
      • Visible UV l = 400-800 nm
      • Transition caused is moving an e- from one MO to higher one
slide15
UV-Vis Spectrometry
    • Sample usually dissolved in solvent having no absorption itself

EtOH, MeOH, cyclohexane

    • Spectrometer Schematic
  • Organic Molecules and UV-Vis Spectroscopy
    • s-bond MO’s are separated by large energy gaps (overlap is very good for bonding MO, very bad for antibonding MO)
    • p-bond MO’s are more closely spaced
    • pp* and np* transitions occur with UV and Visible light energies
slide18
Molar Extinction Coefficient (e = A/C)
    • A = absorbance
    • C = Molar concentration
    • Units of e are L/mol
    • Since concentration is figured in, e is the same for any solution of a particular molecule (can identify an unknown)
  • Wavelength (l) of absorption is indicative of kind of bond absorbing
    • e is taken at lmax (the highest peak)
    • More double bonds lowers the energy of absoption = longer l
    • Fewer double bonds increases the energy = shorter l (Table 14-2)
    • Absorption above 400 nm make a compound visible
      • Dyes often have conjugated p systems
      • b-carotene is bright orange due to conjugation
      • White light contains all wavelengths (colors) of light
      • We see the colors that are not absorbed

R O Y G B I V

Increasing Energy of Colors

b-carotene absorbs here, so we see orange