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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

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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Extended Pi Systems Linear Multiple Conjugated p -bonds 1,3,5-Hexadiene H 2 C=CH—CH=CH—CH=CH 2

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  1. 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

  2. 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

  3. 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

  4. 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)

  5. Diels-Alder Mechanism: • T.S. stabilized like benzene • 3 weak p-bonds broken, 1 weak p-bond and 2 strong s-bonds formed

  6. Diels-Alder Reactions are Stereospecific • Stereochemistry at the dienophile is retained • Stereochemistry of diene is retained • The Endo Rule

  7. 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:

  8. 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)

  9. 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)

  10. Line-structure depictions b) Butadiene light activated ring closing is disrotatory trans, trans cis, trans

  11. Hexatriene thermal ring closing is disrotatory • Cyclohexadiene light activated ring opening is conrotatory • Summary:

  12. 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

  13. 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

  14. 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

  15. 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

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