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Conjugated Systems

Conjugated Systems. Chapter 20. 20.1 Conjugated Dienes, Table 20.1. Comparison of heats of hydrogenation and relative stabilities of conjugated and unconjugated dienes. Stability of Conjugated Dienes.

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Conjugated Systems

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  1. Conjugated Systems Chapter 20

  2. 20.1 Conjugated Dienes, Table 20.1 Comparison of heats of hydrogenation and relative stabilities of conjugated and unconjugated dienes

  3. Stability of Conjugated Dienes • conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol.

  4. Conjugated Carbonyls: ,-unsaturated • systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds.

  5. Orbitals of Conjugated Dienes • the pi system of butadiene is derived from the combination of four 2p atomic orbitals; there are two bonding MOs and two antibonding MOs.

  6. Orbitals of Conjugated Dienes bond order three nodes -3 (3 antibonding) two nodes -1 (2 antibonding and 1 bonding) one node 1 (1 antibonding and 2 bonding) no nodes 3 (3 bonding)

  7. 20.2 A.1,2- and 1,4-Addition (HBr) • Addition of 1 mol of HBr to butadiene at -78°C gives a mixture of two constitutional isomers. • we account for these products by the following two-step mechanism.

  8. 1,2- and 1,4-Addition (HBr) • the key intermediate is a resonance-stabilized allylic carbocation.

  9. -15° C B r C H = C H - C H = C H 2 2 2 B r B r B r B r C H - C H - C H = C H C H - C H = C H - C H 2 2 2 2 3,4-Dibromo-1-butene 1,4-Dibromo-2-butene (54%) (46%) (1,2-addition) (1,4-addition) 1,2- and 1,4-Addition (Br2) • Addition of 1 mole of Br2 to butadiene at -15 °C also gives a mixture of two constitutional isomers. • we account for the formation of these 1,2- and 1,4-addition products by a similar mechanism. • Note: at +40°C distribution favors the 1,4-product. + 1,3-Butadiene +

  10. Experimental Information • for addition of HBr at -78°C and Br2 at -15°C, the 1,2-addition products predominate; at higher temperatures (40° to 60°C), the 1,4-addition products predominate. • if the products of the low temperature addition are warmed to the higher temperature, the product composition becomes identical to the higher temperature distribution; the same result can be accomplished using a Lewis acid catalyst, such as FeCl3 or ZnCl2. • if either pure 1,2- or pure 1,4- addition product is dissolved in an inert solvent at the higher temperature and a Lewis acid catalyst added, an equilibrium mixture of 1,2- and 1,4-product forms; the same equilibrium mixture is obtained regardless of which isomer is used as the starting material.

  11. B.1,2- and 1,4-Addition • We interpret these results using the concepts of kinetic and thermodynamic control of reactions. • Kinetic control: the distribution of products is determined by their relative rates of formation • in addition of HBr and Br2 to a conjugated diene, 1,2-addition occurs faster than 1,4-addition.

  12. 1,2- and 1,4-Addition • Thermodynamic control: the distribution of products is determined by their relative stabilities. • in addition of HBr and Br2 to a butadiene, the 1,4-addition product is more stable than the 1,2-addition product.

  13. 1,2- and 1,4-Addition • Figure 20.3: Kinetic vs thermodynamic control

  14. 1,2- and 1,4-Addition • Is it a general rule that where two or more products are formed from a common intermediate, that the thermodynamically less stable product is formed at a greater rate? • No. • whether the thermodynamically more or less stable product is formed at a greater rate from a common intermediate depends very much on the particular reaction and reaction conditions.

  15. 20.3 A.UV-Visible Spectroscopy • Absorption of radiation in these regions give us information about conjugation of carbon-carbon and carbon-oxygen double bonds and their substation.

  16. UV-Visible Spectroscopy, Fig 20.4 • typically, UV-visible spectra consist of one or a small number of broad absorptions

  17. UV-Visible Spectroscopy • Beer-Lambert law: the relationship between absorbance, concentration, and length of the sample cell (cuvette). • A = absorbance (unitless): a measure of the extent to which a compound absorbs radiation of a particular wavelength. • e = molar absorptivity (M-1cm-1): a characteristic property of a compound; values range from zero to 106 M-1cm-1 • l = length of the sample tube (cm).

  18. UV-Visible Spectroscopy • the visible spectrum of b-carotene (the orange pigment in carrots) dissolved in hexane shows intense absorption maxima at 463 nm and 494 nm, both in the blue-green region.

  19. B. UV-Visible Spectroscopy • Absorption of UV-Vis energy results in move-ment of an electron from a lower-energy occupied MO to a higher-energy unoccupied MO. • the energy of the uv-vis is sufficient to promote electrons from a pi (p) bonding MO to a pi antibonding (p*) MO. • but is generally not sufficient to affect electrons in the much lower-energy sigma bonding (s) MOs • following are three examples of conjugated systems.

  20. B. UV-Visible Electronic Transitions ` s* ----------------------- p* ----------------------- n ----------------------- p ----------------------- s -----------------------

  21. UV-Visible Spectroscopy • UV-Visible spectroscopy of carbonyls. • simple aldehydes and ketones show only weak absorption in the UV due to an n to p* electronic transition of the carbonyl group. • if the carbonyl group is conjugated with one or more carbon-carbon double bonds, intense absorption occurs due to a p to p* transition.

  22. UV-Visible Spectroscopy • Figure 20.5: A p to p* transition in excitation of ethylene

  23. UV-Visible Spectroscopy • Figure 20.6: A p to p* transition in excitation of 1,3-butadiene

  24. UV-Visible Spectroscopy, Table 20.3 • Wavelengths and energies required for p to p* transitions of ethylene and three conjugated polyenes.

  25. 24.6 Diels-Alder Reaction • Diels-Alder reaction: a cycloaddition reaction of a conjugated diene and certain types of double and triple bonds • dienophile: diene-loving • Diels-Alder adduct: the product of a Diels-Alder reaction

  26. C O O E t C O O E t C O O E t C O O E t 1,3-butadiene Diethyl (a diene) 2-butynedioate (a dienophile) Diels-Alder Reaction • alkynes also function as dienophiles • cycloaddition reaction:a reaction in which two reactants add together in a single step to form a cyclic product + Diels-Alder adduct

  27. A. Diels-Alder Reaction • the conformation of the diene must be s-cis

  28. 200° C 140° C 30° C 2,3-Dimethyl- 1,3-butadiene B. Diels-Alder Reaction • reaction is facilitated by a combination of electron-withdrawing substituents on one reactant and electron-releasing substituents on the other + pressure 1,3-Butadiene Ethylene Cyclohexene O O + 1,3-Butadiene 3-Buten-2-one O O + 3-Buten-2-one

  29. Diels-Alder Reaction

  30. room temperature Dicyclopentadiene (endo form) C. Diels-Alder Reaction • the Diels-Alder reaction can be used to form bicyclic systems H + 170°C H Diene Dienophile

  31. the double bond derived from relative to the diene the double bond Diels-Alder Reaction • exo and endo are relative to the double bond derived from the diene exo (outside) endo (inside)

  32. Diels-Alder Reaction • for a Diels-Alder reaction under kinetic control, endo orientation of the dienophile is favored

  33. D. Diels-Alder Reaction • the configuration of the dienophile is retained

  34. E. Diels-Alder Reaction • the configuration of the diene is retained

  35. Diels-Alder Reaction • Figure 24.1 Mechanism of the Diels-Alder reaction

  36. F. Diels-Alder Reaction • The 1,3 substitution product is not observed.

  37. F. Diels-Alder Reaction • A 1,3 substitution product is not observed.

  38. Hexatriene Molecular Orbitals • 1,3,5-hexatriene two nodes five nodes one node four nodes no nodes three nodes

  39. Conjugated Systems End Chapter 20

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