1. Conjugated Systems
CHEM 2425
Chapter 14

2. Isolated and Conjugated Dienes

3. Conjugated systems Benzene
Commonly found in nature as pigments, hormones
Isoprene

4. outline
Structure

5. I. Structure Consider molecules with multiple double bonds…
Example: pentadienes
cumulated (allenes) conjugated isolated
separated by: 0 single bonds single bond or more single bonds
Which is most stable?
Why?
Long answer = molecular orbital theory
Short answer = orbital overlap

6. 1,3-butadiene stability
Unusually strong s bond from greater s-character of C atoms

7. 1,3-butadiene stability Overlap of p atomic orbitals
In its most stable conformation, the p orbitals are parallel
Allows for p electron delocalization
Single bond has partial C=C character, which also contributes to stability

8. Structure, cont. Conformations of 1,3-butadiene:
s-cis (single cis) = double bonds on same side of single bond
s-trans (single trans) = double bonds on opposite sides of single bond
The two conformations easily interconvert at room temperature
Which conformation is more stable?

9. Conformations of conjugated dienes
Mild steric hindrance in s-cis conformation

10. Butadiene Which conformation appears to be more stable? a. s-cis
a. s-cis
b. s-trans
c. neither

11. Estimating the Relative Stability of Resonance Structures
The more covalent bonds a structure has, the more stable it is

12. Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid
this carbon has
6 electrons
this carbon has
8 electrons

13. Charge separation decreases stability

14. Alkadienes and Polyunsaturated Hydrocarbons
Alkadienes (“Dienes”)

15. Alkatrienes (“Trienes”)

16. The Stability of Conjugated
Dienes
Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes

18. outline
Reactions

19. Reactions of Isolated Dienes

20. The Mechanism

21. Double Bonds can have Different Reactivities

22. Electrophilic Attack on Conjugated Dienes: 1,4 Addition

23. Mechanism X
(a)
(b)

24. Reactions of Conjugated Dienes

25. 1,2-Addition and 1,4-Addition

26. Reaction of a Conjugated Diene
Mechanism for the
Reaction of a Conjugated Diene

27. A. Kinetic Control versus Thermodynamic Control of a Chemical Reaction

30. The Diels–Alder Reaction forms a Six-Membered Ring

31. The Mechanism

32. Faster if there is an Electron Withdrawing Group on the Dienophile

33. The Electron Withdrawing Group makes the Electrophile a better Electrophile

34. Another Diels–Alder Reaction

35. Alkynes can also be Dienophiles
The cyclic product has two double bonds.

36. The Stereochemistry of the
Diels–Alder Reaction
The product will be a racemic mixture.

37. How to Determine the Reactants of a Diels–Alder Reaction

38. The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes

39. e.g.

40. A. Factors Favoring the Diels–Alder Reaction
Type A and Type B are normal Diels-Alder reactions

41. Type C and Type D are Inverse Demand Diels-Alder reactions

42. Relative rate

43. Relative rate

44. Steric effects

45. B. Stereochemistry of the Diels–Alder Reaction
The Diels–Alder reaction is stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in the product

47. The diene, of necessity, reacts in the s-cis rather than in the s-trans conformationX

48. e.g.

49. Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction
Relative rate

50. The Diels–Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled
R is exo
longest bridge
R is endo

51. Alder-Endo Rule
If a dienophile contains activating groups with p bonds they will prefer an ENDO orientation in the transition state

52. e.g.

53. Stereospecific reaction

54. Stereospecific reaction

55. Examples

56. Diene A reacts 103 times faster than diene B even though diene B has two electron-donating methyl groups

57. Examples

58. Examples
Rate of Diene C > Diene D (27 times), but Diene D >> Diene E
In Diene C, tBu group  electron donating group  increase rate
In Diene E, 2 tBu group  steric effect, cannot adopt s-cis conformation

59. Ultraviolet and Visible Spectroscopy
UV/Vis spectroscopy provides information
about compounds with
conjugated double bonds.

60. An Electronic Transition
Only organic compounds with π electrons
can produce UV/Vis spectra.
A UV spectrum is obtained when
UV light (180 to 400 nm) is absorbed.
A visible spectrum is obtained when
visible light (400 to 780 nm) is absorbed.

61. A UV Spectrum

62. UV/Vis Absorption Bands are Broad
UV/Vis absorption bands are broad because an electronic state has vibrational sublevels.

63. All these compounds have the same UV spectrum.
Chromophore
All these compounds have the same UV spectrum.
A chromophore is that part of a molecule that is responsible for a UV/Vis spectrum.

64. The Beer–Lambert Law A = ε c l A = absorbance of the sample
c = concentration of substance in solution
l = length of the light path in cm
ε = molar absorptivity of the sample
(characteristic of the compound)

65. Cells Used for Taking UV/Vis Spectra

66. The More Conjugated Double Bonds, the Longer the Wavelength

67. The More Conjugated Double Bonds, the Longer the Wavelength

68. Conjugation Makes the Electronic Transition Easier

69. Colored Compounds Absorb Visible Light (> 400 nm)
β-Carotene is found in carrots, apricots, and flamingo feathers.
Lycopene is found in tomatoes, watermelon, and pink grapefruit.

70. Auxochrome An auxochrome is a substituent that
alters the position and intensity of the absorption.

71. Common Dyes

72. Anthocyanins Responsible for the red, purple, and blue colors
of many flowers and fruits.

73. UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction

74. UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction

75. UV/Vis Spectroscopy Can Be Used to Determine a pKa Value
The phenolate ion absorbs at 287 nm, but phenol does not.

76. UV/Vis Spectroscopy Can Be Used to Determine the Melting Temperature of DNA
The temperature increases with
increasing numbers of G-C base pairs.