1. Chapter 14

2. Conjugated Dienes and Ultraviolet Spectroscopy

3. Conjugated Dienes Multiple Bonds Alternating with Single Bonds
1,3 Butadiene
1,4 Pentadiene
H2C=CH-CH=CH2
H2C=CH-CH2-CH=CH2
CONJUGATED
NOT CONJUGATED !!!

4. Examples of Conjugated Dienes
Lycopene – a conjugated polyene C O H 3
Benzene – a cyclic conjugated molecule
Progesterone – a conjugated enone

5. Preparation and Stability of Conjugated Dienes

6. Based Induced Elimination of HX
Diene Preparation
Based Induced Elimination of HX
Cyclohexene
3-Bromocyclohexene
1,3-Cyclohexadiene

7. Diene Preparation
Thermal cracking of butane using a chromium oxide/aluminum oxide catalyst
CH3CH2CH2CH3
H2C=CHCH=CH2 +2 H2

8. Acid-catalyzed double dehydration

9. Special Properties of Conjugated Dienes
Length of the central single bond is shorter than non-conjugated similar molecule
Comparison of 1,3-Butadiene and Butane
148 pm
153 pm
H2C=CH-CH=CH2
CH3-CH2-CH2-CH3
1,3-Butadiene
Butane
Shorter
Bond

10. Special Properties of Conjugated Dienes
Unusual stability evidenced by heats of hydrogenation
More highly substituted alkenes are more stable than less substituted ones
More highly substituted alkenes release less heat on hydrogenation because they contain less energy to start with

11. Molecular Orbital Description of 1,3 Butadiene

12. Stability of Conjugated Dienes is due to orbital hybridization
Typical C-C single bonds result from sigma overlap of sp3 orbitals on both carbons
CH3-CH2-CH2-CH3
Bonds formed by overlap of sp3 orbitals
Conjugated dienes have a central C-C bond that results from sigma overlap of sp2 orbitals on both carbons
H2C=CH-CH=CH2
Bonds formed by overlap of sp2 orbitals

13. Stability of conjugated dienes is due to orbital hybridization
Since sp2 orbitals have more s character (33%) than sp3 orbitals (25% s), the electrons in sp2 orbitals are closer to the nucleus and the bonds they form are shorter and stronger
The “extra” stability of conjugated dienes result from the greater amount of s character in the bonds forming the C-C bond

14. Stability of conjugated dienes is due to orbital hybridization
Antibonding (3 nodes)
Antibonding (2 nodes)
ENERGY
Bonding (1 node)
Four isolated p orbitals
Bonding (0 nodes)

15. Why is the conjugated bond stronger?
П electrons are “delocalized” over the entire П framework rather than localized between two specific nuclei.
П certain amount of double bond character exists in a conjugated bond over the single bond area.
Compare 1,3-Butadiene with 1,4 Pentadiene
Partial double bond character
1,3-Butadiene
a conjugated diene
1,4-Pentadiene
a non-conjugated diene

16. Electrophilic Additions to Conjugated Dienes: Allylic Carbocations
Electrophilic addition to 1,3-Butadiene yields a mixture of two products:
1,2 addition
1,4 addition

17. Non-conjugated alkene addition reactions
Tertiary Carbocation
2-Methylpropene
2-Chloro-2-methylpropane

18. Conjugated diene 1,2 and 1,4 addition reactions
3-Bromo-1-butene (71%; 1,2 addition)
1-Bromo-2-butene (29%; 1,4 addition)
1,3-Butadiene (a conjugated diene)
1,4-Dibromo-2-butene (45%; 1,4 addition)
3,4-Dibromo-1-butene (55%; 1,2 addition)
1,3-Butadiene

19. 1,4 addition products are due to allylic carbocation intermediates
HBr
Secondary, allylic
Primary, nonallylic (NOT formed)

20. Kinetic versus thermodynamic control of reactions

21. At room temperature, electrophilic addition to a conjugated diene leads to a product mixture where the 1,2 adduct predominates over the 1,4 adduct.
At high temperatures, the product ratio changes and the 1,4 adduct predominates
1,2 adduct
1,4 adduct
At 0oC: 71% %
At 40oC: 15% %

22. Kinetic Versus Thermodynamic Control
Kinetic control dominates reactions where the product of an irreversible reaction is the one that forms fastest
Thermodynamic control dominates reactions where the product of a readily reversible reaction depends on thermodynamic stability

23. A Kinetic Control Reaction
B forms faster because it requires less energy
C is more stable, but requires more energy
The reaction occurs under mild conditions and is irreversible
No equilibrium is reached

24. A Thermodynamic Control Reaction
This reaction is held under higher temperatures and equilibrium is reached
Since C is more stable than B, C is the major product
The product of a readily reversible reaction depends only on thermodynamic control

25. The Diels-Alder Cycloaddition Reaction
Conjugated diene
Dienophile
Diels-Alder reaction:
* Stereospecific
* Prefer Endo product to Exo product

26. Conjugated diene Contain alternating double and single bond:
Adopt S-cis conformation :
More stable than non-conjugated diens.

27. Examples of conjugated diens
1,3-Butadiene
1,3-Pentadiene
1,3-Cyclopentadiene

28. Conjugated diene VS. Non-conjugated diene

29. Non-conjugated diene
Conjugated diene

30. Non-conjugated diene
Conjugated diene

31. Non-conjugated diene
Conjugated diene

32. S-cis conformation of diens
S-trans
S-cis

33. Severe steric strain
in s-cis form
S-trans

34. Dienophile
Has carbon carbon double or triple bond that is next to the positively polarized carbon of a electron-withdrawing substituent group
Reactive and uncreative

35. _ +
Reactive
Propena
(Acrolein)

36. _ +
Reactive
Ethyl propenoate
(Ethyl acrylate)

37. _ +
Reactive + _
Maleic anhydride

38. Reactive
Benzoquinone

39. _ +
Reactive
Propenenitrile
(Acrylonitrile)

40. _ +
Reactive
Methyl propynoate

41. Unreactive

42. Unreactive

43. Unreactive

44. Diels-Alder reaction
Conjugated
diene
Dienophile

45. Stereospecific
The stereochemistry of the starting dienophile is maintained during the reaction, and a single product stereoisomer results.
Example:

48. Endo & Exo product
Endo
Exo

49. Diene Polymers
cis-Polybutadiene In
1,3-Butadiene
trans-Polybutadiene

50. Natural rubber (Z) In
Isoprene
Gutta-percha (E)

51. In
Neoprene (Z)
Chloroprene

52. Ultraviolet Spectrum of 1,3-Butadiene

53. Ultraviolet Excitation of 1,3- Butadiene
When irradiated with UV energy, electrons absorb the energy and are
Promoted from a П bonding molecular orbital to an antibonding П *
Molecular orbital
Ψ4*
(lowest unoccupied molecular orbital)
П *
Ψ3*
LUMO
ENERGY hv
UV irradiation Ψ2
HOMO П
Four p atomic orbitals
(highest occupied molecular orbital) Ψ1
Excited State
Ground State

54. Ultraviolet Spectrum
A UV spectrum is recorded by irradiating a sample with UV light of continously changing wavelength.
When the wavelength corresponds to the energy level required to excite an electron to a higher level, energy is absorbed
This absorption is detected and displayed on a chart that plots wavelength versus absorbance

56. Structure determination in Conjugated systems
Ultraviolet Spectroscopy
Energy
Ultraviolet
Near
infrared
Vacuum
ultraviolet
Visible
X-rays
Infrared
= 200nm
= 400nm

58. Ultraviolet spectrum of Beta-carotene
The absorption occurs in the visible region