Conjugated Dienes and

1
Chapter 14
Conjugated Dienes and Ultraviolet Spectroscopy
2
Introduction

• Compounds can have more than one double or triple
  bond.
• Dienes are compounds with two double bonds
• If they are separated by only one single bond,
  they are conjugated and their orbitals
  interact

3

• There are three types of dienes
• Conjugated -CC-CC- alternating double and
  single bonds
• Cumulated -CCC- consecutive double bonds
  (no intervening single bond)
• Isolated -CC-C-CC- double bonds separated
  by more than one single bond (more than one
  intervening single bond)

4

• Conjugated systems have distinctive properties.
• Example The conjugated diene 1,3-butadiene has
  properties that are very different from those of
  the nonconjugated diene, 1,4-pentadiene

5

• The term "conjugated" or "conjugated system"
  typically is applied to extended systems.
• Polyenes - are compounds with many alternating
  single and double bonds
• - are conjugated hydrocarbons with
  many double bonds
• Examples
• a. beta-carotene/ vitamin A
• b. lycopene the red pigment in tomatoes

6

• Conjugated systems are common in nature and in
  biologically important molecules.

enone (Alkene Ketone)

7

• Ultraviolet Spectroscopy (UV) determines if a
  conjugated p electron system is present
• One of the characteristics of conjugated systems
  is the absorbance of u.v. light by the p electron
  system.

Red pigment
8
1. Preparation and Stability of Conjugated
Dienes

• Conjugated dienes are generally prepared by
• base-induced elimination of HX from an allylic
  halide
• industrial catalytic dehydrogenation
• industrial scale dehydration

9

• Base-induced elimination of HX from an allylic
  halide
• Example Allylic bromination of Cyclohexene
  with NBS followed by elimination (tBOC)

10

• Industrial catalytic dehydrogenation
• Example 1,3- Butadiene, a substance used
  industrially to make polymers, is prepared by
  thermal cracking of butane in the presence of a
  catalyst (Chromium oxide/ aluminum oxide)

11

• Industrial scale dehydration
• Example Preparation of Isoprene via
  acid-catalyzed double dehydration of
  3-Methyl-1,3-butanediol

12
Bond Length

• The central single bond in a conjugated diene is
  shorter than the single bond in a nonconjugated
  diene or an alkane.
• Example The C2-C3 single bond in 1,3-butadiene
  is shorter than the C2-C3 bond in butane

13
Stability

• Conjugated dienes are more stable than
  nonconjugated dienes as evidenced by their heats
  of hydrogenation.

14

• More highly substituted alkenes are more stable
  (release less heat of hydrogenation) than less
  substituted ones.

15
Stability 1,3-butadiene vs 1,4-pentadiene

• Hydrogenating 1,3-butadiene releases 16 kJ/mol
  less heat than 1,4-pentadiene

16

• Conjugation of the double bonds in 1,3-butadiene
  gives the extra stability of approximately 16
  kJ/mol

DHo 2(-126 kJ/mol) -252 kJ/mol) DHo
-236 kJ/mol)

• The unusual stability of 1,3-butadiene (and also
  other conjugated systems) is due to resonance
  energy (also called resonance stabilization or
  delocalization energy).

17
Practice Problem Allene, H2CCCH2, has a heat
of hydrogenation of -298 kJ/mol
(-71.3 kcal/mol). Rank a conjugated
diene, a nonconjugated diene, and an
allene in order of stability
18
2. Molecular Orbital Description of
1,3-Butadiene

• The unusual stability of conjugated dienes can
  be explained by
• Valence Bond Theory
• Molecular Orbital Theory

19
Valence Bond Theory

• According to the valence bond theory, the
  stability of conjugated dienes is due to orbital
  hybridization

25 s character 33 s character
Electrons in sp2 orbitals are closer to the
nucleus. Thus sp2-sp2 s bonds are shorter and
stronger.
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Molecular Orbital Theory

• According to the molecular orbital theory, the
  stability of conjugated dienes is due to
  interaction between the p orbitals of the two
  double bonds
• The bonding ?-orbitals are made from 4 p orbitals
  that provide greater delocalization and lower
  energy than in isolated CC
• The single bond between the two double bonds is
  strengthened by overlap of p orbitals

21

• Two p orbitals combine to form two p molecular
  orbitals Bonding and Antibonding
• Both electrons occupy the low energy, bonding
  orbital, forming a stable bond.

higher in energy
lower in energy
22

• In a conjugated diene, four adjacent p orbitals
  combine to form four p molecular orbitals two
  bonding and two antibonding

fully additive
23

• The four p electrons occupy the two bonding
  orbitals

The number of nodes between nuclei increases as
the energy level of the orbital increases
24
p Molecular Orbitals 1,3-butadiene vs
1,4-pentadiene

• In a conjugated diene (1,3-butadiene), the
  lowest-energy p MO (y1) has a favorable bonding
  interaction between C2 and C3 that is absent in a
  nonconjugated diene
• C2-C3 bond has partial double-bond character
• C2-C3 bond is stronger and shorter than a typical
  single bond

25

• In a conjugated diene, the p electrons are
  delocalized or spread out over the entire p
  framework rather than localized between two
  specific nuclei.
• Electron delocalization always leads to greater
  stability.

26

• Systems containing conjugated double bonds, not
  just those of dienes, are more stable than those
  containing nonconjugated double bonds.

2-Cyclohexenone (more stable)
3-Cyclohexenone (less stable)
27
3. Electrophilic Additions to Conjugated
Dienes Allylic Carbocations

• Conjugated dienes undergo electrophilic addition
  reactions via a different mechanism than that
  observed in nonconjugated dienes 1,4-addition

28
General Mechanism of electrophilic addition
reaction

• Attack on electrophile (such as HX) by a ? bond
  of alkene (nucleophile)
• Formation of carbocation and halide ion
• Reaction of nucleophilic halide ion with
  carbocation (an electrophile)

29
Markovnikovs Regiochemistry

• In the addition of HX to alkene
• The H attaches to the carbon with the most Hs
  and X attaches to the carbon with the most alkyl
  substituents
• The more highly substituted (more stable)
  carbocation is formed as the intermediate rather
  than the less highly substituted one

30
Electrophilic additions Alkenes and
Nonconjugated Dienes

• Alkenes and nonconjugated dienes give
  Markovnikovs products

31
Electrophilic additions Conjugated Dienes

• Conjugated dienes give mixtures of products 1,2
  adduct and 1,4 adduct

1,2 adduct 1,4 adduct
Constitutional isomers
32

• Conjugated dienes give mixtures of products 1,2
  adduct and 1,4 adduct

1,4 adduct 1,2 adduct
Constitutional isomers
33
Carbocations from Conjugated Dienes

• Two possible carbocations of electrophilic
  addition to conjugated dienes
• secondary allylic carbocation
• primary nonallylic carbocation (NOT formed)

34

• The key intermediate formed is the delocalized
  secondary allylic carbocation because
• it is more stable, stabilized by resonance
  between two forms
• it forms faster than a nonallylic carbocation

35
Products of Addition to Delocalized Carbocation

• Nucleophile can add to either cationic site
• 1,2 and 1,4 addition products result
• The transition states for the two possible
  products are not equal in energy

36
Practice Problem Give the structures of the
likely products (both 1,2 adducts and
1,4 adducts) for this reaction
37
Practice Problem Give the structures of both
1,2- adducts and 1,4 adducts resulting
from reaction of 1 equivalent of HCl
with 1,3-pentadiene.
38
Practice Problem Look at the possible
carbocation intermediates produced
during addition of HCl to 1,3-
pentadiene, and predict which 1,2 adduct
predominates. Which 1,4 adduct
predominates?
39
Practice Problem Give the structures of both
1,2 and 1,4 adducts resulting from
reaction of 1 equivalent of HBr with
the following substance
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4. Kinetic vs Thermodynamic Control of
Reactions

• Electrophilic addition to a conjugated diene
  leads to a mixture of 1,2 and 1,4 addition
  products in varying amounts depending on the
  reaction conditions
• 1,2 adduct is usually formed faster and is said
  to be the product of kinetic control
• 1,4 adduct is usually more stable and is said to
  be the product of thermodynamic control

41

• Example The addition reaction of HBr to
  1,3-butadiene has unusual temperature dependence
• At low temperatures (0 C), the 1,2-addition
  products predominate
• At higher temperatures (40C), the 1,4-addition
  products predominate

42
A reaction energy diagram for two competing
reactions

• B forms faster than C DGB lt DGC
• C is more stable than B DGoC gt DGoB

43
Kinetic Control

• Under kinetic control, the product of an
  irreversible reaction depends on relative rates
  of formation.
• If a reaction is irreversible or far from
  equilibrium, then the relative concentrations of
  products depend on how fast each forms, which is
  controlled by the relative free energies of the
  transition states leading to each (Kinetic
  Control)

44
Thermodynamic Control

• Under thermodynamic control, the product of a
  readily reversible reaction depends on
  thermodynamic stability
• At completion, all reactions are at equilibrium
  and the relative concentrations are controlled by
  the differences in free energies of reactants and
  products (Thermodynamic Control)

45
Example Electrophilic Addition of HBr to
1,3-butadiene
46

• Under kinetic control (at lower temperatures),
  there is a limited amount of energy available,
  sufficient only to overcome the lowest activation
  energy barrier, which leads to the 1,2-addition
  product.

47

• Under thermodynamic control (higher
  temperatures), there is enough energy to overcome
  the larger activation energy barrier, which leads
  to the 1,4 adduct.

48

• If the temperature is high enough, both reaction
  can reach equilibrium, in which case the more
  stable product (1,4-addition according to
  Zaitsev's rule) will predominate.

49
Practice Problem The 1,2 adduct and the 1,4
adduct formed by reaction of HBr with
1,3-butadiene are in equilibrium at
40oC. Propose a mechanism by which
the interconversion of products takes
place.
50
Practice Problem Why do you suppose 1,4 adducts
of 1,3- butadiene are generally more
stable than 1,2 adducts?
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5. The Diels-Alder Cycloaddition Reaction

• The Diels-Alder cycloaddition reaction is unique
  to conjugated dienes.
• Conjugated dienes can combine with alkenes to
  form six-membered cyclic compounds
• Example

52

• The Diels-Alder reaction
• is a cycloaddition reaction, i.e one in which two
  reactants add together in a single step to form a
  cyclic product
• was discovered by Otto Paul Hermann Diels and
  Kurt Alder in Germany in the 1930s (awarded the
  1950 Nobel Prize)

53

• The Diels-Alder cycloaddition reaction
• forms two C-C bonds in one step.
• is one of only a few ring-forming reactions.
• is said to be "pericyclic," not polar or
  free-radical (Woodward and Hoffman in 1965)
• is a single, one-step process with no
  intermediates (concerted formation of two bonds)

54

• The Diels-Alder cycloaddition
• involves orbital overlap, change of hybridization
  and electron delocalization in transition state

sp2
sp3
sp2
sp2
sp2
sp2
sp2
sp3
Head on (s) overlap of two alkene p orbitals
with two p orbitals on C1 and C4 of the diene
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6. Characteristics of the Diels-Alder
Reaction

• The Diels-Alder cycloaddition
• involves a dienophile and a diene
• is stereospecific and regioselective

a diene a dienophile Diels-Alder
adduct
56

• The Dienophile
• is diene-loving
• has an alkene (CC) or alkyne (C?C) component
  conjugated to an electron-withdrawing group, such
  as CO or C?N
• has a double (CC) or triple (C?C) bond next to
  the positively polarized carbon of an
  electron-withdrawing substituent

57

• The Dienophile
• has an alkene (CC) or alkyne (C?C) component
  conjugated to an electron-withdrawing group, such
  as CO or C?N

58

• The Dienophile
• has a double (CC) or triple (C?C) bond next to
  the positively polarized carbon of an
  electron-withdrawing group

The electron-withdrawing group makes the double
bond carbons less negative
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Stereospecificity of the Diels-Alder Reaction

• The Diels-Alder reaction is stereospecific
• It maintains relative relationships from reactant
  to product
• There is a one-to-one relationship between
  stereoisomeric reactants and products

60

• The Diels-Alder reaction is stereospecific
• The two carbons of the dienophile add to the same
  face of the diene.
• The stereochemistry of the dienophile is
  maintained, and a single product stereoisomer
  results

cis dienophile reactant gives cis-substituted
cyclohexene product
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Cis dienophile reactant gives cis-substituted
cyclohexene product Trans dienophile reactant
gives trans-substituted cyclohexene product
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Regiospecificity of the Diels-Alder Reaction

• The Diels-Alder reaction is regiospecific
• The diene and dienophile reactants align to
  produce endo (rather than exo) product

Endo and exo are relative to the double bond
derived from the diene
63

• Endo and exo indicate relative stereochemistry in
  bicyclic structures
• Substituent on one bridge is
• exo if it is anti (trans) to the larger of the
  other two bridges
• endo if it is syn (cis) to the larger of the
  other two bridges

64

• Endo products are formed because orbital overlap
  increases when the diene and dienophile reactants
  align so that the electron-withdrawing group of
  the dienophile is underneath the diene.

65
Practice Problem Predict the product of the
following Diels- Alder reaction
66
Practice Problem Predict the product of the
following Diels- Alder reaction
67

• The Diene
• must have the s-cis conformation (cis-like
  about the single bond) to undergo the Diels-Alder
  reaction

(higher in energy) (lower in energy)
68

• The Diene
• must have the s-cis conformation because only in
  the s-cis conformation are C1 and C4 of the diene
  close enough to react through a cyclic transition
  state (overlap with dienophile p orbitals)

69

• Dienes that cannot adopt the s-cis conformation
  are unreactive in the Diels-Alder reaction
• Examples

70

• Other dienes that are fixed in the s-cis
  conformation are highly reactive in the
  Diels-Alder reaction
• Example Dimerization of 1,3-cyclopentadiene

71

• The Diels-Alder cycloaddition
• is facilitated by a combination of
  electron-withdrawing substituents on one reactant
  and electron-releasing substituents on the other
• Example

a diene a dienophile Diels-Alder
adduct
72

• The Diels-Alder cycloaddition
• is facilitated by a combination of
  electron-withdrawing substituents on one reactant
  and electron-releasing substituents on the other

73
Practice Problem Which of the following alkenes
would you expect to be good
Diels-Alder dienophiles?
74
Practice Problem Which of the following dienes
have an s-cis conformation, and which
have an s-trans conformation? Of the
s-trans dienes, which can readily
rotate to s-cis?
75
Practice Problem Predict the product of the
following Diels- Alder reaction
76
7. Diene Polymers Natural and Synthetic
Rubbers

• Conjugated dienes can be polymerized

cis
trans
77

• Polymerization is 1,4 addition of growing chain
  to conjugated diene monomer
• The initiator for the reaction can be
• a radical or
• an acid

78
Natural Rubber

• Two naturally occurring polymers of isoprene are
• natural rubber (Z isomer)
• gutta-percha (E isomer)
• The repeating unit has 5 carbons

head-to-tail polymer of isoprene
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Synthetic Rubber

• Synthetic rubbers are produced commercially by
  diene polymerization
• Example Neoprene (a polymer of chloroprene)

Synthetic rubber (weather resistant)
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Vulcanization

• Natural and synthetic rubbers are too soft to be
  used in products unless hardened by vulcanization
  
• Vulcanization
• was discovered by Charles Goodyear
• involves heating the crude polymer with small
  amount of sulfur to produce strong material

81

• Sulfur forms bridges between hydrocarbon chains
  (cross-links), locking the chains

82
Practice Problem Draw a segment of the polymer
that might be prepared from
2-phenyl-1,3-butadiene.
83
Practice Problem Show the mechanism of the
acid-catalyzed polymerization of
1,3-butadiene.
84
8. Structure Determination in Conjugated
Systems Ultraviolet Spectroscopy

• Mass Spectrometry (MS) determines the size and
  formula
• Infrared (IR) Spectroscopy determines the kinds
  of functional groups present
• Nuclear Magnetic Resonance Spectroscopy (NMR)
  determines the carbon-
  hydrogen framework
• Ultraviolet Spectroscopy (UV) determines if a
  conjugated p electron system is present

85

• The ultraviolet (UV) region is higher in photon
  energy than visible light
• The region from 200 to 400 nm (2 x 10-7 m to 4 x
  10-7 m) is most useful in organic chemistry

86

• Conjugated compounds can absorb light in the
  ultraviolet region of the spectrum
• The energy absorbed corresponds to the amount
  necessary to promote an electron from one orbital
  to another
• The electrons in the highest occupied molecular
  orbital (HOMO) undergo a transition to the lowest
  unoccupied molecular orbital (LUMO)

87
Practice Problem Calculate the energy range of
electromagnetic radiation in the UV
region of the spectrum from 200 to 400
nm. Recall the equation
NA hc
1.20 x 10-4 kJ/mol
E NA e
l
l
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Practice Problem How does the energy you
calculated in the previous problem for
UV radiation compare with the values
calculated previously for IR and NMR
spectroscopy?
89
9. Ultraviolet Spectrum of 1,3- Butadiene

• 1,3-butadiene has four ? molecular orbitals with
  the two lower-energy MOs occupied (y1 and y2)

90

• When 1,3-butadiene absorbs UV light, a p electron
  in the highest occupied molecular orbital (HOMO)
  is promoted to the lowest unoccupied molecular
  orbital (LUMO).
• This corresponds to a p ? p excitation
• This transition requires 217 nm UV light (lmax)

91

• A UV spectrum is a plot of absorbance versus
  wavelength.
• A UV spectrum of purified molecule is obtained by
  irradiating a sample with a linearly changing
  wavelength of UV light and measuring the amount
  of light absorbed at each wavelength.

Io
A log
I
92

• The Beer-Lambert Law gives

A e x C x l
where A Absorbance log( of light
transmitted through the sample) e molar
absorptivity (extinction coefficient) in M-1cm-1
C concentration in mol/L l pathlength in
cm

• Absorbance for a particular compound in a
  specific solvent at a specified wavelength is
  directly proportional to its concentration

93
Practice Problem If a pure vitamin A sample has
an absorbance at 325 nm of 0.735 in a
1.00 cm cell and e is known to be
50,100 M-1 cm-1. What is its
concentration?
A e x C x l Rearranging the equation,
A
C
e x l
0.735
C
50,100 M-1 cm-1 x 1.00 cm
C 1.47 x 10-5 M
94
10. Interpreting Ultraviolet Spectra The
Effect of Conjugation

• tlmax is the wavelength where UV absorbance for a
  compound is greatest
• It depends on
• the energy difference between HOMO and LUMO
• the extent of conjugation

95

• llmax increases as conjugation increases (lower
  energy)
• Energy difference between HOMO and LUMO decreases
  as the extent of conjugation increases
• Substituents on ? system increase ?max

96
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97
Practice Problem Which of the following
compounds would you expect to show
ultraviolet absorptions in the 200 to
400 nm range?
98
11. Conjugation, Color, and the Chemistry
of Vision

• The visible region is about 400 to 800 nm,
  adjacent to the UV region.
• Extended systems of conjugation absorb in visible
  region they are colored

99

• Example b-Carotene, 11 double bonds in
  conjugation, ?max 455 nm

100

• Example b-Carotene is yellow-orange it absorbs
  the blue wavelength and transmits the rest

101

• Light-sensitive molecules responsible for vision
  are conjugated systems.

(dietary)
(in liver)
102

• Visual pigments are responsible for absorbing
  light in eye and triggering nerves to send signal
  to brain

trans-rhodopsin
sends signal to brain
found in rod cells (light-sensitive receptor
cells responsible for dim light vision)
103
Chapter 14
The End