Conjugated Dienes and Ultraviolet Spectroscopy
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Conjugated Dienes and Ultraviolet
Spectroscopy
2
Key Words
• Conjugated Diene
• Resonance Structures
• Dienophiles • Concerted Reaction
• Pericyclic Reaction • Cycloaddition Reaction
• Bridged Bicyclic Compound
• Cyclic Compounds • Endo
• Exo
3
What are Conjugated Dienes?
• Conjugated Dienes are carbon structures which
maintain 2 double bond
separated by a single bond.
• Conjugated Dienes can
be found in many different molecules as
shown.
Examples of Conjugated Dienes
4
Conjugated and Nonconjugated Dienes
• If Di = two and ene = double bond then Diene = two double bonds.
• If double bonds are separated by only ONE single bond, they are conjugated and their orbitals interact.
• The conjugated diene 2,4-heptadiene has properties that are very different from those of the nonconjugated diene, 1,5-heptadiene
Conjugated Diene Non-Conjugated Diene
5
Polyenes
• Compounds with many alternating single and double
bonds.
• Extended conjugation leads to absorption of visible light,
producing color.
• Conjugated hydrocarbons with many double bonds are
polyenes (lycopene is responsible for red color in
tomatoes)
• Extended conjugation in ketones (enones) found in
hormones such as progesterone.
6
Lycopene
O
O
H
H
H
Progesterone
Benzene
Examples of Conjugated Dienes
7
Preparation and Stability of Conjugated Dienes
• Typically by elimination in allylic halide
• Specific industrial processes for large scale production of
commodities by catalytic dehydrogenation and dehydration.
NBS = N-Bromosuccimide (You add a bromine (halogen))
KOC(CH3)3 is a strong base (dehydrohalogenation)
8
Dehydration of Alcohols
Preparation Conjugated Dienes
Removal of hydrogens
9
Stability of Dienes
• Conjugated dienes are more stable than nonconjugated dienes based on heats of hydrogenation.
• Hydrogenating 1,3-butadiene releases 15 kJ/mol less heat than 1,4-pentadiene.
10
Molecular Orbital Description of 1,3-
Butadiene
• The single bond between the conjugated double bonds is shorter and stronger than sp3
11
Molecular Orbital Description of
1,3-Butadiene
• The bonding -orbitals are made from 4 p
orbitals that provide greater delocalization
and lower energy than in isolated C=C
• The 4 molecular orbitals include fewer
total nodes than in the isolated case (See
Figures 14-1 and 14-2)
12
13
14
Molecular Orbital Description of
1,3-Butadiene • In addition, the single bond between the two double
bonds is strengthened by overlap of p orbitals
• In summary, we say electrons in 1,3-butadiene are delocalized over the bond system
– Delocalization leads to stabilization
15
Electrophilic Additions to Conjugated Dienes:
Allylic Carbocations
• Review: addition of electrophile to C=C
– Markovnikov regiochemistry via more stable carbocation
16
Carbocations from Conjugated Dienes
• Addition of H+ leads to delocalized secondary
allylic carbocation
17
18
Products of Addition to Delocalized
Carbocation
• Nucleophile can add to either cationic site
• The transition states for the two possible products are
not equal in energy
19
Practice Problem 14.1: Products?
20
Kinetic vs. Thermodynamic Control
of Reactions
• 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)
• If a reaction is irreversible or if a reaction is far from equilibrium, then the relative concentrations of products depends on how fast each forms, which is controlled by the relative free energies of the transition states leading to each (Kinetic Control)
21
Kinetic and Thermodynamic
Control Example • Addition to a conjugated diene at or below room
temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct
• At higher temperature, product ratio changes and 1,4 adduct predominates (See Figures 14-4 and 14-5)
22
23
24
Conjugated Diene Dienophile
+
Product
What is the Diels-Alder Reaction?
The Diels-Alder reaction uses a conjugated diene and a
dienophile to produce cyclic and bicyclic carbon
structures.
This is also called the [4 + 2] cycloaddition reaction for
the reaction of 4 pi electrons (diene) and 2 pi electron
(dienophile).
25
Properties of Conjugated Dienes
Rotation
s-trans s-cis
• Conjugated Dienes can undergo resonance
which is the movement
of a double bond from
• Conjugated Dienes can
often rotate to either
form the s-cis or s-trans (s = single)
26
What are Dienophiles?
O
O
O
O
O
O
O
O
O
C
C
OO
OO
• Dienophiles are molecules which
maintains a double
bond or triple bond.
• They are normally
bound to electron
withdrawing groups or neutral groups.
27
Diels-Alder Reaction
• The Diels Alder reaction uses the resonance
movement of electrons of
the conjugated diene in
the s-cis configuration
with a dienophile to create a cyclicaddition or
bridge bicyclic structure.
• This reaction works as a concerted reaction or all
in one step similar to an
SN2 reaction.
New Bond
New Bond
+
28
29
Limitations of Diels-Alder
Reaction
• Does not react with s-trans
configuration
• Does not react well with dienophiles
with electron donating groups.
30
Products of Diels-Alder Reactions
• The products of Diels-Alder reaction are cyclic or ring compounds.
• It is also possible to form Bridged Bicyclic Compound by starting with diene found inside ring structures.
+
+
31
Cyclic Product
• The reaction
produces only one
product.
• If the reaction occurs
with a cis dienophile
then the product will
be a cis product.
• If the reaction occurs
with a trans
dienophile then the
product will be a
trans product.
+
H3C
H
CH3
H
+
H
H
CH3
CH3
32
33
Bridged Bicyclic Products
• Often the attachment
to the diene moves
up creating a bridge
while the dienophile
binds beneath it.
• The diene can bind
three ways 1) without
stereoselectivity 2)
endo and 3) exo.
O
O
O
H
H
O
H
HO
O
H
H
H
H
No stereoselectivity
Endo
Exo
34
Endo Product
• This is where the dienophile attaches (down) opposite the bridge or functional groups.
• Of the Diels-Alder reactions with stero selectivity the Endo product is preferred due to decreased steric strain.
O
O
O
H
H
Endo
35
Exo Product
• This is where the dienophile attaches (up) same the bridge or functional groups.
• Of the Diels-Alder reactions with stero selectivity the Exo product is less favorable due to increased steric strain.
O
H
HO
O
Exo
36
Diels-Alder Examples
O
O
O
O
+
EndoMajor Product
CO2Et
CO2Et
CO2Et
CO2Et
+
H
H
+ X NR
Not in the s-cis config
+
The molecules rotates into the s-config
37
Easy Retrosynthesis
• Find the double bond
• Remove the double bond.
• Add double bonds to the adjacent bonds.
• Move 2 bond in both directs, remove these new bonds.
• Add a double bond to the final bond.
O
O
O
O
O
O
O
O
O
+
O
O
O
O
O
O
+
38
Diels Alder Reaction
• Can create carbon carbon single bonds by reacting
conjugated diene and a dienophile to produce cyclic and
bicyclic carbon structures.
• Reacts with electron withdrawing dienophiles or neutral
groups.
• Works with conjugated dienes in the s-cis configuration.
• The Diels-Alder reaction is stereoselective giving cis
and trans configuration to the product.
39
40
Regiochemistry of the Diels-Alder
Reaction
• Reactants align to produce endo (rather than exo) product
– 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 and endo if it is syn (cis) to the larger of the other two bridges
– If the two bridges are equal, the product with the substituent endo to the new double bond is formed.
41
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Conformations of Dienes in the
Diels-Alder Reaction
• The relative positions of the two double bonds in the diene are the “cis” or “trans” two each other about the single bond (being in a plane maximizes overlap)
• These conformations are called s-cis and s-trans (“s” stands for “single bond”)
• Dienes react in the s-cis conformation in the Diels-Alder reaction
43
Practice Problem 14.2:
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Solution:
45
Problem 14.7 (p. 478):
46
Reaction Mechanism:
47
Solution:
48
Unreactive Dienes
49
Reactive Diene: cyclopentadiene
50
Experiment 49:
51
Problem 14.33:
Diels-Alder Products?
52
Problem 14.40:
Diels-Alder Reactants?
53
Problem 14.45:
Structure of Product?
54
First Diels-Alder Reaction:
55
Second Diels-Alder Reaction:
56
Diene Polymers: Natural and
Synthetic Rubber
• Conjugated dienes can be polymerized
• The initiator for the reaction can be a radical, or an acid
• Polymerization: 1,4 addition of growing chain to conjugated diene monomer
57
58
Natural Rubber
• A material from latex, in plant sap
• In rubber, the repeating unit has 5 carbons and Z stereochemistry of all C=C double bonds
• Gutta-Percha is natural material with E in all C=C
• They are head-to-tail polymers of isoprene (2-methyl-1,3-butadiene)
59
60
Vulcanization
• Natural and synthetic rubbers are too soft to be
used in products
• Charles Goodyear discovered heating with small
amount of sulfur produces strong material
• Sulfur forms bridges between hydrocarbon
chains (cross-links)
61
Vulcanization:
62
Synthetic Rubber
• Chemical polymerization of isoprene does not
produce rubber (stereochemistry is not
controlled)
• Synthetic alternatives include neoprene,
polymer of 2-chloro-1,3-butadiene
• This resists weathering and solvents better
than rubber
63
Neoprene:
64
Structure Determination in Conjugated
Systems: UV Spectroscopy
• Conjugated compounds can absorb light in the ultraviolet
region of the spectrum
• The region from 2 x 10-7m to 4 x 10-7m (200 to 400 nm) is
most useful in organic chemistry
65
Structure Determination in Conjugated
Systems: UV Spectroscopy
• The electrons in the highest occupied molecular orbital (HOMO)
undergo a transition to the lowest unoccupied molecular orbital
(LUMO)
66
Structure Determination in Conjugated
Systems: UV Spectroscopy
• A plot of absorbance (log of the ratio of the intensity of light in over light transmitted) against wavelength in this region is an ultraviolet spectrum – see 1,3-butadiene below
67
Ultraviolet Spectrum of 1,3-
Butadiene
• Example: 1,4-butadiene has four molecular orbitals with the lowest two occupied
• Electronic transition is from HOMO to LUMO at 217 nm (peak is broad because of combination with stretching, bending)
68
69
Quantitative Use of UV Spectra
• Absorbance for a particular compound in a specific solvent at a specified wavelength is directly proportional to its concentration
• You can follow changes in concentration with time by recording absorbance at the wavelength (kinetic experiment)
• Beers’ law: absorbance (A) = ecl – “e” is molar absorptivity (extinction coefficient – “c” is concentration in mol/L
– “l” is path of light through sample in cm
70
Interpreting UV Spectra: Effect of
Conjugation
• max: wavelength where UV
absorbance for a compound is
greatest
• Energy difference between HOMO
and LUMO decreases as the extent of
conjugation increases
71
Interpreting UV Spectra: Effect of
Conjugation
• max increases as conjugation
increases (lower energy)
– 1,3-butadiene: 217 nm
– 1,3,5-hexatriene: 258 nm
• Substituents on system increase
max
• See Table 14-2 for examples
72
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Conjugation, Color and the Chemistry of
Vision
• Visible region is about 400 to 800 nm
• Extended systems of conjugation absorb in visible region
• b-Carotene, 11 double bonds in conjugation – max = 455 nm
74
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Conjugation, Color and the Chemistry of
Vision
b-Carotene is converted to Vitamin A, which is converted to 11-cis-retinal:
76
Conjugation, Color and the Chemistry of
Vision
• 11-cis-retinal is converted to rhodopsin in the rod cells of the retina.
• Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain
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