Transcript 14. Conjugated Dienes and Ultraviolet Spectroscopy

Conjugated Dienes and Ultraviolet
Spectroscopy
Key Words
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Conjugated Diene
Resonance Structures
Dienophiles
Concerted Reaction
Pericyclic Reaction
Cycloaddition Reaction
Bridged Bicyclic Compound
Cyclic Compounds
Endo
Exo
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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
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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, 1,5heptadiene
Conjugated Diene
Non-Conjugated Diene
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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.
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Examples of Conjugated Dienes
Lycopene
O
H
H
H
Benzene
O
Progesterone
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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)
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Preparation Conjugated Dienes
Dehydration of Alcohols
Removal of hydrogens
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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.
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Molecular Orbital Description of 1,3Butadiene
• The single bond between the
conjugated double bonds is shorter
and stronger than sp3
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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)
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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
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Electrophilic Additions to Conjugated Dienes:
Allylic Carbocations
• Review: addition of electrophile to C=C
– Markovnikov regiochemistry via more stable carbocation
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Carbocations from Conjugated Dienes
• Addition of H+ leads to delocalized secondary
allylic carbocation
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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
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Practice Problem 14.1: Products?
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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)
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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)
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What is the Diels-Alder Reaction?
+
Conjugated Diene
Dienophile
Product
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).
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Properties of Conjugated Dienes
• 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)
Rotation
s-trans
s-cis
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What are Dienophiles?
O
• Dienophiles are
molecules which
maintains a double
bond or triple bond.
• They are normally
bound to electron
withdrawing groups or
neutral groups.
O
O
O
O
C
O
O
O
C
O
O
O
O
O
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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.
New Bond
+
New Bond
• This reaction works as a
concerted reaction or all
in one step similar to an
SN2 reaction.
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Limitations of Diels-Alder
Reaction
• Does not react with
s-trans
configuration
• Does not react well
with dienophiles
with electron
donating groups.
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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.
+
+
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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.
H
+
H
CH3
CH3
H3C
+
H
H
CH3
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Bridged Bicyclic Products
• Often the attachment
to the diene moves
up creating a bridge
while the dienophile
binds beneath it.
H
H
H
H
No stereoselectivity
H
O
H
• The diene can bind
three ways 1) without
stereoselectivity 2)
endo and 3) exo.
O
Endo
O
O
O
H
O
Exo
H
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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.
H
O
H
O
Endo
O
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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
O
H
O
Exo
H
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Diels-Alder Examples
O
H
+
O
H
O
O
Endo
Major Product
CO2Et
CO2Et
+
CO2Et
CO2Et
X
+
NR
Not in the s-cis config
+
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The molecules rotates into the s-config
Easy Retrosynthesis
O
• 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
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O
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.
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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.
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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
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Practice Problem 14.2:
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Solution:
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Problem 14.7 (p. 478):
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Reaction Mechanism:
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Solution:
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Unreactive Dienes
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Reactive Diene: cyclopentadiene
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Experiment 49:
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Problem 14.33:
Diels-Alder Products?
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Problem 14.40:
Diels-Alder Reactants?
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Problem 14.45:
Structure of Product?
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First Diels-Alder Reaction:
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Second Diels-Alder Reaction:
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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
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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 (2methyl-1,3-butadiene)
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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)
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Vulcanization:
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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
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Neoprene:
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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
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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)
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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
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Ultraviolet Spectrum of 1,3Butadiene
• 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)
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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
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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
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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
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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
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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:
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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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