Conjugated Dienes and Ultraviolet Spectroscopy

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Conjugated Dienes and Ultraviolet Spectroscopy
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Key Words

• 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, 1,5-heptadiene

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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
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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 1,3-Butadiene

• 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 CC
• 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 CC
• 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?
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)

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What are Dienophiles?

• Dienophiles are molecules which maintains a
  double bond or triple bond.
• They are normally bound to electron withdrawing
  groups or neutral groups.

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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.
• 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.

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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.

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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.

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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.

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Diels-Alder Examples
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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.

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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 14.40Diels-Alder Reactants?
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Problem 14.45Structure 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 CC double bonds
• Gutta-Percha is natural material with E in all
  CC
• They are head-to-tail polymers of isoprene
  (2-methyl-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 1,3-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)

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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