Chapter 20 Conjugated Systems

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Chapter 20 Conjugated Systems
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Conjugated Dienes Heats of Hydrogentaion

• From heats of hydrogenation, we can compare
  relative stabilities of conjugated and
  unconjugated dienes.

Longer chain has little effect.
Number of substituents.
Steric Effects
Conjugation stabilizes.
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Conjugated Dienes Butadiene

• Conjugation of the double bonds in 1,3-butadiene
  gives an extra stability of approximately 17 kJ
  (4.1 kcal)/mol .

If double bonds independent
Experimental data does not agree Conjugation is
important and stabilizing.
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Conjugated Dienes Butadiene

• Conjugation of double bonds in butadiene gives
  the molecule an additional stability of
  approximately 17 kJ/mol.

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Conjugated Systems

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

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Structure of Butadiene MOs

• Combination of four parallel 2p atomic orbitals
  gives two p-bonding MOs (this screen) and two
  p-antibonding MOs (the next screen).

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Structure of Butadiene MOs

• the two p-antibonding MOs of butadiene (higher in
  energy).

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How do we form the orbitals of the pi system

• First count up how many p orbitals contribute to
  the pi system. We will get the same number of pi
  molecular orbitals.

Three overlapping p orbitals. We will get three
molecular orbitals.
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If atomic orbitals overlap with each other they
are bonding, nonbonding or antibonding
Anti-bonding, destabilizing. Higher Energy
But now a particular, simple case distant atomic
orbitals, on atoms not directly attached to each
other. Their interaction is weak and does not
affect the energy of the system. Non bonding
If atoms are directly attached to each other the
interactions is strongly bonding or antibonding.
Bonding, stabilizing the system. Lower energy.
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Molecular orbitals are combinations of atomic
orbitals. They may be bonding, antibonding or
nonbonding molecular orbitals depending on how
the atomic orbitals in them interact.
Example Allylic radical
Two antibonding interactions.
Only one weak, antibonding (non-bonding)
interaction.
All bonding interactions.
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Allylic Radical Molecular Orbital vs Resonance
Molecular Orbital. We have three pi electrons
(two in the pi bond and the unpaired electron).
Put them into the molecular orbitals.
Note that the odd electron is located on the
terminal carbons.
Resonance Result
Again the odd, unpaired electron is only on the
terminal carbon atoms.
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But how do we construct the molecular orbitals of
the pi system? How do we know what the molecular
orbitals look like?
Key Ideas
For our linear pi systems different molecular
orbitals are formed by introducing additional
antibonding interactions. Lowest energy orbital
has no antibonding, next higher has one, etc.
2 antibonding interactions
1 weak antibonding Interaction, non-bonding
Antibonding interactions are symmetrically
placed.
0 antibonding interactions
This would be wrong.
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Another example hexa-1,3,5-triene
Three pi bonds, six pi electrons. Each atom is
sp2 hybridized.
Have to form bonding and antibonding combinations
of the atomic orbitals to get the pi molecular
orbitals.
Expect six molecular orbitals. molecular
orbitals atomic orbitals
Start with all the orbitals bonding and create
additional orbitals. The number of antibonding
interactions increases as we generate a new
higher energy molecular orbital.
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1,2- and 1,4-Addition

• Addition of one mol of HBr to butadiene at -78C
  gives a mixture of two constitutional isomers.
• We account for these products by the following
  two-step mechanism.

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1,2- and 1,4-Addition

• The key intermediate is a resonance-stabilized
  allylic carbocation.

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1,2- and 1,4-Addition

• Addition of one mole of Br2 to butadiene at -15C
  also gives a mixture of two constitutional
  isomers.
• We account for the formation of these 1,2- and
  1,4-addition products by a similar mechanism.

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Experimental Information

• For addition of HBr at -78C and Br2 at -15C,
  the 1,2-addition products predominate at higher
  temperatures (40 to 60C), the 1,4-addition
  products predominate.
• If the products of the low temperature addition
  are warmed to the higher temperature, the product
  composition becomes identical to the higher
  temperature distribution. The same result can be
  accomplished using a Lewis acid catalyst, such as
  FeBr3 or ZnBr2.
• If either pure 1,2- or pure 1,4- addition product
  is dissolved in an inert solvent at the higher
  temperature and a Lewis acid catalyst added, an
  equilibrium mixture of 1,2- and 1,4-product
  forms. The same equilibrium mixture is obtained
  regardless of which isomer is used as the
  starting material.

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1,2- and 1,4-Addition

• We interpret these results using the concepts of
  kinetic and thermodynamic control of reactions.
• Kinetic control The distribution of products is
  determined by their relative rates of formation.
• In addition of HBr and Br2 to a conjugated diene,
  1,2-addition occurs faster than 1,4-addition.

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1,2- and 1,4-Addition

• Thermodynamic control The distribution of
  products is determined by their relative
  stabilities.
• In addition of HBr and Br2 to a butadiene, the
  1,4-addition product is more stable than the
  1,2-addition product.

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1,2- and 1,4-Addition

• Kinetic vs thermodynamic control. A plot of Gibbs
  free energy versus reaction coordinate for Step 2
  of addition of HBr to butadiene.

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1,2- and 1,4-Addition

• Is it a general rule that where two or more
  products are formed from a common intermediate,
  that the thermodynamically less stable product is
  formed at a greater rate?
• No
• Whether the thermodynamically more or less stable
  product is formed at a greater rate from a common
  intermediate depends very much on the particular
  reaction and reaction conditions.

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UV-Visible Spectroscopy

• Absorption of radiation in these regions give us
  information about conjugation of carbon-carbon
  and carbon-oxygen double bonds and their
  substitution.

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UV-Visible Spectroscopy

• Typically, UV-visible spectra consist of one or a
  small number of broad absorptions.

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UV-Visible Spectroscopy

• Beer-Lambert law The relationship between
  absorbance, concentration, and length of the
  sample cell (cuvette)
• A absorbance (unitless) A measure of the
  extent to which a compound absorbs radiation of a
  particular wavelength.
• e molar absorptivity (M-1cm-1) A
  characteristic property of a compound values
  range from zero to 106 M-1cm-1.
• I length of the sample tube (cm)

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UV-Visible Spectroscopy

• The visible spectrum of b-carotene (the orange
  pigment in carrots) dissolved in hexane shows
  intense absorption maxima at 463 nm and 494 nm,
  both in the blue-green region.

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UV-Visible Spectroscopy

• A p to p transition in excitation of ethylene.

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UV-Visible Spectroscopy

• A p to p transition in excitation of
  1,3-butadiene

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UV-Visible Spectroscopy

• Wavelengths and energies required for p to p
  transitions of ethylene and three conjugated
  polyenes

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UV-Visible Spectroscopy

• Absorption of UV-Vis radiation results in
  promotion of electrons from a lower-energy,
  occupied MO to a higher-energy,unoccupied MO.
• The energy of this radiation is sufficient to
  promote electrons in a pi- bonding (p) MO to a
  pi-antibonding (p) MO.
• Electrons in sigma bonding MOs are lower in
  energy and the UV radiation energy is no longer
  sufficient to promote the electrons to the empty
  anti-bonding MOs.
• Following are three examples of conjugated
  systems.

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UV-Visible Spectroscopy

• UV-Visible spectroscopy of carbonyls.
• Simple aldehydes and ketones show only weak
  absorption in the UV due to an n to p electronic
  transition of the carbonyl group.
• If the carbonyl group is conjugated with one or
  more carbon-carbon double bonds, intense
  absorption occurs due to a p to p transition.

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Diels Alder Reaction/Symmetry Controlled Reactions
Quick Review of formation of chemical bond.
Electron donor
Electron acceptor
Note the overlap of the hybrid (donor) and the s
orbital which allows bond formation.
For this arrangement there is no overlap. No
donation of electrons no bond formation.
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Diels Alder Reaction of butadiene and ethylene to
yield cyclohexene.
We will analyze in terms of the pi electrons of
the two systems interacting. The pi electrons
from the highest occupied pi orbital of one
molecule will donate into an lowest energy pi
empty of the other. Works in both directions A
donates into B, B donates into A.
B HOMO donates into A LUMO
Note the overlap leading to bond formation
LUMO acceptor
LUMO acceptor
A HOMO donates into B LUMO
HOMO donor
HOMO donor
Note the overlap leading to bond formation
B
A
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Try it in another reaction ethylene ethylene
? cyclobutane
LUMO
LUMO
Equal bonding and antibonding interaction, no
overlap, no bond formation, no reaction
HOMO
HOMO