Conjugated Molecules Part 1

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Conjugated Molecules - Part 1

• Lecture Supplement Thinkbook page 19

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What is Conjugation?
For any molecule best structure lowest energy

• Structure dominated by minimization of electron
  repulsion
• Familiar examples

Square planar or tetrahedral?
Staggered or eclipsed?
Methane
Ethane

• Other factors may also influence structure
  conjugation, aromaticity

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What is Conjugation?
Case 1 Relative stability of C4H6 isomers

• Review these useful terms...
• Isomers same formula, different structure
• Heat of formation (enthalpy of formation DHof)
  hypothetical enthalpy change when a substance
  is synthesized from elements in their standard
  states
• Example

DHof 29.9 kcal mol-1
DHof 47.7 kcal mol-1

• DHof reveals stability lower DHof more stable
  isomer

• DHof comparisons only valid among isomers

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What is Conjugation?Case 1 Relative stability
of C4H6 isomers

• Why this order?
• Ring strain?

Two pi bonds

• Number of pi bonds?

• Position of pi bonds?
• pi-sigma-pi bond sequence

One pi bond
Two pi bonds
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What is Conjugation?Case 2 Catalytic
Hydrogenation of 1,3-Dienes versus 1,4-Dienes
Catalytic hydrogenation Addition of H2 to a pi
bond with a catalyst
DH -30 kcal mol-1

• Thermodynamics
• Lose H-H sigma bond, C-C pi bond
• Gain 2 x C-H sigma bond
• Bond strength generalization sigma gt pi
• Therefore catalytic hydrogenation is exothermic
  (DH lt 0)

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What is Conjugation?Case 2 Catalytic
hydrogenation of 1,3-dienes versus 1,4-dienes
How can we use catalytic hydrogenation to probe
C4H6 isomer stability?
2 H2
2 H2
DH -65.1 kcal mol-1
DH -56.5 kcal mol-1
Same molecule same H
Fact 1,3-butadiene more stable than
3-butyne Observation DH (1,3-butadiene ? butane)
lt DH (3-butyne ? butane) Conclusion Lowest DH
(for cat H2) belongs to most stable isomer
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What is Conjugation?Case 2 Catalytic
hydrogenation of 1,3-dienes versus 1,4-dienes
The experiment Use DH (cat H2) to compare
pi-sigma-pi (1,3-diene) versus pi-sigma-sigma-pi
(1,4-diene)
1-Pentene (alkene energy benchmark)
DH -30 kcal mol-1
1,4-Pentadiene (a 1,4-diene)
Predict DH 2 x (-30) -60 kcal mol-1 Observe
DH -60 kcal mol-1 Conclusion No special
stability for 1,4-diene
1,3-Butadiene (a 1,3-diene)
Predict DH -60 kcal mol -1 if stability
1,4-diene Observe DH -56.5 kcal
mol-1 Conclusion 1,3-diene more stable than
1,4-diene
General observation 1,3-dienes more stable than
similar 1,4-dienes
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1,3-Butadiene A Closer Look
What is origin of special 1,3-diene stability?
s-cis
s-trans
Major conformation?
Torsional strain
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1,3-Butadiene A Closer Look
What is origin of special 1,3-diene stability?
s-cis
s-trans
Major conformation? Torsional strain
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1,3-Butadiene A Closer Look
What is origin of special 1,3-diene stability?
s-cis
s-trans
95
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Major conformation? Torsional strain
s-trans lt s-cis
Stability s-trans gt s-cis
Planarity?
Nearly always planar
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1,3-Butadiene A Closer Look
Rotation around the C2-C3 bond
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1,3-Butadiene A Closer Look
Rotation around Csp2-Csp2 bond

Highest energy point
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1,3-Butadiene A Closer Look
Rotation around Csp2-Csp2 bond

Highest energy point
Conclusion
More than just torsional strain!
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1,3-Butadiene Resonance Model

• What is the origin of this extra stability,
  planarity, and barrier to rotation?
• Resonance is often a strong influence on
  molecular structure, so start there

• Observations
• Partial C2-C3 pi bond

? explains barrier to rotation

• C2-C3 pz orbital overlap

? explains planarity
Resonance hybrid
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1,3-Butadiene Resonance Model
The resonance model looks useful, but simplistic.
Is it accurate? The resonance model predicts...
C2-C3 bond length (Å)
C2-C3 barrier to rotation (kcal mol-1)
Conclusion
resonance model is accurate despite its
simplicity.
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1,3-Butadiene Resonance Model
How does the resonance explain why a 1,3-diene is
more stable than a 1,4-diene?
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1,3-Butadiene Resonance Model
How does the resonance explain why a 1,3-diene is
more stable than a 1,4-diene?
Has some resonance
No significant resonance
Pi electrons roam over four carbons
Pi electrons confined between two carbons
Pi electrons have longer wavelength
Pi electrons have shorter wavelength (?
wavelength ?energy)
Molecule is more stable
Molecule is less stable
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Conjugated Molecules - Part 2

• Lecture Supplement Thinkbook page 23

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Part 1 Summary

• Adjacent, overlapping p orbitals allows for...
• ...more resonance
• ...more electron delocalization
• ...lower electron energy
• ...greater stability

• Consequences of p orbital overlap
• Atoms with p orbitals must be planar
• Partial pi bond(s)
• Barrier to rotation

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Limited to 1,3-Dienes?

• Is this special stabilization limited to
  1,3-dienes?
• 1,3-diene has four adjacent p orbitals
• Three adjacent p orbitals is enough to provide
  extra stability

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Extra Stability Limited to 1,3-Dienes?Another
special stabilization example an amide
Amide resonance contributors
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Extra Stability Limited to 1,3-Dienes?Another
special stabilization example an amide

• Predict four attachments sp3
• sp3 lacks p orbital needed for resonance
• Therefore sp2 to accommodate resonance
• Therefore sp2 to increase stability

• I thought hybridization is controlled only by the
  number of attachments!
• Energy causes geometry geometry causes
  hybridization

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Extra Stability Limited to 1,3-Dienes?More
adjacent p orbitals larger electron playground
Amide has three adjacent, parallel p orbitals
Build your own model
Compare with 1,3-butadiene four adjacent,
parallel p orbitals
Build your own model
In general adjacent, parallel p orbitals improve
molecular stability
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Conjugation A DefinitionFinally!
Special stability
provided by three or more adjacent, parallel,
overlapping p orbitals.
Conjugation
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Consequences of Conjugation
Conjugation influences widespread Molecular
structure Physical properties Chemical
reactivity etc.
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Consequences of Conjugation
Consequence 1 Molecules prefer to be conjugated

• Influences distribution of products in chemical
  reaction
• Reaction products ? stability ? amount produced

Example Determine major product of this reaction.
or
Ten conjugated p orbitals
Six conjugated p orbitals
Which isomer is more stable?
More stable
Less stable
Produced in greatest amount
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Consequences of Conjugation
Consequence 2 Partial pi bond character

• Resistance to conformational change (barrier to
  rotation)
• Causes planarity of atoms conjugated p orbitals
• Example Amide linkage between two amino acids in
  a protein

Barrier to rotation and planarity critical to
protein function.
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Consequences of Conjugation
Consequence 3 Highly conjugated molecules may
be colored
Examples
Chlorophyll
Lycopene
b-Carotene

• Origin of color
• Some portion of visible (white) light spectrum is
  absorbed
• Brain perceives remaining light as color
• How does molecular structure control energy of
  photons absorbed?

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Consequences of ConjugationHow does molecular
structure control energy of photons absorbed?
Controlled by electron energies

• Molecule absorbs photon (hn)

• Energy of photon absorbed must equal orbital
  energy difference (DE)

• Electron is excited to higher energy molecular
  orbital

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Consequences of ConjugationHow does molecular
structure control energy of photons absorbed?

• ? number of conjugated p orbitals ? DE
• If DE low enough, photons of visible light
  absorbed
• Unabsorbed portion of visible light spectrum
  perceived as color

Photon absorbed
Observed color
Ultraviolet (UV)
Colorless
Yellow Orange Red Violet Indigo Blue Green
Visible light
Infrared (IR)
Colorless
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Consequences of ConjugationHow does molecular
structure control energy of photons absorbed?
Example Molecules
Four conjugated p orbitals DE ultraviolet Color
colorless
Six conjugated p orbitals DE ultraviolet Color
colorless
Eight conjugated p orbitals DE
ultraviolet Color colorless
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Consequences of ConjugationHow does molecular
structure control energy of photons absorbed?
Example Molecules
Twelve conjugated p orbitals DE indigo Color
orange
Ten conjugated p orbitals DE violet Color
yellow
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Consequences of ConjugationHow does molecular
structure control energy of photons absorbed?
Example Molecules
22 conjugated p orbitals DE blue Color red