Organic Chemistry

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Organic Chemistry

• Chapter 7 - Resonance

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Electron Delocalization and Resonance

• Localized electrons restricted to a
  particular region
• Delocalized electrons do not belong to a single
  atom or exclusively to a bond between 2 atoms

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Kekule Structure

• Benzene C6H6
• Rapid Equilibrium between 2 structures
• Proposed by Fredrich Kekule (1865 German chemist)

Rapid Equilibrium
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Kekules Dream

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Benzene Structure

• Kekule Structures of Benzene were accepted in the
  1930s when X-ray studies showed ALL SIX C-H
  bonds equal and ALL SIX C-C bonds equal!

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Bonding in Benzene

• Each C is sp2 hybridized
• Each C has an unhybridized p orbital
  perpendicular to the plane of the ring
• The 6 p orbitals overlap to form a ? cloud

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Resonance Hybrid

• A compound with delocalized e- is said to have
  resonance
• resonance contributor
• resonance structure
• contributing resonance structure

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Resonance Hybrid

• Benzene
• contributing resonance structures

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Resonance Hybrids

• Drawing resonance hybrids
• 1) Only e- move (not atoms)
• 2) Only ? and non-bonding e- move
• 3) Total e- stays same (as does unpaired e-)

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Resonance Hybrids

• e- can be moved only by
• ? e- move toward or toward ? bond

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Resonance Hybrids

• e- can be moved only by
• Nonbonding pair e- toward a ? bond

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Resonance Hybrids

• e- can be moved only by
• Nonbonding single e- toward a ? bond

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Resonance Hybrids

• Drawing resonance hybrids

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Resonance Hybrids
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Practice Drawing Resonance

• Practice
• Practice
• Practice

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• What makes a Resonance Structure Have Decreased
  Stability?
• 1) an atom with an incomplete octet
• 2) a negative charge that is not on the most
  electronegative atom
• 3) a positive charge not on the most
  electropositive atom
• 4) charge separation

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Examples To Examine
Equal Stability
B is less stable than A
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Resonance Energy

• 1) The greater the predicted stability of a
  resonance contributor, the more it contributes to
  the resonance hybrid.
• 2) The greater the number of relatively stable
  resonance contributors, the greater the resonance
  energy.
• 3) The more nearly equivalent the resonance
  contributors, the greater the resonance energy.

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Resonance Energy

• The more nearly equivalent the resonance
  contributors,
• the greater the resonance energy

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Stability of Dienes
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Stability of butadiene
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Butadiene Example
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Allylic Cations

• Allylic cations have delocalized e-
• They are more stable due to resonance
  contributors

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Benzylic Cations

• Benzylic cations have delocalized e-
• They are more stable due to resonance
  contributors

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Stability of Cations

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Consequences

• Because of the allyl and benzyl cations and
  radicals are more stable, some products can be
  formed easier.

Mechanisms Mechanisms Mechanisms Look at how
conjugation affects reactions
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Effects on pKa

• Carboxylic acids are stronger acids than alcohols
  due to resonance effects

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Phenol

• Why is this alcohol as acidic as it is?
• -OH is on an sp2 hybridized C
• In phenol, some resonance contributors have a
  charge on O
• 3 of phenols structures have separated charges

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Molecular Orbital Theory

• Molecular Orbital Theory can also help explain
  increased stability
• READ THROUGH THIS SECTION OF THE BOOK

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Thermodynamic vs. Kinetic Control

• If the kinetic product and thermodynamic product
  differ, the major product will depend on reaction
  conditions.
• If carried out at mild (low-temp) conditions, the
  reaction will be irreversible and the kinetic
  product will be favored.
• If carried out at sufficiently vigorous (hi-temp)
  conditions, the reaction will be reversible and
  the thermodynamic product will be favored.

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Thermodynamic vs. Kinetic Control

• Kinetic product and thermodynamic product

Formed fastest
Most stable
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Thermodynamic vs. Kinetic Control

• Reaction conditions control products!

Kinetic product low temp
Thermodynamic product high temp
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Diels-Alder Reaction

• This reaction makes new carbon carbon bonds
• Uses a conjugated diene and a dienophile

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Diels-Alder Reaction

• Electron withdrawing groups (O or N) increases
  the reactivity of the dienophile and increase the
  reactivity

acts as an electrophile
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Diels-Alder Reaction

• 1,4 addition to a conjugated diene

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Diels-Alder Stereochemistry

• If a chiral center is created, equal amounts of
  R, S produced (racemic)

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Diels-Alder Stereochemistry

• Stereospecific cis dienophile remains cis
• trans dienophile remains trans

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Diels-Alder Examples

• Dienophiles can also be C triple bond C

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Bicycloalkanes

• Nomenclature
• parent is the alkane of the same number of
  carbons as are in two rings
• number from a bridgehead, along longest bridge
  back to the bridgehead, then along the next
  longest bridge, etc.
• show the lengths of bridges in brackets, from
  longest to shortest

1
2
6
7
3
5
4
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Bicyclics - Nomenclature

• Examples

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Bicyclics - Nomenclature

• Examples