Free Radical Reactions

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Free Radical Reactions

• Bond Dissociation Energies
• Halogenations

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Bond BreakingNon-polar and Polar
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Hydrogen atom abstraction
Radicals tend to react in ways that lead to
pairing of their unpaired electron
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Free Radical Chlorination
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Experimental Evidence Helps to Determine Mechanism

• Chlorination does not occur at room temperature
  in the dark.
• The most effective wavelength of light is blue
  that is strongly absorbed by Cl2 gas.
• The light-initiated reaction has a high quantum
  yield (many molecules of product are formed from
  each photon of light).

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Free Radical Species are Constantly Generated
Throughout the ReactionPropagation
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Termination Reaction of any 2 Radicals
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Enthalpy of Reaction (DHo) Measures Difference in
Strength of Bonds Broken and Bonds FormedBond
Dissociation Energy
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Breaking a Bond Requires Energy

• Homolytic Bond Dissociation Energies
• Atoms have higher energy (are less stable) than
  the molecules they can form
• The formation of covalent bonds is exothermic
• Breaking covalent bonds requires energy (i.e. is
  endothermic)

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Stability of Free Radicals

• Homolytic Bond Dissociation Energies and the
  Relative Stabilities of Radicals.
• The formation of different radicals from the same
  starting compound offers a way to estimate
  relative radical stabilities.

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• Likewise the tert-butyl radical is more stable
  than the isobutyl radical.

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Energy Diagrams
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The relative stabilities of radicals follows the
same trend as for carbocations
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The chain propagation steps have overall DHo
-101 kJ mol-1 and are highly exothermic.
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Polyhalogenation is Possible
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Bromination RDS is Significantly Endothermic
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Iodination is Difficult to Carry Out
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Fluorination is Dangerously Exothermic
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Chlorination is not Very Selective
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Hs are not abstracted at the same rate.

• First Reaction
• Primary 45/6H 7.5
• Secondary 55/2H 27.5
• Secondary Primary H abstraction
• occurs at 3.71 ratio
• Second Reaction
• Primary 63/9H 7.0
• Tertiary 37/1H 37.0
• Tertiary Primary H abstraction
• occurs at 5.31 ratio

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Can Predict Composition of Reaction Mixture
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Problem

• Consider the free radical monochlorination of
  2,2,5-trimethylhexane. Assuming the rate of H
  abstraction for 1o 2o 3o is 1 3.7 5.3,
  draw all of the products and predict the percent
  composition of the product mixture.

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Chlorination vs. BrominationBromination is much
more Selective
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Selectivity of Bromination
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Addition of HBr to Alkenes
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