Chapter 4 Free Radical Reactions and Free Energies

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Chapter 4Free Radical Reactions and (Free)
Energies

• Important Topics Free radical chain
  reactions Reaction Mechanism Energy diagrams
  Calculation of enthaply (energy) of reactions
  Kinetics and Thermodynamics transition state,
  intermediates, rate limiting steps determination
  of chemical selectivity

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Chlorination of Methane (Free radical chain
reactions)

• Requires heat or light for initiation.
• A mixture of products is formed.
• All free radical chain reactions involve the
  following 3 steps
• Initiation generates a reactive intermediate.
• Propagation the intermediate reacts with a
  stable molecule to produce another reactive
  intermediate (and a product molecule).
• Termination side reactions that destroy the
  reactive intermediate.

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1. Initiation Step

• A chlorine molecule splits homolytically into
  chlorine atoms (free radicals).

2. Propagation Steps
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3. Termination Steps
And more
Overall Reaction
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Bond-Dissociation Energy (BDE), ?H(A-B)
?H(A-B)

• Table 4.2 gives BDE for homolytic cleavage of
  bonds in a gaseous molecule.

We can use BDE to estimate ?Hrxn for a reaction.
For the reaction we just discussed (you should
use the lecture notes from the class where
I discuss this in a more clear way than this
lecture slides from the text
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Reaction-Energy Diagrams

• For a one-step reactionreactants ? transition
  state ? products

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Energy Diagram for a Two-Step Reaction

• Reactants ? transition state ? intermediate
• Intermediate ? transition state ? product

Many things discussed (write your notes
carefully)
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Chlorination of Propane
Radical Relative Stability and Reaction
Selectivity
1? C
2? C
60
40
Bromination of Propane
1? C
2? C
97
3
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Chlorination Energy Diagram
More selective

• Lower Ea, faster rate, so more stable
  intermediate is formed faster.

Less selective
Hammond postulate

• Energy required to break a C-H bond decreases as
  substitution on the carbon increases.
• Stability of radicals 3? gt 2? gt 1? gt methyl
  DH(kJ) 381, 397, 410, 435
  

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Radical Inhibitors

• Often added to food to retard spoilage.
• Without an inhibitor, each initiation step will
  cause a chain reaction so that many molecules
  will react.
• An inhibitor combines with the free radical to
  form a stable molecule.
• Vitamin E and vitamin C are thought to protect
  living cells from free radicals.

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• O2 UV light -gt 2 O
• O2 M -gt O3 M (where M indicates
  conservation of energy and momentum)
• O O2 -gt O3 or O3 O -gt O2 O2
• CFC (ChloroFlouroCarbons)
• CCL2F2 photon-gtCCLF2CL
• CLO3-gtClO O2
• ClOO-gtCL O2
• CL O3 ClO O -gtClOClO2O2