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Chapter 10 Notes - Chapter10 RadicalReactions Introduction...

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Chapter 10 Radical Reactions
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Introduction Homolytic bond cleavage leads to the formation of radicals (also called free radicals). Radicals are highly reactive, short‐lived species. Single‐barbed arrows are used to show the movement of single electrons. Production of Radicals: Homolysis of relatively weak bonds such as O‐O or X‐X bonds can occur with addition of energy in the form of heat or light. Reactions of Radicals: Radicals tend to react in ways that lead to pairing of their unpaired electron. Hydrogen abstraction is one way a halogen radical can react to pair its unshared.
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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). The homolytic bond dissociation energy is abbreviated DH o . Homolytic Bond Dissociation Energies and Heats of Reaction: Homolytic bond dissociation energies can be used to calculate (estimate) the enthalpy change ( Δ H o ) for a reaction. D H o is positive for bond breaking and negative for bond forming. Example: This reaction below is highly exothermic since Δ H o is a large and negative. Δ H o is not dependant on the mechanism; only the initial and final states of the molecules are considered in determining Δ H o .
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In Class Problem: Estimate the heat of reaction, Δ Ho, for each of the following reactions.
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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. Examples: The propyl radical is less stable than the isopropyl radical. Likewise the tert ‐butyl radical is more stable than the isobutyl radical.
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The energy diagrams for these reactions are shown below.
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The relative stabilities of radicals follows the same trend as for carbocations. The most substituted radical is most stable. Radicals are electron deficient, as are carbocations, and are therefore also stabilized by hyperconjugation. The Reactions of Alkanes with Halogens Alkanes undergo substitution reactions with halogens such as fluorine, bromine and chlorine in the presence of heat or light.
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