Section_8_Radicals_post

Section_8_Radicals_post - Vollhardt. Chap. 3 Sec. 5-7 Free...

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Sec. 8 - Free Radicals 1 Free Radical Reactions Free Radical any atoms or groups of atoms that has one or more unpaired electrons. They usually has do not have a positive or negative charge, but are highly reactive and short lived reaction intermediates 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 homolytic bond cleavage of σ bond Vollhardt. Chap. 3 Sec. 5-7
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Sec. 8 - Free Radicals 2 Recall, the addition of Na and liquid NH 3 to an alkyne generates a trans alkene: This reaction proceeds via a radical reaction mechanism
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3 The second electron transfer takes place faster than any cis/trans equilibrium of the alkenyl radical. The final alkene is stable enough to avoid further reduction by the reagent.
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Sec. 8 - Free Radicals 4 These materials can be made via a free radical mechanism or a cationic and ionic mechanism
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Sec. 8 - Free Radicals 5 Examples of some Vinyl Polymers H 2 C CHCl CH 2 Cl n vinyl chloride monomer PVC polymer
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Sec. 8 - Free Radicals 6 Homolytic Bond Dissociation Energies and Heats of Reaction The homolytic bond dissociation energy is abbreviated DH o Homolytic Bond Dissociation energies can be used to calculate the enthalpy change ( DH o ) for a reaction DH o is positive for bond breaking and negative for bond forming The more stable the bond the more energy required to cleave the covalent bond Example: This reaction below is highly exothermic since D H o is a large and negative D H o is not dependant on the mechanism; only the initial and final states of the molecules are considered in determining D H o
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Sec. 8 - Free Radicals 7 Free Radical Reactions Initiation Cl Cl + 246 kJ mol -1 2 Cl Propagation: The initial formation of a few radicals results in the propagation of new radicals in a self perpetuating reaction called a chain reaction. Cl + H CH 3 CH 3 Cl + Cl HCl + CH 3 CH 3 + Cl Cl Cl CH 2 Cl + H HCl + CH 2 Cl CH 2 Cl Cl Cl + ClCH 2 Cl + Cl Termination: The formation of stable molecules CH 3 Cl + ClCH 3 CH 3 + CH 3 CH 3 CH 3 etc. etc. No bond breaking all exothermic
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Sec. 8 - Free Radicals 8 Hydrogen Abstraction Hydrogen abstraction is usually the rate determining step. The reaction does not follow 1st or 2nd order kinetics because of cyclical process.
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This note was uploaded on 09/13/2011 for the course CHEM 228 taught by Professor Jacobfischer during the Spring '09 term at Waterloo.

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Section_8_Radicals_post - Vollhardt. Chap. 3 Sec. 5-7 Free...

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