8_radicals_post

8_radicals_post - Vollhardt. Chap. 3 Sec. 5-7 Free Radical...

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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 Can be made through a Free radical mechanism or cationic and ionic mechanism
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Sec. 8 - Free Radicals 3 Some vinyl polymers H 2 CC F 2 vinylidine fluoride used in piezoelectric material audio speakers and microphones styrene used in plastics, stryofoam and isolation N 4-vinylpyridine ion exchange resins O O methyl methacrylate plastics, plexiglas O O t -BOC styrene photoresists for microelectronics O HO acrylic acid thickeners adhesives sporting goods O O CH 2 2 OH hydroxy ethyl methacrylate (HEMA) soft contact lenses O O( C H 2 ) 6 OC N cyano-biphenyl mesogenic monomer side chain liquid crystal polymer H 2 C CHCl 2 Cl n vinyl chloride monomer PVC polymer
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Sec. 8 - Free Radicals 4 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 5 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 6 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 07/10/2010 for the course CHEM 266 taught by Professor Pols during the Spring '07 term at Waterloo.

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8_radicals_post - Vollhardt. Chap. 3 Sec. 5-7 Free Radical...

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