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STABILITY of FREE RADICALS

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STABILITY of FREE RADICALS Powered By Docstoc
					STABILITY OF FREE RADICALS
Stability of free radical can be,

        Thermodynamic stability

                   or

            Kinetic stability.
Thermodynamic
    stability
Breaking bonds requires energy, so the
energy required to dissociate the bond is a
good measure of how strong that bond is.

     B     C     energy     B· + C·
Question ?
             energy
B + C                 B· + C·
In which case do you think the product is
  more stable ??

   When the dissociation energy is high
                   Or
    When the dissociation energy is low
So now lets make a criterion,

  ‘’Products (radicals in our case) formed
  from bond dissociations requiring higher
           energy are less stable’’
Have a look on the table below,
Factors affecting stability :
 Conjugation.
 Hyper conjugation.
 Hybridization.
 and
 Captodative effects.
Conjugation:
primary reason for existence of stable radicals.

       allylic radical




       benzylic radical



               CH2              CH2                CH2




                                                   CH2
Hyperconjugation:
   Some times unstable charges on
    molecules are dispersed over the structure
    due to the presence of hydrogens
    attached to stabilize the molecule but if the
    odd charge is negative than the presence
    of hydrogens will de stabilize the system.
Hybridization:
as the p character of a radical increases so does its
thermodynamic stabilization.

           Vinyl and Aryl Radicals


                             sp2 or radical




           cannot be resonance stabilised
                     Very Reactive Radicals
Captodative effect:
Kinetic
stability
Kinetic stability is generally due to steric factors,

				
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posted:8/2/2010
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