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Free radicals
 Introduction of free radicals

Formation of free radicals

Stabilization of free
 radicals                            Moses Gomberg
                                  the founder of radical
                                  chemistry (1866-1947)
Reactions of free radicals
Free radicals
 Radicals are atoms, molecules, or ions with unpaired
  electrons in outer shell configuration.
 Free radicals may have positive, negative or zero
  charge.
   Even though have unpaired electrons, by convention,
    metals and their ions or complexes with unpaired
    electrons are not radicals.
 Unpaired electrons cause radicals to be highly
 reactive.
Electron pairing in the
outermost orbit indicates
stability of the atom. To
maintain stability, each
electron in the outer orbit
must     be   paired  with
another electron.
A free radical is simply an atom with one or more
unpaired electrons in its outer orbit.
What happens when free
radicals bond?
 When a weak bond is split, a free radical may be
  formed.
 In normal circumstances, the body provides
  endogenous substances (free-radical scavengers) to
  combine with the free radicals. If these scavengers
  aren’t available or if overproduction of free radicals, the
  radicals donate to or steal an electron from another
  molecule, leading to a chain reaction that triggers
  formation of more free radicals. The chain reaction
  results in damage to the cell membrane and
  deoxyribonucleic acid (DNA), altered enzyme reactions,
  and damage to collagen and connective tissues.
Oxygen as a free radical
 Probably the most well-known free radical, oxygen is
 the basis for development of most free radicals in the
 body. Inherently, oxygen is an unstable molecule.
 During metabolism, the O2 molecule splits and energy
    is released. To regain stability, the free single oxygen
    atom (oxygen free radical) seeks out or steals
    electrons from other available sources. This may
    result in a bond with dangerous properties:
•
    If oxygen accepts one electron, it becomes superoxide
    anion radical.
•
    If oxygen accepts two electrons, it produces peroxide.
 Although the superoxide radical isn’t very powerful, it
 can easily donate an electron to a nearby iron atom to
 produce the hydroxyl radical (OH*), one of the most
 potent biological free radicals. OH* can react with
 almost any molecule to cause oxidative stress and
 damage. These oxygen free radicals also are called
 reactive oxygen species (ROS).
 Radicals  are believed to be involved in
  degenerative diseases and cancers.
 Free radicals play an important role in
  combustion,         atmospheric      chemistry,
  polymerization, plasma chemistry, biochemistry,
  and many other chemical processes, including
  human physiology.
   For example, superoxide and nitric oxide regulate
    many biological processes including controlling
    vascular tone.
 Covalent bond can be broken down into 2 ways
 If a covalent compound, R:X , is broken down, then
 there are 3 possibilities, which are given as:
ONLY POSSIBLE IN SOLUTION

           water
H   Cl                 H         +    Cl
                   0 electrons        8 electrons in outer shell

                                     H 2O


                        H 3O           Cl
Different molecules require different
energy to split
Important points
 Free radical reactions occur widely in gaseous phase
  but may also occur in non-polar solvents
 Radicals once formed, produce chain reactions,
  hence free radical reaction process consists of
  incitation, propagation and termination
 Produced largely as reaction intermediates
 Most of the radical are short lived but some have
  long life
 Stable radicals
 The prime example of a stable radical is molecular
 oxygen O2. Organic radicals can be long lived if they
 occur in a conjugated π system for example vitamin-E
Persistent radicals
 Persistent radical compounds are those whose
 longevity is due to steric crowding around the radical
 centre, which makes physically difficult for the radical
 to react with another molecule. Examples are
 Potassium nitroso disulfonate (KSO3)2NO·) and
 nitroxides (R2NO·).
Diradicals
 Diradicals are molecules containing two radical
 centres. Atmospheric oxygen naturally exists as a
 diradical in its ground state as a triplet oxygen. This
 state of oxygen results in its paramagnetic character,
 which is demonstrated by its attraction to an external
 magnet.
Short lived radicals (R*)
 Are difficult to handle, most important and are highly
  reactive
 Their stability will be in the order:
  R3C* > R2CH * > RCH2* > CH3*
 The sequence shows that stability increases due to
  increase in hyper-conjugation and decrease due to relief of
  steric strain as the series traversed
 Radical forms having allylic or benzylic are more stable
  through delocalization of pi electrons e.g

CH2=CH-*CH2                     *CH2-CH=CH2
 Degeneracy = 2S+1, where S is the total electron spin
  angular momentum.
 An electron has a spin of +1/2 or –1/2, and an orbital
  can contain up to two electrons but they must be of
  opposite spin: the Pauli exclusion principle.
 Singlet diradicals have a pair of electrons, one spin-up
  and one spin-down [+1/2 and –1/2], in one orbital with
  the second, equal energy orbital, empty.
 The two electron spins are +1/2 and –1/2
 Total electron spin angular momentum, S = 0
 Degeneracy = (2*0) + 1 = 0+1 = 1 = singlet
 Triplet diradicals have two "spin-up" electrons in
  adjacent, degenerate (equal energy) orbitals.
 Two electrons of the same spin, +1/2 and +1/2
 Total electron spin angular momentum, S = 1
 Degeneracy = (2*1) + 1 = 2+1 = 3 = triplet
Methods of preparation
 Photochemical reactions
 Thermal fission
 Oxidation reduction reactions
Thermal (OrganoMetallic initiators)
C-M bonds have low Bond Dissociation Energy, and are
easily homolyzed into radicals
            CH3
                   HEAT
      H3C Pb CH3          Pb   +    4 CH3
            CH3


   FORMATION OF GRIGNARD REAGENT


    Ph Br     Mg          Ph       Mg Br    Ph MgBr
Oxidation reduction (Electron Transfer Processes)
Single Electron Transfer reactions



            SET
 R   X                     R   X     R   +   X


         M +n     M +n+1
Fenton’s reaction




Tertiary butylhydroperoxide
1- Thermodynamic stability
2-Kinetic stability
 To break a bond energy is needed, so the energy
   required to dissociate a bond is a good measure of
   how strong the bond is
It is quantified in terms of the enthalpy of dissociation
   of R-H into R• and H•
Q?
 In which case do you think the product is more
 stable?

A-    When bond dissociation energy is high
                        OR
B-    When bond dissociation energy is low
Answer
 The radicals formed from a bond which needs more
    energy are less stable
   Electron donating or electron pushing species
    towards free radicals stabilize them
   Methane                          105 KCal
   Ethane                           101 KCal
   Isopropane                      99 KCal
   Trimethyl methane                97 KCal
    Tertiary> Secondary > primary> methyl
Factors affecting stability
The main factors which determine
thermodynamic stability are
1- Conjugation
2- Hybridisation
3- Hyperconjugation
4-Captodative effects
Conjugated system
 It is a system of connected p-orbitals with delocalized
 electrons in compounds with alternating single and
 multiple bonds, which in general may lower the
 overall energy of the molecule and increase stability
1. Conjugation or Mesomerism
This is the primary reason for the existence of stable
radicals
    allylic radical




     benzylic radical



            CH2             CH2                CH2




                                               CH2
Hybridization
 Geometry of alkyl radical is considered between sp2
  and sp3 hybridization, and energy required to invert
  pyramid is very small




 One can usually think of alkyl radical as if it is sp2
  hybridized. Both electron donating and pi bond
  stabilize these
Hybridisation
π−Radical is more stable than σ−Radical.
As the π- character of a radical increases, its thermodynamic stabilisation increases




                                                    Vinyl and Aryl Radicals


                                                                    sp2 or σ−radical




                                                    cannot be resonance stabilised
                                                            Very Reactive Radicals
 Hyperconjugation is the interaction of the electrons
 in a sigma bond (usually C–H or C–C) with an
 adjacent empty or partially filled non-bonding p-
 orbital or antibonding π orbital or filled π orbital, to
 give an extended molecular orbital that increases the
 stability of the system.
Hyper-conjugation
 Unstable charges on molecules are dispersed over the
 structure or due to presence of hydrogen attached to
 stabilize the molecule
3. Hyperconjugation

                        H       H                                     H
                                                              H
                  H     C       C                     H       C   C

                        H       H                         H           H

                                    thermodynamic stability


      H    CH3                  H        H                H       H               H
  H   C   C         >       H       C   C       >     H   C       C       >   H   C
      H    CH3                  H        CH3              H       H               H

  9 Hyperconjugatable H s
                        6 Hyperconjugatable H s

                                                    3 Hyperconjugatable H s
The captodative effect is the effect on the stability of a
carbon-centred radical that is determined by the combined
action of a captor (electron-withdrawing) and a dative
(electron-donating) substituent, both attached to the radical
centre
b/ Kinetic Stability
     This is generally due to steric effects
 Steric effects arise from the fact that each atom within a molecule
 occupies a certain amount of space. If atoms are brought too close
 together, there is an associated cost in energy due to overlapping
 electron clouds, and this may affect the molecule's preferred shape
 and reactivity




          H          Methyl radical is less stable as compared to triphenyl
                     methyl radical
         *C     H

          H
Methods of detection
 Lead-mirror- is deposited on the inside wall of a glass
  tubes. These mirrors are disappeared when attacked
  by free radicals. So, by varying distance of mirrors
  from the source of free radical generation and velocity
  of carrier inert gas, free radicals can be detected
 Several radical are colored or produce color reaction
  which can be detected by colorimetry
 Magnetic field is used to detect the free radicals
Reactions of free radicals
Three types of reactions
1- Addition
2- Displacement
3- Rearrangement
1- Addition
                  Cl2

                        hv


   Cl2 C = CCl2 + Cl*        *CCl2-CCl3




                                  Cl2
                             CCl3-CCl3
2-Displacement
        Cl2

              hv


R-H   + Cl*        *R + HCl

                      Cl2

                   R-Cl
3-Rearrangement
                             -CO

Ph2MeC - CH2C* =O                          Ph2MeC - *CH2




3, 3 diphenyl butyric acid
                                   PhMeC* - CH2Ph
Types of free radicals in the body
 The most important free radicals in the body are:
 1- The radical derived from oxygen are known as reactive
   oxygen species, Superoxide radical (O2⁻), Peroxynitrite
   (ONOO⁻)
2- Carbon centred free radical (*CCI3) that arises from the
   attack of an oxidizing radical on organic molecules
3- Hydrogen centred radicals result from attack of on H
   atom (H*)
4- Sulfur centred radical produced in the oxidation of
   glutathione resulting in the thiyl radical (R-S*)
5- Nitrogen- centred radical also exists, for example
the nitric oxide ·NO and phenyl diazine
 A diradical in is a molecular species with two electrons
  occupying two degenerate molecular orbitals.
 They are known by their higher reactivity and shorter
  lifetimes.
 In a broader definition diradicals are even-electron
  molecules that have one bond less than the number
  permitted by the standard rules of valence.
 The electrons can pair up with opposite spin in one MO
  leaving the other empty. This is called a singlet state.
  Alternatively each electron can occupy one MO with
  spins parallel to each other. This is called a triplet state.

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Free radicals

  • 1. Free radicals  Introduction of free radicals Formation of free radicals Stabilization of free radicals Moses Gomberg the founder of radical chemistry (1866-1947) Reactions of free radicals
  • 2. Free radicals  Radicals are atoms, molecules, or ions with unpaired electrons in outer shell configuration.  Free radicals may have positive, negative or zero charge.  Even though have unpaired electrons, by convention, metals and their ions or complexes with unpaired electrons are not radicals.  Unpaired electrons cause radicals to be highly reactive.
  • 3. Electron pairing in the outermost orbit indicates stability of the atom. To maintain stability, each electron in the outer orbit must be paired with another electron.
  • 4. A free radical is simply an atom with one or more unpaired electrons in its outer orbit.
  • 5.
  • 6. What happens when free radicals bond?  When a weak bond is split, a free radical may be formed.  In normal circumstances, the body provides endogenous substances (free-radical scavengers) to combine with the free radicals. If these scavengers aren’t available or if overproduction of free radicals, the radicals donate to or steal an electron from another molecule, leading to a chain reaction that triggers formation of more free radicals. The chain reaction results in damage to the cell membrane and deoxyribonucleic acid (DNA), altered enzyme reactions, and damage to collagen and connective tissues.
  • 7. Oxygen as a free radical  Probably the most well-known free radical, oxygen is the basis for development of most free radicals in the body. Inherently, oxygen is an unstable molecule.
  • 8.  During metabolism, the O2 molecule splits and energy is released. To regain stability, the free single oxygen atom (oxygen free radical) seeks out or steals electrons from other available sources. This may result in a bond with dangerous properties: • If oxygen accepts one electron, it becomes superoxide anion radical. • If oxygen accepts two electrons, it produces peroxide.
  • 9.  Although the superoxide radical isn’t very powerful, it can easily donate an electron to a nearby iron atom to produce the hydroxyl radical (OH*), one of the most potent biological free radicals. OH* can react with almost any molecule to cause oxidative stress and damage. These oxygen free radicals also are called reactive oxygen species (ROS).
  • 10.  Radicals are believed to be involved in degenerative diseases and cancers.  Free radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes, including human physiology.  For example, superoxide and nitric oxide regulate many biological processes including controlling vascular tone.
  • 11.  Covalent bond can be broken down into 2 ways  If a covalent compound, R:X , is broken down, then there are 3 possibilities, which are given as:
  • 12. ONLY POSSIBLE IN SOLUTION water H Cl H + Cl 0 electrons 8 electrons in outer shell H 2O H 3O Cl
  • 13.
  • 14. Different molecules require different energy to split
  • 15. Important points  Free radical reactions occur widely in gaseous phase but may also occur in non-polar solvents  Radicals once formed, produce chain reactions, hence free radical reaction process consists of incitation, propagation and termination  Produced largely as reaction intermediates  Most of the radical are short lived but some have long life
  • 16.  Stable radicals The prime example of a stable radical is molecular oxygen O2. Organic radicals can be long lived if they occur in a conjugated π system for example vitamin-E
  • 17. Persistent radicals  Persistent radical compounds are those whose longevity is due to steric crowding around the radical centre, which makes physically difficult for the radical to react with another molecule. Examples are Potassium nitroso disulfonate (KSO3)2NO·) and nitroxides (R2NO·).
  • 18. Diradicals  Diradicals are molecules containing two radical centres. Atmospheric oxygen naturally exists as a diradical in its ground state as a triplet oxygen. This state of oxygen results in its paramagnetic character, which is demonstrated by its attraction to an external magnet.
  • 19. Short lived radicals (R*)  Are difficult to handle, most important and are highly reactive  Their stability will be in the order: R3C* > R2CH * > RCH2* > CH3*  The sequence shows that stability increases due to increase in hyper-conjugation and decrease due to relief of steric strain as the series traversed  Radical forms having allylic or benzylic are more stable through delocalization of pi electrons e.g CH2=CH-*CH2 *CH2-CH=CH2
  • 20.  Degeneracy = 2S+1, where S is the total electron spin angular momentum.  An electron has a spin of +1/2 or –1/2, and an orbital can contain up to two electrons but they must be of opposite spin: the Pauli exclusion principle.
  • 21.  Singlet diradicals have a pair of electrons, one spin-up and one spin-down [+1/2 and –1/2], in one orbital with the second, equal energy orbital, empty.  The two electron spins are +1/2 and –1/2  Total electron spin angular momentum, S = 0  Degeneracy = (2*0) + 1 = 0+1 = 1 = singlet
  • 22.  Triplet diradicals have two "spin-up" electrons in adjacent, degenerate (equal energy) orbitals.  Two electrons of the same spin, +1/2 and +1/2  Total electron spin angular momentum, S = 1  Degeneracy = (2*1) + 1 = 2+1 = 3 = triplet
  • 23. Methods of preparation  Photochemical reactions  Thermal fission  Oxidation reduction reactions
  • 24.
  • 25.
  • 26. Thermal (OrganoMetallic initiators) C-M bonds have low Bond Dissociation Energy, and are easily homolyzed into radicals CH3 HEAT H3C Pb CH3 Pb + 4 CH3 CH3 FORMATION OF GRIGNARD REAGENT Ph Br Mg Ph Mg Br Ph MgBr
  • 27. Oxidation reduction (Electron Transfer Processes)
  • 28. Single Electron Transfer reactions SET R X R X R + X M +n M +n+1
  • 31.  To break a bond energy is needed, so the energy required to dissociate a bond is a good measure of how strong the bond is It is quantified in terms of the enthalpy of dissociation of R-H into R• and H•
  • 32. Q?  In which case do you think the product is more stable? A- When bond dissociation energy is high OR B- When bond dissociation energy is low
  • 33. Answer  The radicals formed from a bond which needs more energy are less stable  Electron donating or electron pushing species towards free radicals stabilize them  Methane 105 KCal  Ethane 101 KCal  Isopropane 99 KCal  Trimethyl methane 97 KCal Tertiary> Secondary > primary> methyl
  • 34.
  • 35. Factors affecting stability The main factors which determine thermodynamic stability are 1- Conjugation 2- Hybridisation 3- Hyperconjugation 4-Captodative effects
  • 36. Conjugated system  It is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability
  • 37. 1. Conjugation or Mesomerism This is the primary reason for the existence of stable radicals allylic radical benzylic radical CH2 CH2 CH2 CH2
  • 38.
  • 39. Hybridization  Geometry of alkyl radical is considered between sp2 and sp3 hybridization, and energy required to invert pyramid is very small  One can usually think of alkyl radical as if it is sp2 hybridized. Both electron donating and pi bond stabilize these
  • 40. Hybridisation π−Radical is more stable than σ−Radical. As the π- character of a radical increases, its thermodynamic stabilisation increases Vinyl and Aryl Radicals sp2 or σ−radical cannot be resonance stabilised Very Reactive Radicals
  • 41.  Hyperconjugation is the interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty or partially filled non-bonding p- orbital or antibonding π orbital or filled π orbital, to give an extended molecular orbital that increases the stability of the system.
  • 42. Hyper-conjugation  Unstable charges on molecules are dispersed over the structure or due to presence of hydrogen attached to stabilize the molecule
  • 43. 3. Hyperconjugation H H H H H C C H C C H H H H thermodynamic stability H CH3 H H H H H H C C > H C C > H C C > H C H CH3 H CH3 H H H 9 Hyperconjugatable H s 6 Hyperconjugatable H s 3 Hyperconjugatable H s
  • 44. The captodative effect is the effect on the stability of a carbon-centred radical that is determined by the combined action of a captor (electron-withdrawing) and a dative (electron-donating) substituent, both attached to the radical centre
  • 45. b/ Kinetic Stability This is generally due to steric effects Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds, and this may affect the molecule's preferred shape and reactivity H Methyl radical is less stable as compared to triphenyl methyl radical *C H H
  • 46. Methods of detection  Lead-mirror- is deposited on the inside wall of a glass tubes. These mirrors are disappeared when attacked by free radicals. So, by varying distance of mirrors from the source of free radical generation and velocity of carrier inert gas, free radicals can be detected  Several radical are colored or produce color reaction which can be detected by colorimetry  Magnetic field is used to detect the free radicals
  • 47. Reactions of free radicals Three types of reactions 1- Addition 2- Displacement 3- Rearrangement
  • 48. 1- Addition Cl2 hv Cl2 C = CCl2 + Cl* *CCl2-CCl3 Cl2 CCl3-CCl3
  • 49. 2-Displacement Cl2 hv R-H + Cl* *R + HCl Cl2 R-Cl
  • 50. 3-Rearrangement -CO Ph2MeC - CH2C* =O Ph2MeC - *CH2 3, 3 diphenyl butyric acid PhMeC* - CH2Ph
  • 51.
  • 52. Types of free radicals in the body  The most important free radicals in the body are: 1- The radical derived from oxygen are known as reactive oxygen species, Superoxide radical (O2⁻), Peroxynitrite (ONOO⁻) 2- Carbon centred free radical (*CCI3) that arises from the attack of an oxidizing radical on organic molecules 3- Hydrogen centred radicals result from attack of on H atom (H*) 4- Sulfur centred radical produced in the oxidation of glutathione resulting in the thiyl radical (R-S*) 5- Nitrogen- centred radical also exists, for example the nitric oxide ·NO and phenyl diazine
  • 53.
  • 54.  A diradical in is a molecular species with two electrons occupying two degenerate molecular orbitals.  They are known by their higher reactivity and shorter lifetimes.  In a broader definition diradicals are even-electron molecules that have one bond less than the number permitted by the standard rules of valence.  The electrons can pair up with opposite spin in one MO leaving the other empty. This is called a singlet state. Alternatively each electron can occupy one MO with spins parallel to each other. This is called a triplet state.