1. Free Radical substitution reactions and Their applicationsBY
DR. GHULAM ABBAS

2. Heterolytic Fission Homolytic Fission
When bonds break and the atoms get one electron each

3. Peroxides
When R is alkyl, loss of CO2 is very fast. Benzoyl peroxide has a half-life of 1 hour at 90 oC, and is useful, as it selectively decomposes to benzoyl radicals below 150 oC.
Organo Metallic INITIATORS
C-M bonds have low bond Dissociation energy (BDE), and are easily homolyzed into radicals;

4. FORMATION OF GRIGNARD REAGENTS
All the radical initiation pathways so far discussed give very reactive, short-lived radicals (< 10-3s), which are useful in synthesis

5. Very Stable Radicals (Half-life = years)
Thermodynamic Stabilisation is most important. These radicals can be stored on the bench, and handled like other ordinary chemicals, without any adverse reaction in air or light.

6. Chlorination of Methane
carried out at high temperature (400 °C)
CH Cl2  CH3Cl HCl
CH3Cl + Cl2  CH2Cl HCl
CH2Cl Cl2  CHCl HCl
CHCl Cl2  CCl HCl 6

7. Preparation

8. 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

9. The energy diagrams for these reactions are shown below

10. Free Radical Resonance
When drawing resonance for free radicals, use fishhook arrows to show electron movement
An allylic radical is stabilized by resonance

11. Free Radical Resonance
The benzylic radical is a hybrid that consists of 4 contributors
Hybrid δ δ δ δ

12. Allylic Halogenation with NBS
To avoid the competing halogenation addition reaction, NBS can be used to supply Br• radicals
Heat or light initiates the process 12

13. Allylic Halogenation with NBS
Propagation produces new Br• radicals to continue the chain reaction
Where does the Br-Br above come from? The amount of Br-Br in solution is minimal, so the competing addition reaction is minimized 13

14. Sodium Amide, (Na+NH2-) is made by dissolving Na in liquid ammonia, and then waiting until the solution is no longer blue
Ether or THF

15. In aprotic solvents, ketyl radical anions dimerise
Other REDOX reactions
Birch Reduction
Prof. Arthur Birch, ANU
Pinocol Coupling
In aprotic solvents, ketyl radical anions dimerise

16. Radical Polymerization
Radical polymerizations commonly proceed through a chain reaction mechanism.

17. Radical Polymerization

18. Radical Polymerization

19. Free Radical Polymerization

20. Reaction of Chloroalkanes with Ozone
The use of chlorinated organic compounds is being curtailed for two main reasons.
Some of the compounds are potentially carcinogenic (cancer forming) and they also have the potential to lower
the concentration of ozone in the ozone layer.
Ozone is continually being formed and depleted in the ozone layer between about 120 km and 50 km above the surface of the Earth.
The strong double bond in oxygen is broken by high energy
Ultraviolet light from the Sun to form radicals. One oxygen radical can then react with an oxygen
molecule to form ozone.

21. Overall, the rate of production of ozone is equal to the rate of ozone destruction – this is known as
a steady state. However, compounds containing C—Cl bonds can alter this steady state.
Chlorinated hydrocarbons tend to be rather unreactive until they reach the stratosphere, where the ultraviolet light causes the C—Cl bond to break homolytically.
The chlorine radicals can then react with ozone molecules by a chain reaction. This causes a substantial lowering of the ozone
concentration, resulting in ‘holes’ in the ozone layer, particularly over the polar regions.

23. THE END