Free Radicals organic chemistry unit.pptx

Nice ppt of free radicals.

1. Free Radicals Dr. Bandita Datta

2. Structure A radical is paramagnetic and so can be observed by ESR (electron spin resonance) spectroscopy. The free radicals have two structures: sp 2 planar structure B) sp 3 pyramidal structure The ESR studies have shown that the structure of methyl radical is planar and the structure of trifluoromethyl radical is pyramidal.

3. Stability Radicals are electron-deficient because they lack an octet around the carbon atom. Radicals are stabilized by the electron-donating effect of alkyl groups, making more highly substituted radicals more stable.

4. Stability R adicals can be stabilized by resonance. Overlap with the p orbitals of a p bond allows the odd electron to be delocalized over two carbon atoms. Resonance delocalization is particularly effective in stabilizing a radical.

5. Stability The decreasing order of the stability of various radicals as follows:

6. Stability Certain free radicals have rigid molecular structures with fixed bond angles and dihedral angles. These are known as bridge head radicals and have pyramidal structure.

7. Generation of free radicals The homolytic cleavage of a covalent bond generates a pair of free radicals.

8. Thermolysis The radicals are formed by heating the appropriate substrate at suitable temperature. Azobisisobutyronitrile (AIBN)

9. Photolysis Compounds having absorption bands in the visible or UV region may be electronically excited to such an extent that weak covalent bonds undergoes homolysis.

10. Photolysis

11. Single electron transfer (SET) An electron can be removed from an anion and the process is known as the oxidation process. For example phenoxide ion is oxidized by Fe 3+ .