Chemistry of Radicals

1
Chemistry of Radicals
(Chapter 10)
Homolysis (cleaving chemical bond in a
symmetric  fashion)
radicals
Notice the single-barbed arrow
Radicals are unstable species produced by
homolysis each bearing one unpaired electron
2
Formation of radicals

• Thermal cleavage of peroxides
• Photochemical cleavage of halogens

3
Reactions of radicals
Hydrogen Abstraction
.
.
.
.
Organic radical
4
An example of chain reaction
.
.
.
.
.
.
polymers
5
Cleaving a bond requires energy
Dissociation E is the inverse of bond E
DH 243 kJ/mol
DH 435 kJ/mol
6
A practical use of the bond dissociation energy
H-H Cl-Cl 2
H-Cl Edis 435 Ediss 243
Ediss 2 x (431)
862


H-H 2H.
435 Cl-Cl 2Cl.
243 2H. 2Cl.
2HCl - 862
total - 184 Reaction is
Exothermic
7
Lets consider dissociation of C-H bonds
(formation of radicals from hydrocarbons)
primary radical
CH3CH2CH2-H CH3CH2CH2. H.
Ediss 410 (CH3)2CH-H
CH3CHCH3 H. Ediss 395
secondary radical
Thus stability of radicals depends on their
nature 3 gt 2 gt 1
8
Hyperconjugation for relative stabilization of
radicals
Partly vacant p orbital
The partly vacant orbital will receive part of
electron density from the neighboring orbitals.
More atoms around, more stabilization
9
Exercise List the followings radicals in order
of decreasing stability CH3.
(CH3)2CHCH2. CH3CH2C(CH3)2
CH3CH2CHCH3
.
.
10
Exercise List the followings radicals in order
of decresasing stability CH3.
(CH3)2CHCH2. CH3CH2C(CH3)2
CH3CH2CHCH3
.
.
Methyl primary
tertiary secondary
11
Exercise List the followings radicals in order
of decresasing stability CH3.
(CH3)2CHCH2. CH3CH2C(CH3)2
CH3CH2CHCH3
.
.
Methyl primary
tertiary secondary
Mister unstable
Mister stable