1. Wonderful world of radical reactions
Wojnárovits László

2. Elementary particles Fermions Half integer spin Bosons Integer spin
Electron, 1/2
Photon, 1
Positron, 1/2
Antineutrino, 1/2
Neutrino, 1/2
Spin is an intrinsic form of angular momentum carried by elementary particles. All elementary particles of a given kind have the same magnitude of spin angular momentum, which is indicated by assigning the particle a spin quantum number. Spin is given as a dimensionless spin quantum number by dividing the spin angular momentum by the reduced Planck constant ħ.

3. Some reactions with changes of spin
60Co → 60Ni + e- +ve s = 1
2 photons s = 2
18F → 18Ni + e+ +v+ s = 1
The allowed changes of the net spin quantum number(s)
are 0, ± 1, ± 2…..

4. Detection of free radicals
-CH2-CH-CH2-
e
COMPUTER
Magnetic field strength
ESR spectrometer
ESR spectra

5. Inorganic free radicals
1s 2s p s p d s
Cu
Cu +
Cu 2+
Cu2+ + H  Cu+ + H+
Cu+ + OH  Cu2+ + OH- N
O (O)

6. Free radicals produced under milder conditions
By simply heating nitrous acid, nitric acid and chloric acid (or their salts), nitrogen oxide, nitrogen dioxide and chlorine dioxide, respectively, can be formed:
3 HNO2  2 NO + HNO3 + H2O
4 HNO3  4 NO2 + O2 + 2 H2O
8 HClO2  6 ClO2 + Cl2 + 4 H2O
In these reactions the acids on the left side are non-paramagnetic. NO, NO2 and ClO2 are paramagnetic with spin quantum number of ½. The net spin quantum number changes are 1, 2 and 3.

7. Stable organic free radicals
Triphenylmethyl Diphenylpicrylhydrazyl Galvinoxyl.

8. Formation under heavier conditions

9. Peroxyl radicals, self termination
HO + HO  H2O2
HO2 + HO  O2 + H2O
O2 + HO  O2 + OH
+H2O
O2 + HO2  H2O2 + O2 + OH
2 HO2  H2O2 + O2
2 O2  O-O-O-O  O3 + O

10. Reactions of organic radicals with dioxygen
Rate constants of forward (kf) and reversed (kr) peroxyl radical forming/dissociation reactions of radicals and equilibrium constants (K). kf
R + O2 ⇄ ROO  kr
Radical
kf , mol-1 dm3 s-1
kr , s-1
K, mol-1 dm3
Simple alkyl radicals
c-C5H9
3.5  109
CH2OH
4.2  109
Cyclohexadienyl radicals
Anisol
8  108
3.9  104
2.1  104
Toluene
4.8  108
7.5  104
6.4  103
Benzene
3.1  108
1.2  104
2.6  104

11. Reactions of organic radicals with dioxygen
__________ _______________________________
High reactivity Low reactivity

12. Carbenes: Hydrogen elimination from cycloalkanes

13. Biradicals: Isomerization of cycloalkanes to linear alkenes
Biradicals forming in C-C bond rupture in cycloakanes may reclose again to the starting cycloalkane or its cis/trans isomer, rearrange to linear alkanes or fragment.

14. Biradical scavenging [Biradical] + I2  [Biradical-I] + I
[Biradical-I] + I2  Biradical-I2 + I
Irradiation of
cyclpentane + I2 system

15. Indications of the radical character
1. In chemical formula the radical character and in structural formula also the place of unpaired electron can be indicated by a dot. In organic radicals with a conjugated double bonds the unpaired electron may be distributed over a larger molecule volume.
2. The charge sign and the radical sign should be indicated both on radical anions and cations and also on the released electron. IUPAC suggests first the radical sign and then the charge, e.g. H2O+, c-C6H12+, e, O2.
3. The rule related to spin quantum number change (0,  1,  2 etc.) allows checking the correct writing of equations in which radicals are reaction partners. For practical purposes the rule is formulated as follows: when on one side of equation there is one radical, the other should bear also at least one. When on one side there is no radical, or there are 2, 4, etc., the other should contain 0, or an even number. The rule can be incorporated into computer programs used for checking/generating chemical equations.