(b) Allylic and benzylic halogenation using N-Halo-succinimides
(NXS)
Br
O
+
N Br
O
h or In•
NBS
O
+
N H
O
O
O
h or 
N Br
N•
O
+ Br•
O
•
+ Br•
+ HBr
+
OH
O
N Br + HBr
OH
+
N Br + Br-
N Br
O
O
OH
O
OH
+
N Br
Br-
O
NH + Br2
N
O
O
O
Br
•
+ Br2
+ Br•
•
+ Br•
+ HBr Etc.
(c) The Barton Reaction - Regiospecific radical-mediated oxidation to
carbonyl, C=O, of an unactivated CH2 group which is positioned  to
the oxygen atom of an adjacent OH group. (Radical substitution of two
methylene hydrogens in CH2 by an oxygen atom to give C=O):

HO


CH2CH3

O
(i) NOCl (ii) h
C
(iii) H3O+
O
HO
HO
CH3
CH2CH3
NOCl
N
O
CH2CH3
Nitrosyl chloride
(Acid halide of
nitrous acid, HO–N=O)
h
- NO•
Nitrite
ester
•
O
Photolytic
homolysis
Regiospecific intramolecular
hydrogen abstraction via
a favoured six-membered
transition state.
HO N
HO
C
CH3
O N
HO
CH3
CH
Nitroso
compound
Oxime
H 3O +
O
Hydrolysis
HO
HO
+ NO•
Tautomerisation
C
CH3
H CH3
CH
Electron
pairing
KEY
STEP
CH3
•CH
(2) Free radical reduction of organic halides:
Br
R3SnH
In•
R = Me, Bu,
Ph
H
H
Cl
Cl
Cl
2 R3SnH
R3SnH
In•
In•
Cl
R3SnH
In•
Ph
Ph
Br
+ R3SnH
AIBN
Inert solvent

Mechanism - radical chain process:
Bu3SnH + In•
Bu3Sn• + InH
Bu3Sn• + RX
Bu3SnX + R•
Bu3SnH + R•
Bu3Sn• + RH
H
(3) Addition of free radicals to alkenes
Addition of halogens to alkenes:
Cl2
h
2 Cl•
Cl•
Cl
Cl
Cl
Cl
Cl
Cl
Cl
•
Cl
Cl
Cl
•
Cl
Cl
Cl
Cl
Cl
+ Cl2
Cl
Cl
Cl
Cl + Cl•
Cl
Reactivity of X2 in addition reactions: F2 > Cl2 > Br2 > I2
R
H
I2 (catalytic)
R
H
R
R
+
H
- I•
R
I
H
H
R
R
H
H
Equilibrium mixture
+ I•
H
•
R
- I•
R
R
H
H
Addition of HX to alkenes:
Polar solvent - ionic mechanism - Markovnikov addition:
RHC=CH2
HBr
Br
RHC CH3
RHBrC CH3
Non-polar solvent, initiator - free radical mechanism - Anti
Markovnikov addition:
HBr + In
RHC=CH2 + Br
InH + Br
RHC CHBr
HBr
RH2C CHBr
Intramolecular radical addition to alkene C=C can be a useful method
of ring-closure:
+ HCBr3
CBr3
In•
•
CBr3
CBr3
•CBr3 +
HCBr3
Cyclooctene
Further chain
reaction
•
Bu3SnH
In
I
Bu3SnH
+ Bu3SnH
(4) Free radical dimerisation.
Kolbe electrolytic oxidation of carboxylic acids:
RCO2-
-e
Electrolysis
RCO2•
2 R•
CH3(CH2)12CO2H
R• + CO2
R-R
CH3OH
2% Na
[CH3(CH2)12CO2]– Na+ + 1/2H2
Pt Electrodes
CH3(CH2)24CH3
CH3(CH2)11CH2• + CO2 + Na•
(5) Free radical fragmentation - Synthesis of cubane
CO2H
COCl
SOCl2
t
O
BuOOH
O
C
C
O
•
O
OBut
- tBuO•
Mild
Isopropylbenzene
solvent
- CO2
•
H abstraction
from solvent
•
+ PhCMe2
(6) Reactions involving radical anions.
(a) Reduction of carbonyl compounds to alcohols with Na/EtOH
R
1
C O
Na/EtOH
R
1
R
C
R
H
OH
Na e
Note: e + H+ H•
R • _
C O
1
R
R •
C OH
1
R
EtOH
H+
R _
C OH
1
R
Na
e
EtOH H+
Ketyl radical anion
R _
•
C O
R1
R
1
C
R
H
OH
Overall: 2 e + 2 H+ 
(b) The Acyloin Condensation - conversion of two moles of a
carboxylic ester into an -hydroxy ketone:
O
O
(i) Na, Et2O or C6H6
2
C
(ii) H2O
R
OMe
R
C
C
R
OH
H
Acyloin (hydroxyketone)
Mechanism of the acyloin condensation:
O
2
R
C
O
2 Na
OMe
2
R
OMe
_
O
R
C
_
C•
OMe
R
Ketyl radical anion
C
O
OMe
_
- 2 MeOR
C
_
O
_
R • O
C
_ C
O • R
_ C
O
R
R
2 Na
C
C
R
O
O
2 H2O
R
C
HO
C
R
OH
C
O
C
HO
R
H
R
(c) The Birch Reduction of arenes to non-conjugated cyclohexadienes
with sodium and liquid ammonia/ethanol:
OCH3
Na
NH3 (liq.) - EtOH
OCH3
-35° C
CO2H
Na
NH3 (liq.) - EtOH
-35° C
CO2H
Mechanism of the Birch Reduction:
•
Na/NH3
e
•

__•

•
_
•
EtOH
H
_
+
H H
_
•

Na
e
H H
H H
_
H H
H H
_
H H
EtOH
_
H+
H H
H H
H H
(d) Reduction of alkynes to trans-alkenes.
R C C R
Na e
R
_C C •
R
R
"H2"
H
NH3
R
H
C C
R
C C
R
H+
H
NH3
R
R
H+
H
C C
H
Na/Liq. NH3
•
H
R
Na e
_
C C
R