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skip 11.3, 11.5.2.1 p326-327 except NBS, 11.5.2.2 p330-331, 11.5.2.3, 11.6
Free Radical Reactions
1. general properties of radicals
1.1. unpaired electron, slightly pyramidal, small energy barrier to invert pyramidal geometry
1.1.1. planar – 5% s character in orbital containing electron
1.1.2. ESR – orbital connecting to electron and 13C – little significant s character
1.1.3. EWG increase s character, why?
1.2. normally very reactive with short lifetimes
1.3. If they don't react with closed shell molecules, they decay by termination (coupling and
disproportionation: examples)
1.4. acids, bases and polar solvent have minor affect
1.5. stabilization
1.5.1. alkyl substitution, (methyl < primary < secondary < tertiary)
1.5.1.1. hyperconjugation
1.5.1.2. greater reduction in steric interaction when radical forms
1.5.2. resonance stabilization (allyl, benzyl, heteroatom) – related BDE
,
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1.5.3. steric hindrance inhibit radical combination
non planar rings in triphenylmethyl radical
2. Radical formation
2.1. Electron transfer

2.1.1. Chemical reduction A  e  A 
O
O
Na
+ Na
Na
R C C R
R C C R
, H2O2  Ti 3  HO   HO   Ti 4
O
O
O
+
+ Cu
O
O
O +
O
+ Cu+2
O
2.1.2. Electrolytic reduction

2.1.3. oxidation: D  D   e 
2.1.3.1. RCO2H  Pb4  RCO2   Pb3  H , also works with Ce+4, Mn+3 and Ag+2
2.1.3.2. Electrolytic oxidation:
2.2. thermal cleavage – weak bonds: cumulated lone pairs, stabilized products, organometallics
for electronegative metal
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O
RO OR
O
2RO
O
O
2
O
O
N
N
N
C
C
C
N
N
N
N
C
N
+
N
N
2 C
N
2.3. photolysis
h
2.3.1. cumulated lone pairs: Br2, ROOR  2RO , HOX,
2.3.2.  electrons: alkenes and carbonyls
3. Reactions of radicals
3.1. atom abstraction - usually monovalent (H or halogen)
3.1.1. dependent on bond energies, TS can have polar character
3.1.2.
R' O  RH  R' OH  R,R  alkyl,benzyl , allyl
3.1.3.
R3M  R' X  R3MX  R' ,M  Sn, Ge & Si, X  Cl,Br,I
3.1.4. rates can be estimated where abstraction is rate
determining step
BDE
broken
formed
CH3CH2-H (98)
Br-H (88)
CH3CH2-H (98)
F-H (136)
Ea for Br = 13.2 kcal/mol, H* = 12.6 kcal/mol
HOCl
HO
Cl
3
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Ea for F = 0.3 kcal/mol, H* = -0.3 kcal/mol
3.1.5. exothermic processes are diffusion controlled, barrier estimated from activation energy
(2-5 kcal in solution)
endothermic barrier estimated from bond energy difference and reverse barrier (2-5 kcal)
3.2. fragmentation
O
R
O
O
R + CO2
O
R N N
R + N
O
R + C O
N
R
CH3
3.3. Addition
3.3.1. To other radicals: R  O2  RO 2 

3.3.2. To  bonds: R'   R2C  CR2  R' R2 CR2
+
R
+
R
H
3.3.3. To empty orbitals:
O +
OEt
109
P OEt
OEt
isooctane
Et
+
+
Et
P
109
O
O
Et
Et
isooctane
Et
B
109
Et
isooctane
4. destruction of radicals
OEt
P OEt
OEt
Et
P
Et
O
Et
O
O
+
P
OEt
P OEt
OEt
Et
+ Et
Et
Et
B Et
Et
O
Et
B + Et
Et
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4.1. preceding reactions have “exchanged” radicals
4.2. coupling (recombination): R  R  R  R

4.3. disproportionation:

R'2 CH C H 2  R'2 CH C H 2  R'2 CHCH 3  R'2 C  CH 2
4.4. oxidation: R   Mn  R  M(n1) , M+n = Pb+4, Ce+4, Mn+3, Ag+2
5. chain reactions: path combination
5.1. example substitution: methane halogenation
5.2. propagation (chain reaction) can occur many times per initiation: 10 6 for Cl
initiation
propagation
or h
X 2  
 2X 
step
F
Cl
Br
I
1
-31
2
17
34
2
-70
-26
-24
-21
X  CH4  XH  CH3 
CH3   X 2  CH3 X  X 
termination
Enthalpies of propagation steps
X   X  X 2
5.3. termination rate constant is fast, but it is bimolecular and radical concentrations are low
5.4. propagation steps can compete because substrate concentration is high
5.5. feasibility determined by propagation steps, e. g. for methane bromination and iodination
does not occur at an appreciable rate (rate related to enthalpy of propagation step, difference
in bond energies
5.6. at 25 C, bromination of alkanes (per H): R3C-H (1600) > R2CH-H (80) > RCH2-H(1)
5.7. Chlorination of alkanes (per H): R3CH (6.7) > R2CH2 (4.4) > RCH3 (1)
5.8. polar effects- chlorination more sensitive than bromination
5.8.1. the -CH bonds are weakest but are also relatively electron deficient
5.8.2. the chlorine radical is electrophilic so it avoids the elecron deficient -CH
CH3CH2CH2CN + Cl2
ClCH2CH2CH2CN + CH3CHClCH2CN + HCl
31%
69%
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Chlorination of chlorobutane:
6. radical addition
6.1. HBr addition:
H (kcal/mol) of X addition and H abstraction steps
X + CH2=CH2
XCH2CH2 + HX
HF
-45
37
HCl
-26
5
HBr
-5
-11
HI
7
-27
For halogen acids only HBr is exothermic for both reactions
6.2. RSH, Br3CH, Cl3CH are the same mechanism
6.3. BrCCl3, and CCl4 have a carbon or silicon radical initiation
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6.4. X2
6.5. Polymerization
6.5.1. Chain length determines concentrations and rates of steps
6.5.2. Branching can occur from H abstraction
7. addition to nitroso or nitrone compounds
7.1. stable radicals are formed
7.2. two-atom 3-electron bonds, what are MO’s for NO?
R
R
N O + R'
R'
N O
R'
O
O
N
N
+ R'
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8. autooxidation
Ri
ROOH 
RO   RO 
initiation (source of ROOH?) OH!!!
RO  RH  ROH  R 
R  O2  RO 2 
propagation
RO 2  RH  RO 2H  R 
propagation
2RO 2   RO 4R
termination
9. inhibitors - abstraction or addition forms unreactive radicals
OH
OH
tBu
O
HO
tBu
2R
O
R + O2
RO2
R
RO2
OH
Me
O
RO4R
-tocopherol
BHT
hydroquinones
O
RSH
-
OH
+
N
-
O
R
+
N
N+
-
O
R
tBu
R
-
thiol
O
O
+
N
O-
tBu
R
ascorbate
spin traps
spin adducts
10. selected reactions
10.1. hydroxylation arenes
H2O2
Fe2+
H2O2
HO - + HO
+ HO - + HO
OH
OH
H
H
10.2. silicon hydride (or tin hydride) reduction of alkyl halides.???
ROOR  2RO 
RO   Et3 SiH  ROH  Et3 Si 
Et3 Si   RX  Et3 SiX  R 
R   Et3 SiH  RH  Et3 Si 
10.3. diacyl peroxide reduction, catalytic in copper (only for R = alkyl)
OH
8
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10.3.1. cation may rearrange or react with added nucleophile
ast
RCO 2   f

 R   CO2
R   Cu2  R  Cu
R  RCO 2  RCO 2R
10.4. rearrangements
10.4.1. phenyl migration, delocalization provide low energy
Ph
Ph
Ph
Ph
Ph
Ph
10.4.2. Alkyl radical rearrangements rare! Compare carbon cation transition state or
intermediate structure
10.5. Dissolving metal reduction
10.5.1. olefins, alkynes, aromatics
10.5.2. note that C radicals oxidize electropositive metals
10.5.3. RX + 2Na  R2
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H O R'
Na + R
R
C C
C C R
H
+ Na
R
R
R
C C R + Na
C C
H
R
H O R'
11. esters, aldehydes and ketones (acyloin)
+ Na
R
C C
H
+ Na + O R'
R
R C C H
+ Na + O R'
H
R
10