Oxidation State Of Organic Molecules

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Chem. 343/345
Reich/Nelsen
Oxidation State of Organic Molecules
The most reduced form of carbon is CH
, the most oxidized is CO
. Thus the oxidation state of a one-carbon
4
2
fragment is unambiguous and defined by the number of C-H bonds that have been replaced by C-X bonds, where X
= any electronegative element (see periodic table on previous page). Replacing C-H bonds by C-Metal bonds is not
a redox process. A C=O double bond is equivalent to two C-O single bonds (C(OH)
).
2
M = H, Li, MgBr, BH
Ox
2
R M
R X
X = F, Cl, Br, I, OH, NH
Red
2
Reduction - Each step involves C-X → C-H
Each step involves C-H → C-X, Oxidation
0
1
2
3
4
Sum:
Reduction
CH
CH
-Cl
CH
Cl
CHCl
CCl
4
3
2
2
3
4
Oxidation
CH
-MgBr
CH
-OH
CH
=O
H-C(=O)OH
CO
3
3
2
2
CH
-NH
CH
=NH
H-C(=O)NH
O=C=NH
3
2
2
2
CH
(OH)
H-C≡N
O=CCl
2
2
2
+
C ≡O
When there are C-C bonds involved, the situation is more complicated. Now, in addition to replacement of C-H by
C-X, oxidations can also involve the removal of H
by introduction of C-C and C-X double and triple bonds. To go
2
left and right in the scheme below requires reduction and oxidation, respectively. Compounds in the same column
are at the same oxidation state, and can be interconverted by nucleophilic substitutions (C-X by C-Y), electrophilic
substitutions (C-H by C-M), additions or eliminations (of HX or MX fragments) and rearrangements.
Reduction - Each step involves C-X → C-H or addition of H
2
Each step involves C-H → C-X or loss of H
, Oxidation
2
Sum:
0
1
2
3
4
5
6
CH
-CH
CH
=CH
HC≡CH
HC≡COH
HOC≡COH
3
3
2
2
CH
-CH
-Li
CH
-CH
Cl
CH
-CHCl
CH
-CCl
ClCH
-CCl
Cl
CH-CCl
Cl
C-CCl
3
2
3
2
3
2
3
3
2
3
2
3
3
3
O
O
O
O
O
O
O
O
CH
-CH
-BH
CH
-CH
-OH
CH
-C
-H
HOCH
-C
-H
H-C
-C
-H
H-C
-C
-OH
HO-C
-C
-OH
3
2
2
3
2
3
2
O
O
O
O
O
OH
CH
-C
-OH
HOCH
-C
-OH
H-C
-CCl
Cl-C
-C
-Cl
H
C=C
3
2
3
2
H
O
O
O
HOCH
-CH
OH
CH
=C=O
H
NCH
-C
-OH
Cl
CH-C
-OH
HO-C
-C≡N
2
2
2
2
2
2
O
CH
-CH
-NH
CH
-CH=NH
CH
-C≡N
HO-CH
-C≡N
H-C
-C≡N
N≡C-C≡N
3
2
2
3
3
2
The oxidation level of any carbon fragment can then be defined as the sum of the number of C-X bonds and
number of π bonds to carbon (C=C, C≡C, C=O, C=N, C≡N). Two molecules with the same number of carbons and
the same sum are at the same oxidation state, and can, in principle, be interconverted without any redox chemistry,
or by an equal number of reductions and oxidations.

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