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OXIDATION AND REDUCTION IN ORGANIC CHEMISTRY
In ionic and free radical reactions, oxidation and reduction are defined as processes by which an element
undergoes a net loss or gain of electrons, respectively. The concept as applied to organic covalent compounds,
where elements share electrons rather than losing or gaining them is the same, but it’s frequently simplified
and narrowed down to make it easier to recognize these processes. Therefore it must be kept in mind that,
while the following definition is grossly simplified, it serves the goal of quickly identifying oxidation and
reduction processes in many organic reactions.
In reference to organic molecules, oxidation is a process by which a carbon atom gains bonds to more
electronegative elements, most commonly oxygen. Reduction is a process by which a carbon atom gains
bonds to less electronegative elements, most commonly hydrogen.
The following chart summarizes these concepts when applied to organic transformations. [ox] stands for
oxidation, and [red] stands for reduction.
OXIDATION
H
[ox]
H C H
H C OH
H
[red]
alkane
O
[ox]
H
[red]
H
H
C
O
[ox]
H
H
[red]
Most highly
reduced state
OH
O C O
[red]
(CO2)
carboxylic
acid
aldehyde
alcohol
C
[ox]
carbon
dioxide
Most highly
oxidized state
REDUCTION
Some points must be noted. First, a gain or loss of bonds means simply more or less bonds. The double bond
counts as two bonds and the triple bond counts as three bonds. Thus in the carbonyl group (C=O) carbon
is considered to have two bonds to oxygen. Therefore, this carbon has a higher oxidation state than the
alcohol carbon, which has only one bond to oxygen. In routine terminology, it is said that the aldehyde is
a more highly oxidized functional group than the alcohol. Oxidation reactions are therefore those in which
the central carbon of a functional group is transformed into a more highly oxidized form, and reduction
reactions are those in which the central carbon is transformed into a more highly reduced form.
Second, there can be several functional groups where the central carbon has the same oxidation state. For
example, the carbons bonded to oxygen in alcohols and ethers have the same oxidation state. Likewise for
aldehydes and their hydrated forms, and for carboxylic acids and their derivatives. However, most references
to oxidation and reduction reactions in organic chemistry textbooks involve the functional groups presented
in the chart above.
H
H C OH
H
alcohol
H
H
H C O C H
H
H
ether
OH
O
H
C
H C H
H
aldehyde
OH
aldehyde hydrate
The chart presented before shows the oxidation and reduction states for a molecule that contains only one
carbon. But most organic compounds contain more than one carbon. The maximum oxidation state that a
particular carbon can attain depends on how many other carbons it must remain attached to. For example,
a molecule with two carbon atoms could not be oxidized all the way to carbon dioxide because the carbon
atom in CO2 must have four bonds to oxygen, leaving no room for bonds to other carbons. The maximum
oxidation state that a carbon can attain decreases gradually as the number of bonds to other carbons
increases. Thus, the maximum oxidation state possible for a carbon that’s bonded to one other carbon is
the carboxylic acid stage, and so on. The following chart illustrates this idea.
ONE SINGLE CARBON
H
H C H
O
[ox]
H
[ox]
H C OH
H
[red]
[red]
H
alcohol
alkane
(methane)
H
C
O
[ox]
H
H
[red]
C
[ox]
OH
O C O
[red]
carboxylic
acid
aldehyde
carbon dioxide
(CO2)
CARBON BONDED TO ONE OTHER CARBON
H
[ox]
[ox]
[red]
H
[red]
H3C
1o alcohol
alkane
[ox]
O
H3C C OH
H3C C H
H
H
C
H
[red]
(1 carbon)
CARBON BONDED TO TWO OTHER CARBONS
H3C C
[ox]
H3C C
CH3
[red]
H
OH
[ox]
CH3
H
[red]
2o alcohol
alkane
(2o carbon)
CARBON BONDED TO THREE OTHER CARBONS
CH3
H3C C
[ox]
H3C C
CH3
H
alkane
(3o carbon)
CH3
[red]
CH3
OH
3o alcohol
O
H3C
C
H3C
C
OH
carboxylic
acid
aldehyde
o
H
O
CH3
ketone