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Introduction to Oxidation and Reduction

Oxidation and Reduction in Organic Chemistry

In organic chemistry, the concepts of oxidation and reduction are less defined and handled less systematically than in inorganic chemistry. In most cases, only those reactions are considered oxidations which formally include the removal of H or the addition of O.

Fig.1: Addition of H

Alkenes to alkanes

Fig.2: Addition of H

Acetaldehyde to ethanol

Fig.3: Substitution of O by H

Acetic acid to ethanol

Fig.4: Removal of H

Isopropanol to acetone

Fig.5: Addition of O

Acetaldehyde to acetic acid

Principally, this historic view is outmoded as evidenced in the formal writing where [O] and [H] are used for oxidations and reductions, respectively, and in which the corresponding reagents are not defined specifically. Example:

Fig.6: A is being reduced to B

Strictly speaking, the following reactions could also be considered oxidations:

Fig.7: Substitution

Methane to tetrachloromethane

Fig.8: Addition

Alkene to dibromoalkane

Usually, the first reaction would be classified mechanistically as "substitution" (exchange of H by Cl) and the second one as "addition" (addition of bromine).

The removal (oxidation) or addition (reduction) of electrons leading to the formation of charged species is called electron transfer.

Fig.9: Oxidation

Fig.10: Reduktion

One of the outstanding examples is the reduction of $C_{60}$ (Buckminsterfullerene) with extremely strong reducing agents (e.g. ${C_{20} H_{10}}^{4 -}$ ) or electrochemically to the tetraanion.

Fig.11

As a basic rule, the following definitions apply to oxidation and reduction reactions in organic chemistry:

Definition of reduction in organic chemistry: Reactions are classified as reductions when the oxidation state of C-atoms in the substrate or the oxidation number of a heteroatom decrease.

Definition of oxidation in organic chemistry: Reactions are classified as oxidations when the oxidation state of C-atoms in the substrate or the oxidation number of a heteroatom increase.