Oxidizing agent

An oxidizing agent (also called an oxidant or oxidizer) can be defined as either:

1. a chemical compound that readily transfers oxygen atoms, or
2. a substance that gains electrons in a redox chemical reaction

The former definition is not applicable to what most people read about, but it is the sense in which most organic chemists use the term. In both cases, the oxidizing agent becomes reduced in the process.

In simple terms:

• The oxidizing agent is reduced.
• The reducing agent is oxidized.
• All atoms in a molecule can be assigned an oxidation number. This number changes when an oxidant acts on a substrate.
• Redox reactions occur when electrons are exchanged.

A mnemonic for differentiating the reactions is "OIL RIG": Oxidation Is Loss, Reduction Is Gain (of electrons) or "LEO the lion says GER" (Lose Electrons: Oxidation, Gain Electrons: Reduction)

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Example of oxidation

The formation of iron(III) oxide;

4Fe + 3O₂ → 2Fe₂O₃

In the above equation, the Iron (Fe) has an oxidation number of 0 before and 3+ after the reaction. For oxygen (O) the oxidation number began as 0 and decreased to 2−. These changes can be viewed as two "half-reactions" that occur concurrently:

1. Oxidation Half Reaction: Fe⁰ → Fe³⁺ + 3e⁻
2. Reduction Half Reaction: O₂ + 4e⁻ → 2 O²⁻

Iron III (Fe) has been oxidized because the oxidation number increased and is the reducing agent because it gave electrons to the oxygen (O). Oxygen (O) has been reduced because the oxidation number has decreased and is the oxidizing agent because it took electrons from iron (Fe)

Alternate meanings

Because the process of oxidation is so widespread (explosives, chemical synthesis, corrosion), the term oxidizing agent has acquired multiple meanings.

One definition, an oxidizing agent receives - or accepts - electrons from a reagent. In this context, the oxidizing agent is called an electron acceptor. A classic oxidizing agent is the ferrocenium ion [Fe(C₅H₅)₂]⁺ which accepts an electron to form Fe(C₅H₅)₂. Of great interest to chemists are the details of the electron transfer event, which can be described as inner sphere or outer sphere.

In another more colloquial usage, an oxidizing agent transfers oxygen atoms to the substrate. In this context, the oxidizing agent can be called an oxygenation reagent or oxygen-atom transfer agent. Examples include [MnO₄]⁻ permanganate, [CrO₄]²⁻ chromate, OsO₄ osmium tetroxide, and especially ClO₄ perchlorate. Notice that these species are all oxides, and in fact, polyoxides. In some cases, these oxides can also serve as electron acceptors, as illustrated by the conversion of [MnO₄]⁻ to [MnO₄]²⁻, manganate.

The strict dangerous goods definition of an oxidizing agent are substances that, while in themselves not necessarily combustible, may, generally by yielding oxygen, cause, or contribute to, the combustion of other material. (Australian Dangerous Goods Code 6th Edition) By this definition some materials that are classified as oxidizing agents by analytical chemists are not classified as oxidizing agents in a dangerous goods sense. An example is potassium dichromate which does not pass the dangerous goods test of an oxidizing agent.

Common oxidizing agents

• Hypochlorite and other hypohalite compounds such as Bleach
• Iodine and other halogens
• Chlorite, chlorate, perchlorate, and other analogous halogen compounds
• Permanganate salts
• Ammonium cerium(IV) nitrate and probably related Cerium(IV) compounds
• Hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide, Pyridinium chlorochromate (PCC), and chromate/dichromate compounds
• Peroxide compounds
• Tollen's Reagent
• Sulfoxides
• Persulfuric acid
• Ozone
• Osmium tetroxide (OsO₄)
• Nitric acid
• Nitrous oxide (N₂O)

Common oxidizing agents and their products

See also

• Reducing agent
• Organic oxidation