Reducing agent

Reducing agent (also called a reductant or reducer) is an element or compound that loses (or "donates") an electron to an electron recipient (oxidizing agent) in a redox chemical reaction.

A reducing agent is thus oxidized when it loses electrons in the redox reaction. Reducing agents "reduce" (or, are "oxidized" by) oxidizing agents. Oxidizers "oxidize" (that is, are reduced by) reducers.

Historically, reduction referred to the removal of oxygen from a compound, hence the name 'reduction'. The modern sense of donating electrons is a generalisation of this idea, acknowledging that other components can play a similar chemical role to oxygen.

In their pre-reaction states, reducers have extra electrons (that is, they are by themselves reduced) and oxidizers lack electrons (that is, they are by themselves oxidized). A reducing agent typically is in one of its lower possible oxidation states and is known as the electron donor. Examples of reducing agents include the earth metals, formic acid, oxalic acid, and sulfite compounds.

For example, consider the overall reaction for aerobic cellular respiration:

C₆H₁₂O₆(s) + 6O₂(g) → 6CO₂(g) + 6H₂O(l)

The oxygen (O₂) is being reduced, so it is the oxidizing agent. The glucose (C₆H₁₂O₆) is being oxidized, so it is the reducing agent.

In organic chemistry, reduction usually refers to the addition of hydrogen to a molecule, though the aforementioned definition still applies. For example, benzene is reduced to cyclohexane in the presence of a platinum catalyst:

C₆H₆ + 3 H₂ → C₆H₁₂

Characteristics

Consider the following reaction:

2 [Fe(CN)₆]⁴⁻ + Cl
₂ → 2 [Fe(CN)₆]³⁻ + 2 Cl⁻

The reducing agent in this reaction is ferrocyanide ([Fe(CN)₆]⁴⁻). It donates an electron, becoming oxidized to ferricyanide ([Fe(CN)₆]³⁻). Simultaneously, the oxidizer chlorine is reduced to chloride.

Strong reducing agents easily lose (or donate) electrons. An atom with a relatively large atomic radius tends to be a better reductant. In such species, the distance from the nucleus to the valence electrons is so long that these electrons are not strongly attracted. These elements tend to be strong reducing agents. Good reducing agents tend to consist of atoms with a low electronegativity, the ability of an atom or molecule to attract bonding electrons, and species with relatively small ionization energies serve as good reducing agents too. The measure of a material to reduce, or gain electrons, is known as its reduction potential.[1] The table below shows a few reduction potentials (which can be changed to oxidation potentials by reversing the sign). Reducing agents can be ranked by increasing strength by ranking their reduction potentials. The reducing agent is stronger when it has a more negative reduction potential and weaker when it has a more positive reduction potential. The following table provides the reduction potentials of the indicated reducing agent at 25 °C. For example, among Na, Cr, Cu⁺ and Cl⁻, Na is the strongest reducing agent and Cl⁻ is the weakest one.

{\begin{array}{|rl|r|}{\text{Oxidizing agent}}&\qquad {\text{Reducing agent}}&{\text{Reduction potential (V)}}\\\hline {\ce {{Li+}+e-}}&{\ce {<=>Li}}&-3.04\\{\ce {{Na+}+e-}}&{\ce {<=>Na}}&-2.71\\{\ce {{Mg^{2+}}+2e-}}&{\ce {<=>Mg}}&-2.38\\{\ce {{Al^{3+}}+3e-}}&{\ce {<=>Al}}&-1.66\\{\ce {{2H2O(l)}+2e-}}&{\ce {<=>{H2(g)}+2OH-}}&-0.83\\{\ce {{Cr^{3+}}+3e-}}&{\ce {<=>Cr}}&-0.74\\{\ce {{Fe^{2+}}+2e-}}&{\ce {<=>Fe}}&-0.44\\{\ce {{2H+}+2e-}}&{\ce {<=>H2}}&0.00\\{\ce {{Sn^{4+}}+2e-}}&{\ce {<=>Sn^{2+}}}&+0.15\\{\ce {{Cu^{2+}}+e-}}&{\ce {<=>Cu+}}&+0.16\\{\ce {{Ag+}+e-}}&{\ce {<=>Ag}}&+0.80\\{\ce {{Br2}+2e-}}&{\ce {<=>2Br-}}&+1.07\\{\ce {{Cl2}+2e-}}&{\ce {<=>2Cl-}}&+1.36\\{\ce {{MnO4^{-}}+{8H+}+5e-}}&{\ce {<=>{Mn^{2+}}+4H2O}}&+1.49\\{\ce {{F2}+2e-}}&{\ce {<=>2F-}}&+2.87\end{array}}

[2]

Common reducing agents include metals potassium, calcium, barium, sodium and magnesium, and also compounds that contain the H⁻ ion, those being NaH, LiH,[3] LiAlH₄ and CaH₂.

Some elements and compounds can be both reducing or oxidizing agents. Hydrogen gas is a reducing agent when it reacts with non-metals and an oxidizing agent when it reacts with metals.

2 Li_(s) + H₂_(g) → 2 LiH_(s)[lower-alpha 1]

Hydrogen acts as an oxidizing agent because it accepts an electron donation from lithium, which causes Li to be oxidized.

H₂_(g) + F₂_(g) → 2 HF_(g)[lower-alpha 2]

Hydrogen acts as a reducing agent because it donates its electrons to fluorine, which allows fluorine to be reduced.

Importance

Reducing agents and oxidizing agents are the ones responsible for corrosion, which is the "degradation of metals as a result of electrochemical activity".[1] Corrosion requires an anode and cathode to take place. The anode is an element that loses electrons (reducing agent), thus oxidation always occurs in the anode, and the cathode is an element that gains electrons (oxidizing agent), thus reduction always occurs in the cathode. Corrosion occurs whenever there’s a difference in oxidation potential. When this is present, the anode metal begins deteriorating, given there is an electrical connection and the presence of an electrolyte.

Example of redox reaction

The formation of iron(III) oxide;

4Fe + 3O₂ → 4Fe³⁺ + 6O²⁻ → 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:

Oxidation half reaction: Fe⁰ → Fe³⁺ + 3e⁻
Reduction half reaction: O₂ + 4e⁻ → 2 O²⁻

Iron (Fe) has been oxidized because the oxidation number increased. Iron is the reducing agent because it gave electrons to the oxygen (O₂). Oxygen (O₂) has been reduced because the oxidation number has decreased and is the oxidizing agent because it took electrons from iron (Fe).

Common reducing agents

Lithium aluminium hydride (LiAlH₄), a very strong reducing agent
Nascent (atomic) hydrogen
Hydrogen without or with a suitable catalyst e.g. a Lindlar catalyst
Sodium amalgam (Na(Hg))
Sodium-lead alloy (Na + Pb)
Zinc amalgam (Zn(Hg)) (reagent for Clemmensen reduction)
Diborane
Sodium borohydride (NaBH₄)
Compounds containing the Fe²⁺ ion, such as iron(II) sulfate
Compounds containing the Sn²⁺ ion, such as tin(II) chloride
Sulfur dioxide (sometimes also used as an oxidizing agent), Sulfite compounds
Dithionates, e.g. Na₂S₂O₆
Thiosulfates, e.g. Na₂S₂O₃ (mainly in analytical chemistry)
Iodides, e.g. KI (mainly in analytical chemistry)
Hydrogen peroxide (H
₂O
₂) – mostly an oxidant but can occasionally act as a reducing agent (typically in analytical chemistry.)
Hydrazine (Wolff-Kishner reduction)
Diisobutylaluminium hydride (DIBAL-H)
Oxalic acid (C
₂H
₂O
₄)
Formic acid (HCOOH)
Ascorbic acid (C₆H₈O₆)
Reducing sugars
Phosphites, hypophosphites, and phosphorous acid
Dithiothreitol (DTT) – used in biochemistry labs to avoid S-S bonds
Carbon monoxide (CO)
Cyanides in hydrochemical metallurgical processes
Carbon (C)
Tris-2-carboxyethylphosphine hydrochloride (TCEP)

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Notes

Half reactions: 2 Li⁰_(s) → 2 Li⁺_(s) + 2 e⁻ ::::: H₂⁰_(g) + 2 e⁻ → 2 H⁻_(g)
Half reactions: H₂⁰_(g) → 2 H⁺_(g) + 2 e⁻ ::::: F₂⁰_(g) + 2 e⁻ → 2 F⁻_(g)

References

"Electrode Reduction and Oxidation Potential Values". www.siliconfareast.com. Retrieved 29 March 2018.
"Standard Electrode Potentials". hyperphysics.phy-astr.gsu.edu. Retrieved 29 March 2018.
Aufray M, Menuel S, Fort Y, Eschbach J, Rouxel D, Vincent B (2009). "New Synthesis of Nanosized Niobium Oxides and Lithium Niobate Particles and Their Characterization by XPS Analysis" (PDF). Journal of Nanoscience and Nanotechnology. 9 (8): 4780–4789. doi:10.1166/jnn.2009.1087. PMID 19928149.

External links

Table summarizing strength of reducing agents at the Wayback Machine (archived June 11, 2011)

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[4] Half reactions: 2 Li⁰_(s) → 2 Li⁺_(s) + 2 e⁻ ::::: H₂⁰_(g) + 2 e⁻ → 2 H⁻_(g)

[5] Half reactions: H₂⁰_(g) → 2 H⁺_(g) + 2 e⁻ ::::: F₂⁰_(g) + 2 e⁻ → 2 F⁻_(g)

[sfe-1] "Electrode Reduction and Oxidation Potential Values". www.siliconfareast.com. Retrieved 29 March 2018.

[2] "Standard Electrode Potentials". hyperphysics.phy-astr.gsu.edu. Retrieved 29 March 2018.

[3] Aufray M, Menuel S, Fort Y, Eschbach J, Rouxel D, Vincent B (2009). "New Synthesis of Nanosized Niobium Oxides and Lithium Niobate Particles and Their Characterization by XPS Analysis" (PDF). Journal of Nanoscience and Nanotechnology. 9 (8): 4780–4789. doi:10.1166/jnn.2009.1087. PMID 19928149.