Reducing agent
Encyclopedia
A reducing agent is the element or compound in a reduction-oxidation (redox
) reaction that donates an electron to another species
; however, since the reducer loses an electron we say it is "oxidized". This means that there must be an "oxidizer"; because if any chemical is an electron donor (reducer), another must be an electron recipient (oxidizer). Thus reducers are "oxidized" and oxidizers are "reduced". For example, consider the following reaction:
The reducing agent in this reaction is ferrocyanide
. It donates an electron, becoming oxidized to ferricyanide
, simultaneously the oxidizer chlorine
is reduced to chloride
.
In organic chemistry
, reduction more specifically 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:
In organic chemistry, good reducing agents are reagents that deliver H2.
s 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 oxidize or lose electrons is known as its oxidation potential". The table below shows a few reduction potentials that could easily be changed to oxidation potential by simply reversing the sign. Reducing agents can be ranked by increasing strength by ranking their oxidation potentials. The reducing agent is stronger when it has a more positive oxidation potential and weaker when it has a negative oxidation potential. The following table provides the reduction potentials of the indicated reducing agent at 25 °C.
To tell which is the strongest reducing agent, one can change the sign of its respective reduction potential to make it oxidation potential. The bigger the number, the stronger the reducing agent. For example, among Na, Cr, Cu and Cl−, Na is the strongest reducing agent and Cl− is the weakest one.
Common reducing agents include metals potassium, calcium, barium, sodium and magnesium, and also compounds that contain the H− ion, those being NaH
, LiH
, LiAlH4 and CaH2.
Some elements and compounds can be both reducing or oxidizing agent
s. Hydrogen gas is a reducing agent when it reacts with non-metals and an oxidizing agent when it reacts with metals.
Hydrogen acts as an oxidizing agent because it accepts an electron donation from lithium, which causes Li to be oxidized.
Half reactions: 2 Li(s)0 → 2 Li(s)+ + 2 e−::::: H20(g) + 2 e− → 2 H−(g)
Hydrogen acts as a reducing agent because it donates its electrons to fluorine, which allows fluorine to be reduced.
Half reactions: H20(g) → 2 H+(g) + 2 e−:::::
F20(g) + 2 e− → 2 F−(g)
, which is the “degradation of metals as a result of electrochemical activity”. 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
.
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-reaction
s" that occur concurrently:
Iron (Fe) has been oxidized because the oxidation number increased and 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)
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
) reaction that donates an electron to another species
Chemical species
Chemical species are atoms, molecules, molecular fragments, ions, etc., being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a...
; however, since the reducer loses an electron we say it is "oxidized". This means that there must be an "oxidizer"; because if any chemical is an electron donor (reducer), another must be an electron recipient (oxidizer). Thus reducers are "oxidized" and oxidizers are "reduced". For example, consider the following reaction:
- 2 + → 2 + 2
The reducing agent in this reaction is ferrocyanide
Ferrocyanide
Ferrocyanide is the name of the anion Fe64−. In aqueous solutions, this coordination complex is relatively unreactive. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe6....
. It donates an electron, becoming oxidized to ferricyanide
Ferricyanide
Ferricyanide is the anion [Fe6]3−. It is also called hexacyanoferrate and in rare, but systematic nomenclature, hexacyanidoferrate...
, simultaneously the oxidizer chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
is reduced to chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
.
In organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, reduction more specifically refers to the addition of hydrogen to a molecule, though the aforementioned definition still applies. For example, benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
is reduced to cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
in the presence of a platinum catalyst:
- C6H6 + 3 H2 → C6H12
In organic chemistry, good reducing agents are reagents that deliver H2.
Characteristics of reducing agents
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 electronValence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
s 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
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
, the ability of an atom or molecule to attract bonding electrons, and species with relatively small ionization energies
Ionization energy
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
serve as good reducing agents too. "The measure of a material to oxidize or lose electrons is known as its oxidation potential". The table below shows a few reduction potentials that could easily be changed to oxidation potential by simply reversing the sign. Reducing agents can be ranked by increasing strength by ranking their oxidation potentials. The reducing agent is stronger when it has a more positive oxidation potential and weaker when it has a negative oxidation potential. The following table provides the reduction potentials of the indicated reducing agent at 25 °C.
| Oxidizing agent | Reducing agent | Reduction potential (V) |
|---|---|---|
| Li+ + e− = | Li | −3.04 |
| Na+ + e− = | Na | −2.71 |
| Mg2+ + 2e− = | Mg | −2.38 |
| Al3+ + 3e− = | Al | −1.66 |
| 2H2O(l) + 2e− = | H2(g) + 2OH − | −0.83 |
| Cr3+ + 3e− = | Cr | −0.74 |
| Fe2+ + 2e− = | Fe | −0.44 |
| 2H+ + e− = | H2 | 0.00 |
| Sn4+ + 2e− = | Sn2+ | |
| Cu2+ + e− = | Cu+ | |
| Ag+ + e− = | Ag | |
| Br2 + 2e− = | 2Br− | |
| Cl2 + 2e− = | 2Cl− | |
| MnO4− + 8H+ + 5e− = | Mn2+ + 4H2O |
To tell which is the strongest reducing agent, one can change the sign of its respective reduction potential to make it oxidation potential. The bigger the number, the stronger the reducing agent. For example, among Na, Cr, Cu and Cl−, Na is the strongest reducing agent and Cl− is the weakest one.
Common reducing agents include metals potassium, calcium, barium, sodium and magnesium, and also compounds that contain the H− ion, those being NaH
Nah
Nah, nah, or NaH may refer to:*Nah, Iran, a city in South Khorasan Province, Iran* Sodium hydride, whose chemical formula is NaH* The language code nah, which stands for Nahuatl...
, LiH
LIH
LIH may refer to:* Andrew Lih, American author of The Wikipedia Revolution* Lihue Airport, an airport on the island of Kauai, Hawaii, United States of America* Lithium hydride, or LiH, a Lithium compound in chemistry...
, LiAlH4 and CaH2.
Some elements and compounds can be both reducing or oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
s. 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)
Hydrogen acts as an oxidizing agent because it accepts an electron donation from lithium, which causes Li to be oxidized.
Half reactions: 2 Li(s)0 → 2 Li(s)+ + 2 e−::::: H20(g) + 2 e− → 2 H−(g)
- H2(g) + F2(g) → 2 HF(g)
Hydrogen acts as a reducing agent because it donates its electrons to fluorine, which allows fluorine to be reduced.
Half reactions: H20(g) → 2 H+(g) + 2 e−:::::
F20(g) + 2 e− → 2 F−(g)
Importance of reducing and oxidizing agents
Reducing agents and oxidizing agents are the ones responsible for corrosionCorrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
, which is the “degradation of metals as a result of electrochemical activity”. Corrosion requires an anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
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
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
.
Example of redox reaction
The formation of iron(III) oxide;- 4Fe + 3O2 → 2Fe2O3
In the above equation, the Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(Fe) has an oxidation number of 0 before and 3+ after the reaction. For oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O) the oxidation number began as 0 and decreased to 2−. These changes can be viewed as two "half-reaction
Half-reaction
A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.-Example:...
s" 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 and 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 hydrideLithium aluminium hydrideLithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters,...
(LiAlH4) - Nascent (atomic) hydrogenNascent hydrogenAtomic hydrogen is the species denoted by H , contrasted with dihydrogen, the usual 'hydrogen' commonly involved in chemical reactions. It is claimed to exist transiently but long enough to effect chemical reactions...
- Sodium amalgamSodium amalgamSodium amalgam, commonly denoted Na, is an alloy of mercury and sodium. The term amalgam is used for alloys, intermetallic compounds, and solutions involving mercury as a major component. Sodium amalgam is often used in reactions as strong reducing agents with better handling properties compared...
- Sodium borohydrideSodium borohydrideSodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
(NaBH4) - Compounds containing the Sn2+ ion, such as tin(II) chlorideTin(II) chlorideTin chloride is a white crystalline solid with the formula 2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent , and in electrolytic baths for tin-plating...
- SulfiteSulfiteSulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
compounds - HydrazineHydrazineHydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
(Wolff-Kishner reductionWolff-Kishner reductionThe Wolff–Kishner reduction is a chemical reaction that fully reduces a ketone to an alkane.The method originally involved heating the hydrazine with sodium ethoxide in a sealed vessel at about 180 °C. Other bases have been found equally effective...
) - Zinc-mercury amalgam (Zn(Hg)) (Clemmensen reductionClemmensen reductionClemmensen reduction is a chemical reaction described as a reduction of ketones to alkanes using zinc amalgam and hydrochloric acid. This reaction is named after Erik Christian Clemmensen, a Danish chemist....
) - Diisobutylaluminum hydride (DIBAH)
- Lindlar catalystLindlar catalystA Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead. The lead additive serves to deactivate the palladium sites. A variety of "catalyst poisons" have been used including lead acetate and lead oxide. The...
- Oxalic acidOxalic acidOxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...
(C2H2O4) - Formic acidFormic acidFormic acid is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early...
(HCOOH) - Ascorbic acidAscorbic acidAscorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form of vitamin C. The name is derived from a- and scorbutus , the...
(C6H8O6) - PhosphitePhosphiteA phosphite is a salt of phosphorous acid. The phosphite ion is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3...
s, hypophosphiteHypophosphiteThe hypophosphite ion is H2PO2−, hypophosphorous acid minus one hydrogen ion.Hypophosphites are the compounds containing this ion, with phosphorus in oxidation state +1, or similar covalent ones....
s, and phosphorous acidPhosphorous acidPhosphorous acid is the compound described by the formula H3PO3. This acid is diprotic , not triprotic as might be suggested by this formula. Phosphorous acid is as an intermediate in the preparation of other phosphorus compounds.-Nomenclature and tautomerism:H3PO3 is more clearly described with... - DithiothreitolDithiothreitolDithiothreitol is the common name for a small-molecule redox reagent known as Cleland's reagent. DTT's formula is C4H10O2S2 and the molecular structure of its reduced form is shown at the right; its oxidized form is a disulfide-bonded 6-membered ring . Its name derives from the four-carbon...
(DTT) – used in biochemistry labs to avoid S-S bonds - Compounds containing the Fe2+ ion, such as iron(II) sulfateIron(II) sulfateIron sulfate or ferrous sulfate is the chemical compound with the formula FeSO4. Known since ancient times as copperas and as green vitriol, the blue-green heptahydrate is the most common form of this material...
Several common reducing agents and their products
| Agent | Product |
|---|---|
| Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly... |
H+, H2O |
| Metals | metal ions |
| Hydrocarbons | CO2 carbon dioxide Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... , H2O |
See also
- Organic reduction
- Oxidizing agentOxidizing agentAn oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
- ElectronegativityElectronegativityElectronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
- ElectrochemistryElectrochemistryElectrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
- CorrosionCorrosionCorrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
- ElectrolyteElectrolyteIn chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
- RedoxRedoxRedox reactions describe all chemical reactions in which atoms have their oxidation state changed....
Further reading
- "Chemical Principles: The Quest for Insight", Third Edition. Peter Atkins and Loretta Jones p. F76