Reducing agents are molecules or atoms that themselves undergo oxidation and reduce other elements. Let us discuss some reducing agents and their property in brief.
There are few examples of reducing agents are listed below.
- Sodium Borohydride
- Lithium Aluminium hydride
- Diisobutyl aluminium hydride
- Wilkinson’s Catalyst
- Lindlar’s Catalyst
- Raney Nickel
- Sodium amalgam
- Zinc amalgam
- Sodium in Liq NH3
- Nascent hydrogen
- Sn/ HCl
- Aluminium isopropoxide
- Triphenyl Phosphine
- HI/Red P
- Sodium dithionate
- Ascorbic Acid
1. Sodium borohydride
Sodium Borohydride can easily reduce the carbonyl functionality especially ketones and aldehydes. It reduced ketones and aldehydes to -OH functionality. NaBH4 is weak in reducing agents, so it can reduce only ketones and aldehydes functionality. It can also reduce the ester group but in some experimental conditions.
2. Lithium aluminium hydride
Lithium Aluminium hydride can reduce carbonyl compounds as well as carboxylic acid, ester even nitriles also. It can reduce epoxides, acid, and ester molecules to the corresponding -OH group. If we need to selectively reduce some particular functionality, LiAlH4 reduced all functionalities.
3. Diisobutyl aluminium hydride
DIBAL- H can reduce different functionality at different temperature,
- At higher temperatures, it can reduce carboxylic acid and ester to the corresponding -OH group.
- At a lower temperature, it can also reduce carboxylic acid to aldehyde, lactones to lactols, Nitriles to aldehyde, and amines (2 equivalent of DIBAL-H).
- DIBAL-H also reduces α, β- unsaturated ester to corresponding allylic -OH by using 2 equivalents of reagent.
4. Wilkinson’s Catalyst
Wilkinson’s catalyst is an organometallic compound of Rh. It can reduce alkenes to alkanes and alkynes to alkenes. Wilkinson’s catalyst is a bulky molecule so it can reduce only less hindrance moiety. It is a 16 electron , planar geometry system.
5. Lindlar’s Catalyst
The reduction is done by hydrogen on the surface of the Pd metal. First alkynes are reduced by Lindlar’s catalyst to alkene and after that, it is reduced to alkane. If quinoline was used then the reaction was stopped after the formation of alkene. Then it is used as selective reduction.
There is electrons deficiency present over B, thus two borane units come closer to form diborane via a three-center two-electron bond. Diborane can reduce the acid group to the corresponding -OH functionality. Diborane makes aldehyde or ketones to borate ester and then borate esters quenched to -OH group.
7. Raney Nickel
Raney nickel can remove the protective sulfur group and reduce the moiety. It can also reduce the ketones to alkane by the process of protecting the group via thiol. Raney Ni can reduce the heteroatoms bond like Hydrazines, Nitrosamines, and Nitro group to simple alkanes.
8. Sodium amalgam
Sodium amalgam is often referred to as Na(Hg), it is allowed and easy to handle than free Na. It is a very powerful reducing agent. Sodium amalgam is mainly reduced to different types of ketones to the corresponding -OH groups. Na metal can easily release an electron which is further used for the reduction process.
9. Zinc amalgam
Sodium amalgam is used along with hydrochloric acid reduces the carbonyl group. Zn(Hg) reduces ketones or aldehydes to -CH3 or -CH2 moiety depending upon the surrounding environment. Zn can easily release two electrons and those are absorbed in the Hg surface and then it reduces the carbonyl molecules.
10. Sodium in Liq NH3
The solvated Na ion in ammonia released an electron and form a radical. That Na radical which release from the Na participates in the reduction reaction. Na in Liq NH3 reduces the alternated double bonds from the aromatic rings and reduces the molecule to diene. On reduction of naphthalene by Na in Liq NH3 we get 1,4-dihydro naphthalene.
11. Nascent hydrogen
Nascent hydrogen is more reactive than hydrogen because when nascent hydrogen is produced it is associated with higher energy. Nascent hydrogen can reduce the saturated moiety like a double bond or triple bond to unsaturated or less saturated. The geometry of the product after being reduced by the nascent hydrogen is cis.
It is the oxyanion of S. due to the presence of a negative charge it can behave as a reducing agent. It is a moderate reducing agent. It can reduce various nitro groups to aniline moiety and also be used as protecting group for several functions.
Sn can easily release electrons which can be used for reduction. It can reduce the aromatic nitro group to corresponding aniline moiety. It is a weak reducing agent or partial reducing agent.
14. Aluminium isopropoxide
Aluminium isopropxide is being used for Meerwein-Ponndorf-Verley reduction. It can be used for reduction of aromatic ketone to aromatic -OH group. In the presence of ethanol the reduction was take placed. It is the reverse of the oppenauer oxidation reaction.
15. Triphenyl Phosphine
Triphenyl phosphine is a ligand rather than a reagent and cab be accepted electrons. It can reduced phenyl hydrazine to phosphinimine. The reaction took place in presence of water molecule. It is named as Staudinger reaction.
when Hydrazine is used along with KOH for reduction of aromatic ester then the reaction knows as Wolff–Kishner reduction. This reagent can be used for reduction of aromatic ester to aromatic hydrocarbon, it can reduce the C=O functionality to -CH2 moeity. Strong base will be needed for this reduction reaction.
17. HI/Red P
I containing species can be used for the reduction , because I has higher reduction potential. HI along with Red P is being used for reduction of alkene to alkane functionality. IT can also reduce any functionality to alkane moiety.
18. Sodium dithionate
Sodium dithionate is a good reducing agent in analytical chemistry. The S-S bond in Na2S2O6 can easily be cleaved and reduced several reducible moiety. It can reduced several metal oxide to free metal.
19. Ascorbic Acid
Ascorbic acid is known as Vitamin C. It is a necessary reagent for human life. It can reduce graphene oxide under a mild condition.
Oxidizing reaction example
Any reactions done by reducing agent is known as oxidising reaction. There are some examples are listed below,
- 4Fe + 3O2 = 2Fe2O3
- C + O2 = CO2
- Fe + S = FeS
- 2KI + H2O2 → I2 + 2 KOH
- H2S + Br2 → 2 HBr + S
Reducing agents oxidized themselves to reduce other elements or molecules. Every reaction has two redox reactions one is an oxidizing reaction and the other reducing reaction. Reducing agents are involved in the oxidizing reactions only.