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Electrons are transferred from one chemical substance to another in a chemical reaction. Oxidation-reduction reactions or redox reactions are termed this way because of their electron-transfer characteristics. Different meanings can undergo oxidation and reduction reactions by adding oxygen or hydrogen simultaneously. Oxidation occurs when a reaction causes electrons to be lost. By gaining electrons during a reaction, reduction occurs. In a reduction-oxidation reaction, oxidation and reduction both co-occur. We often use various organic chemical reactions to synthesise essential compounds in organic chemistry. Two such reactions are the Clemmensen and Wolff Kishner reductions.
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Wolff Kishner Oxidation-Reduction
Organic chemistry uses the Wolff-Kishner reduction to generate methylene groups from carbonyl functionalities. N. Kishner and Ludwig Wolff independently discovered the Wolff–Kishner reduction in 1911 and 1912, respectively. In a reaction of a pre-formed hydrazone with hot potassium hydroxide that contained a crumpled platinized porous plate, Kishner observed the production of corresponding hydrocarbons.
Using ethanol solutions of semicarbazones or hydrazones, Wolff achieved the same result by heating them in a sealed tube at 180°C under the influence of sodium ethoxide. While Kishner’s method does not require a sealed box, both methodologies failed to demonstrate reliability.
Mechanism
Aldehydes or ketones are subjected to hydrazine. This produces hydrazone for the process.
The terminal nitrogen atom deprotonates and forms a double bond with its nitrogen atom. Once released into the immediate environment, a proton attaches to the hydroxide ion, including water.
Since oxygen is an electron-withdrawing, the water molecule protonates the carbon.
Once again, the terminal nitrogen is deprotonated to form a triple bond with its neighbour. Two triple-bonded nitrogens are thus released as nitrogen gas from a carbanion.
Water protonates carbon, forming the desired hydrocarbon. As a result, the aldehyde or ketone is converted into an alkane. The reaction rate is determined as the terminal carbon forms a hydrogen bond.
Mildly electron-withdrawing substituents help carbon-hydrogen bonds forming. Negative charges on the terminal nitrogen are decreased by electron-withdrawing substituents, making it harder to break the N-H bond.
Applications
Multiwalled carbon nanotubes are a perfect example of this reaction’s broad applicability.
Huang Minlon’s modification of the Wolff-Kishner reduction allowed Pettus and Green to reduce tricyclic carbonyl compounds in 2011. Decarbonylation of tricyclic allylic acetate containing ketone failed despite several attempts, but now the acetate functionality must be removed for the Wolff-Kishner reduction to succeed. Finally, oxyplumbation was then used to prepare the allylic alcohol.
It was also used to make a functionalized imidazole substrate on a kilogram scale using the Wolff-Kishner reduction.
To improve the Wolff-Kishner reduction, much effort has been devoted to increasing the yield of the hydrazone intermediate by removing water and accelerating hydrazone decomposition by raising reaction temperature. A range of newer modifications has been developed, which provide significant advancements and allow reactions to occur under relatively mild conditions.
Clemmensen Oxidation Reduction
The first report of this reaction was made by Clemmensen in 1913. That is why Erik Christian Clemmensen, a Danish chemist, is credited with inventing the Clemmensen reduction. Carbonyl groups (in aldehydes and ketones) are reduced to form methylene groups.
It is a reaction carried out using zinc amalgam and hydrochloric acid and is the Clemmensen reduction. Clemmensen reduction, such as those created during Friedel-Crafts acylation, works particularly well for reducing aryl-alkyl ketones. Clemmensen reductions are often accomplished by converting Friedel-Crafts acyl benzenes into alkylbenzenes. However, they may also be performed with acid-insensitive aldehydes or ketones.
With the carbonyl compound and amalgamated zinc, it is heated up. The reduction occurs as a result of complex mechanisms on the zinc surface. When strong acids are present, zinc and mercury are used in the Clemmensen reduction.
Mechanism
Two different hypotheses are proposed for the mechanism of this reaction;
Carbanionic mechanism: It appears that the zinc attacks the protonated carbon directly in the carbanionic agent.
Carbenoid mechanism: A carbenoid mechanism reduces the events on zinc metal surfaces. Reduction occurs on the zinc catalyst surface. Alcohols are not considered intermediates in this reaction, as alkanes are not formed when these same reaction conditions are applied to corresponding alcohols.
Wolff-Kishner reductions containing a solid base can react with acid-sensitive base substances if they are milder than Mozingo reductions. Substances that are sensitive to acids are not for this reaction.
Even though the mechanism is ancient, it remains obscure, and studies on it are complex because of its heterogeneous nature. In addition, there have been few studies on the reaction proposed, including possible organozinc intermediates and zinc carbenoids.
Applications
Carbonyl groups are commonly converted into ethylene groups through this reaction.
Polycyclic aromatics and aromatics containing unbranched side chains can also be prepared using this method.
This reaction accomplishes the reduction of aliphatic and mixed aliphatic-aromatic carbonyl compounds.
Conclusion
During organic synthesis and biochemical transformation, oxidation and reduction are significant reactions. Therefore, knowing the reagents and methods of oxidizing and reducing the various types of organic compounds is very useful. Oxidation occurs when a functional group within a molecule is elevated from a lower oxidation state to a higher oxidation state, and reduction occurs when the opposite happens. For example, Clemmensen and Wolff-Kishner drops can reduce aldehydes and ketones to hydrocarbons.
