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Wolff-Kishner Reduction Mechanism

Introduction to Wolff-Kishner Reduction Mechanism

The Wolff-Kishner reduction process starts with the formation of a hydrazone anion, which then releases the nitrogen atom, resulting in the formation of a carbanion. After reacting with the water in the environment, the carbanion forms a hydrocarbon. Diethylene glycol is commonly used as a solvent in this form.

This article will study Wolff-Kishner's reduction mechanism, Wolff-Kishner reaction examples, and Wolff-Kishner's reagent.


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The percent is an organic reaction that converts aldehydes and ketones into alkanes. Since certain carbonyl compounds are stable under strongly basic conditions, they can be reduced to alkanes quickly (The carbon-oxygen double bond becomes two carbon-hydrogen single bonds). While the process normally starts with the condensation of hydrazine to form a hydrazone, using a preformed hydrazone may have benefits such as shorter reaction times, room-temperature reactions, or very mild reaction conditions. Different solvents and reaction temperatures are needed for the preformed hydrazone substrates that can be used in this reduction.

In organic chemistry, the Wolff–Kishner reduction reaction is used to transform carbonyl functionalities into methylene groups. The Wolff–Kishner reduction is incompatible with base-sensitive substrates because it necessitates extremely simple conditions. In certain cases, sterically hindered carbonyl groups will not form the necessary hydrazone, preventing the reaction. For compounds with acid-sensitive functional groups, such as pyrroles, and for high-molecular-weight compounds, this approach may be superior to the associated Clemmensen reduction.

N. Kishner and Ludwig Wolff separately discovered the Wolff Kishner reaction in 1911 and 1912. Kishner discovered that when pre-formed hydrazone was added to hot potassium hydroxide containing crushed platinized porous plate, the corresponding hydrocarbon was formed.


Wolff Kishner Reduction Reaction Mechanism

Step 1: Szmant and colleagues investigated the mechanism of the Wolff–Kishner reduction. The formation of a hydrazone anion 1 by deprotonation of the terminal nitrogen by MOH, according to Szmant's study, is the first step in this reaction. As semicarbazones are used as substrates, they are first converted into the corresponding hydrazone, then deprotonated. The formation of a new carbon-hydrogen bond at the carbon terminal in the delocalized hydrazone anion appears to be the rate-determining step, according to a variety of mechanistic results.


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Step 2: The terminal nitrogen atom is deprotonated and forms a double bond with the nitrogen atom next to it. The proton that has been released binds to the hydroxide ion in the basic environment to form water.


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Step 3: Since oxygen withdraws more electrons than carbon, the water molecule protonates the carbon, as seen below.


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Step 4: The terminal nitrogen is deprotonated once more, forming a triple bond with the nitrogen atom next to it. The two triple-bonded nitrogens are released as nitrogen dioxide, resulting in the formation of a carbanion. The ejected proton forms water with the basic atmosphere, similar to phase 2.


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Step 5: The carbon is protonated by water, similar to step 3 of the Wolff Kishner reduction process, resulting in the formation of the desired hydrocarbon product as shown. As a result, the aldehyde or ketone is transformed into an alkane.


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The formation of a bond between the terminal carbon and hydrogen is the rate-determining step in this reaction (in the hydrazone anion). Mildly electron-withdrawing substituents aid in the formation of carbon-hydrogen bonds. The negative charge of the terminal nitrogen is reduced by highly electron-withdrawing substituents, making it more difficult to split the N-H bond. The Wolff Kishner reduction has been adapted into many techniques, each with its own set of benefits and drawbacks. For example, the Huang Minlon modification (which uses the carbonyl compound, 85 percent hydrazine, and potassium hydroxide as the reagent) allows for faster reaction times and higher temperatures, but it necessitates distillation.


Did You Know?

Clemmensen reduction is a chemical reaction in which ketones (or aldehydes) are converted to alkanes with the aid of zinc amalgam and concentrated hydrochloric acid. Erik Christian Clemmensen, a Danish chemist, was the inspiration for this reaction. Aryl-alkyl ketones, such as those produced in a Friedel-Crafts acylation, react well to the original Clemmensen reduction conditions. A classic technique for primary alkylation of arenes is a two-step sequence of Friedel-Crafts acylation followed by Clemmensen reduction. Changed Clemmensen conditions using activated zinc dust in an anhydrous solution of hydrogen chloride in diethyl ether or acetic anhydride are much more successful with aliphatic or cyclic ketones.


Difference between Clemmensen Reduction and Wolff-Kishner Reaction

Both Clemmensen Reduction and Wolff-Kishner processes by reducing the functional groups. That is why both the reactions need to meet specific conditions and catalysts for the process to work properly. Below are the key differences between Clemmensen Reduction and Wolff-Kishner Reactions: 

  • In the Clemmensen reduction reaction, we try to convert ketones and aldehydes into an alkane. On the other hand, the Wolff-Kishner reaction is used to convert a carbon functional group into a methylene group. 

  • In the Clemmensen reduction process, we use a catalyst, named amalgamated zinc. However, you do not need a catalyst to perform the Wolff-Kishner Reduction process.

  • A major difference between Clemmensen and Wolff-Kishner Reactions is that the former uses strong acidic conditions to work, making it unsuitable for acid-sensitive substrates. On the contrary, the Wolff Kishner reduction reaction uses basic conditions, making it unsuitable for base-sensitive substrates. 


Benefits of learning the Wolff-Kishner Reduction Mechanism - Explanation, and FAQs

Learning the Wolff-Kishner Reduction Mechanism will be quite beneficial for you. It is one of the most important concepts of Chemistry, which is why you must have a strong grasp of this topic. Before you start the Wolff-Kishner Reduction Mechanism, you should first go through the textbook explanations of the concept. It will give you an idea of what the concept is and why it is important in Chemistry. Moreover, you should also use the exercise questions to test your knowledge and check whether you have understood the Wolff-Kishner Reduction Mechanism or not. Below are some of the advantages of learning the Wolff-Kishner Reduction Mechanism: 

  • With the Wolff-Kishner Reduction Mechanism, you will get to learn how to convert a carbon functional group into a methylene functional group. 

  • By understanding the concept of the Wolff-Kishner Reduction Mechanism, you can enhance your knowledge of organic chemistry, which is a crucial part of the Chemistry syllabus.

  • Once you have learned the Wolff-Kishner Reduction Mechanism, you won’t have to go through the entire chapter again during your revisions and exam preparations. 

  • The Wolff-Kishner Reduction Mechanism is one of the most important concepts of Chemistry, carrying a significant weightage in the final exam. So, if you strengthen your knowledge of the Wolff-Kishner Reduction Mechanism, you can score well in your exams.

  • When you go through the Wolff-Kishner Reduction Mechanism thoroughly, you will be able to tell the difference between this organic reaction and the Clemmensen reduction reaction. 

  • If you learn the Wolff-Kishner Reduction Mechanism from  Vedantu’s online learning platform, you will get a better understanding of the concept. You don’t have to pay any money to start learning from our website or mobile application.

FAQs on Wolff-Kishner Reduction Mechanism

1. What happens in the Wolff-Kishner reaction?

The Wolff Kishner reduction process starts with the formation of a hydrazone anion, which then releases the nitrogen atom, resulting in the formation of a carbanion. The carbanion reacts with the water in the system to form a hydrocarbon. Aldehydes and ketones are reduced to alkanes through this organic reaction.

2. Which reagent is used in Wolff-Kishner Reduction?

In the Wolff Kishner ketone reduction, hydrazine (NH2NH2) is used as the reducing agent in the presence of a solid base (KOH) in a high-boiling protic solvent (ethylene glycol, HO-CH2-CH2-OH, boiling point 197 °C). The conversion of hydrazine to nitrogen gas is the reaction's driving force. 

3. Is Wolff-Kishner effective at Reducing Amides?

Like the example above shows, amides are normally not suitable substrates for the Wolff–Kishner reduction. However, Coe and colleagues discovered that under Wolff–Kishner conditions, a twisted amide can be efficiently reduced. Both the authors explained this observation using the stereoelectronic bias of the substrate that prevents the formation of anti–Bredt iminium ions and therefore favours the ejection of formation of alcohol and hydrazone. Due to the torsional restraints, resonance stabilization is prevented. That is why the amide functionality is considered isolated amine and ketone functionalities in this strained substrate.

4. What are the limitations of the Wolff-Kishner Reduction Mechanism?

The Wolff-Kishner Reduction Mechanism has three significant limitations. These are as follows: 

  • It cannot be performed at low temperatures.
  • It cannot be used for sterically hindered ketones.
  • Even though the Wolff-Kishner Reduction process is used for carbonyl groups only, sometimes instead of the expected mechanism of the reaction, the migration of the double bond takes place whenever an unwanted product such as alpha, beta-unsaturated carbonyl compounds are formed.

5. How to learn the concept of Wolff-Kishner Reduction Mechanism?

You can start learning the Wolff-Kishner Reduction Mechanism from our e-learning platform i.e. Vedantu.com. For a much better experience, you can download Vedantu’s mobile app from the play store or the app store and start studying the concept of Wolff-Kishner Reduction Mechanism for absolutely free. With an abundance of study materials for the Chemistry subject, Vedantu provides you with a free learning platform to make the learning process smooth and steady. Apart from this topic, you will find more concepts such as Charles’ Law, Condensation, Decantation, and much more only on our website and mobile application.

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