Wolff-Kishner Reduction Mechanism

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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.



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.



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.



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



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.



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.



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.

FAQ (Frequently Asked Questions)

Question 1: What Happens in Wolff-Kishner Reaction?

Ans: 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.

Question 2: Which Reagent is Used in Wolff-Kishner Reduction?

Ans: 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.

Question 3: Is Wolff-Kishner Effective at Reducing Amides?

Ans: As 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.