Wolff–Kishner Reduction Reaction

wolff kishner reduction reaction

Core Concepts

In this organic chemistry tutorial, you will learn about the Wolff–Kishner reduction reaction and its mechanism. You will also see some examples of molecules undergoing the Wolff–Kishner reductions.

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Introduction to the Wolff–Kishner Reduction

The German chemist Ludwig Wolff and the Russian chemist Nikolai Kishner independently discovered the Wolff–Kishner reduction in 1912 and 1911, respectively.

In the Wolff–Kishner reduction, an aldehyde or a ketone is reduced into an alkane. More specifically, the reduction involves the conversion of a carbonyl group (C=O) into a methylene group (–CH2–).

Wolff Kishner reduction of a ketone into an alkane
Figure 1. The Wolff–Kishner Reduction of a Ketone into an Alkane

This reaction occurs in high temperatures (100–200˚C) and under basic conditions. This makes it different from the Clemmensen reduction—another popular reduction reaction that occurs under acidic conditions. To choose which reaction to use, one must assess the sensitivity of one’s substrates: The Wolff–Kishner reduction favors acid-sensitive substrates, while the Clemmensen reduction favors base-sensitive ones.

Mechanism of the Wolff–Kishner Reduction

The Wolff–Kishner reduction involves two major steps: the formation of a hydrazone and the reduction of said hydrazone into an alkane.

The Formation of Hydrazone

Hydrazones are formed when hydrazines (N2H4) react with aldehydes or ketones under mildly acidic conditions. In such a reaction, hydrazines play the role of a nitrogen nucleophile

Conversion of ketone into hydrazone
Figure 2. The Transformation of a Ketone into a Hydrazone

The mechanism of the formation of hydrazone is as follows: First, the hydrazine performs a nucleophilic attack on the carbonyl group. Several proton transfer steps facilitated by the acidic environment follow. Next, water leaves, forming a C=N double bond. The resulting intermediate is then deprotonated, yielding hydrazone.

Mechanism of the formation of hydrazone
Figure 3. The Mechanism of the Formation of Hydrazone

The Reduction of Hydrazone

Under basic conditions, hydrazones are readily reduced into alkanes—the ultimate product of a Wolff–Kishner reduction reaction.

Reduction of hydrazone into alkane
Figure 4. The Reduction of a Hydrazone into an Alkane

The mechanism of the reduction of hydrazone is as follows: First, the basic environment deprotonates the NH2 of the hydrazone, forming a resonance-stabilized intermediate. Then, the negatively charged carbon of that intermediate is protonated. After NH loses a proton, nitrogen gas leaves, forming a carbanion (a carbon with an unshared pair of electrons). Finally, the carbanion is protonated and becomes an alkane

It is important to note that the expulsion of nitrogen gas out of the solution as bubbles forces the reaction to completion (per Le Chatelier’s Principle), giving a very good product yield. 

Mechanism of the reduction of hydrazone
Figure 5. The Mechanism of the Reduction of Hydrazone

Examples of Wolff–Kishner Reductions

In this example, we will reduce butanone (or ethyl methyl ketone) into butane (or n-butane). Notice how the hydrazone has two diastereomeric forms. This is because butanone is asymmetrical.

wolff kishner reduction of butanone into butane
Figure 6. The Wolff–Kishner Reduction of Butanone into Butane

In another example, we will reduce isobutyrophenone into isobutyl benzene. Notice again how the asymmetry of isobutyrophenone results in two diastereomeric hydrazones.

wolff kishner reduction of isobutyrophenone into isobutyl Benzene
Figure 6. The Wolff–Kishner Reduction of Isobutyrophenone into Isobutyl Benzene

Further Reading

In this organic chemistry tutorial, you will learn about the Wolff–Kishner reduction reaction and its mechanism.