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.
Topics Covered in Other Articles
- Carbonyl Functional Group
- Diastereomers
- Leaving Groups
- Lewis Dot Structures
- Nucleophiles
- Resonance Structures
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–).
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.
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.
The Reduction of Hydrazone
Under basic conditions, hydrazones are readily reduced into alkanes—the ultimate product of a Wolff–Kishner reduction reaction.
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.
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.
In another example, we will reduce isobutyrophenone into isobutyl benzene. Notice again how the asymmetry of isobutyrophenone results in two diastereomeric hydrazones.
