Wolff-Kishner Reduction: Mechanism, Components & Process

Instructor: Saranya Chatterjee

Saranya has a masters degree in Chemistry and in Secondary Education. She has taught high school, AP chemistry for 2 years and is teaching undergraduate college chemistry for 3 years.

The lesson discusses the Wolff-Kishner reduction process, which is basically a reduction process that converts a carbonyl functional group to a saturated hydrocarbon. It also talks about the mechanism of the reduction process, including two modifications.

Wolff-Kishner Reduction Scheme

The Wolff-Kishner reduction is the reduction of carbonyl compounds to hydrocarbons by heating them with hydrazine hydrate and a base. In this process the carbonyl group (>C=O) gets converted to >CH2 group via >C=N. NH2 and nitrogen gas evolves out. So, the reaction is basically converting carbonyl groups to methylene groups. To drive out the nitrogen gas, which is a side product of the reaction, it's necessary to use a high-boiling-point solvent like ethylene glycol.

This is a reduction for aldehydes and ketones; other reducible groups such as carbon-carbon double bonds and nitro groups usually remain unaffected under the conditions of this reaction.


Wolff-Kishner reduction scheme
wolff kishner reduction


The first procedure of the reduction is to heat the carbonyl compound with hydrazene hydrate and sodium ethoxide in a sealed tube at 180°C.

Mechanism

The Wolff-Kishner reduction takes place in two stages.

1. The first step of the reaction is the conversion of the carbonyl compound into a hydrazone derivative. This happens in two steps:

a) Nucleophilic attack of the hydrazine to the carbonyl group. It is the slowest step in the entire reaction, which is why it is called the rate determining step abbreviated as RDS.

b) The elimination of the water molecule

2. The second step of the reaction converts >C=N. NH2 into >CH2. This occurs in three steps:

a) Base-catalyzed tautomerization reaction of the hydrazone derivative.

b) Formation of a carbanion with the evolution of nitrogen gas.

c) Protonation of the carbanion by water to form the desired product


Mechanism of Wolff-Kishner reduction
mechanism of wolff kishner


There are three major limitations of the Wolff-Kishner reduction:

a) The reduction requires a high temperature.

b) It cannot be used for sterically hindered ketones.

c) Although the process is specific for carbonyl groups, sometimes migration of the double bond takes place instead of the expected mechanism of the reaction when undesired product like alpha, beta-unsaturated carbonyl compounds are formed in the reaction.

Modifications

Some of the limitations of the original reaction are overcome by modifying the procedure as shown below.

Huang Minlon reduction

1. In this process, the carbonyl compounds are heated strongly with alkali solution (KOH) and hydrazine hydrate in refluxing ethylene glycol. Since Nitrogen gas is formed as one of the side product in this reaction which has to be removed from the reaction vessel soon, so a high boiling solvent like ethylene glycol is necessary which drives away the gas from the reaction vessel. The advantages of this modification are

a) It is easy to carry out the reaction.

b) Pure products can be isolated.

c) Sterically hindered ketones can also be reduced.


Wolff-Kishner Huang modification
Wolff Kishner Huang modification


One disadvantage of the reaction is that the product requires distillation.

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