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Wolff Kishner Reduction

A Wolff-Kishner reduction takes place when a carbanion is formed from a hydrazone anion that has released an atom of nitrogen.

Wolff Kishner Reduction

A Wolff-Kishner reduction takes place when a carbanion is formed from a hydrazone anion that has released an atom of nitrogen. Upon reaction with water in the system, this carbanion produces a hydrocarbon. The solvent usually used in this method is diethylene glycol.

The aldehyde and ketone are reduced to alkanes in this organic process. The double bond between carbon and oxygen in carbonyl compounds can be easily broken, resulting in two carbon hydrogen atoms. As an advantage of pre-formed hydrazone, reduced reaction time and room temperature reactions can be achieved, while mild reaction conditions can be achieved despite the fact that the reaction usually begins with the condensation of hydrazine into hydrazone. Additionally, different solvents as well as reaction temperatures are required for the pre-formed hydrazone substrates.

Wolff Kishner Reduction Mechanism

Step 1

Hydrazine is used to decompose aldehydes or ketones. This produces hydrazone, which is needed for the process. A diagram of the reaction follows.

Step 2

A nitrogen atom at the terminal of a chain deprotonates and forms a double bond with an adjacent nitrogen atom. Hydroxide from the basic environment attaches to the proton released by the reaction to form water.

Step 3

Carbon is protonated by the water molecule since oxygen is less electron withdrawing than carbon.

Step 4

A new deprotonation gives rise to a triple bond between the terminal nitrogen and the neighbouring nitrogen atom. Nitrogen gas is formed by the release of the three triple bonded nitrogen from the carbanion. The proton, along with the basic environment, ejects out of the system in a similar fashion to step 2.

Step 5

A protonation reaction takes place in step 3 of the Wolff Kishner reduction mechanism, producing a hydrocarbon product similar to those shown. Thus, aldehydes or ketones are converted to alkanes.

This reaction is governed by the formation of a hydrogen bond between the terminal carbon and the hydrazone anion. Mildly electron-withdrawing substituents aid the formation of carbon hydrogen bonds. Nitrogen substitutes with high electron withdrawing properties decrease the nitrogen's negative charge, making the N-H bond much more difficult to break.

Wolff-Kishner reductions may be modified into several different techniques, each with its own advantages and disadvantages; for instance, the Huang Minlon modification (using Carbonyl compounds, 85% hydrazine as the reaction reagent, and potassium hydroxide as the catalyst) reduces reaction time, but has the disadvantage of requiring distillation to achieve high temperatures.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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