Wolff Kishner reduction mechanism begins with the formation of a hydrazone anion which then releases the nitrogen atom to form a carbanion. This carbanion then reacts with the water in the system to give a hydrocarbon. Typically, diethylene glycol is used as a solvent for this method.
This reduction is an organic reaction where aldehydes and ketones are reduced to alkanes. Some carbonyl compounds are stable in strongly basic conditions, Hence they can be easily reduced to alkanes (The carbon oxygen double bond becomes two carbon hydrogen single bonds). Although the mechanism usually begins with the condensation of hydrazine to give a hydrazone, the usage of a pre formed hydrazone can have advantages such as reduced reaction time, reactions that proceed at room temperature or very mild reaction conditions. The pre formed hydrazone substrates which can be used in this reduction also require different solvents and reaction temperatures.
Wolff Kishner Reduction Mechanism
The aldehyde or ketone is subjected to hydrazine. This yields the hydrazone required for the process. The reaction is illustrated below.
The terminal nitrogen atom is deprotonated and it proceeds to form a double bond with the neighbouring nitrogen atom. The released proton attaches itself to the hydroxide ion from the basic environment to form water.
Since oxygen is more electron withdrawing than carbon, the carbon is protonated by the water molecule as shown below.
The terminal nitrogen is deprotonated again, this time forming a triple bond with its neighbouring nitrogen atom. This results in the formation of a carbanion where the two triple bonded nitrogens are released as nitrogen gas. Similar to step 2, the ejected proton forms water along with the basic environment.
Similar to step 3 of the Wolff Kishner reduction mechanism, the carbon is protonated by water, resulting in the formation of the desired hydrocarbon product as shown. Thus the aldehyde or ketone is converted to an alkane.
The rate determining step of this reaction is the bond formation of the terminal carbon with hydrogen (in the hydrazone anion). The carbon hydrogen bond formation is helped by mildly electron withdrawing substituents. Highly electron withdrawing substituents decrease the negative charge of the terminal nitrogen, making it difficult to break the N-H bond. The Wolff Kishner reduction has been modified into several techniques, each with their own advantages and disadvantages, for example – the Huang Minlon modification (using the carbonyl compound, 85% hydrazine and potassium hydroxide as the reagent) offers reduced reaction time and the achievement of higher temperatures but requires distillation.