What is the mechanism of Wolff-Kishner reductions?
The formation of a carbanion through the release of a nitrogen atom from a hydrazone anion is the initial process of the Wolff Kishner reduction mechanism. This carbanion forms a reaction with water and this takes place in the system from which a hydrocarbon is formed. For this method, the solvent that is typically used in the reaction is diethylene glycol.
This type of reduction mechanism is an organic type where alkanes are obtained from ketones and aldehydes through the process of reduction. Carbonyl compounds that are stable in strongly basic conditions can be reduced to alkanes with minimal effort. Two carbon-hydrogen single bonds are formed from the carbon-oxygen double bond. While the process typically starts with the condensation of hydrazine to obtain hydrazone, there are many advantages to using pre-formed hydrazone like reduction of reaction time, reactions that take place at mild reaction conditions or room temperature. Different temperatures and solvents are required for the reaction to take place for the substrates of pre-formed hydrazone which can be useful in the reduction process.
Working Mechanism of Wolff-Kishner Reduction
First Step
The ketone and aldehyde are added to the solution of hydrazine. From this step, hydrazone is yielded which is required for the reduction process to happen.
Second Step
A double bond is formed along with the adjacent nitrogen atom through the process of deprotonation of the terminal nitrogen atom. The proton that was released merges itself with the hydroxide ion due to the basic nature of the environment to form H2O (water).
Third Step
Since carbon does not draw more electrons than oxygen, the water molecule protonates the carbon.
Fourth Step
Deprotonation of the terminal nitrogen takes place once again and this time a triple bond is formed and merges with its adjacent nitrogen atom. This process ultimately yields a carbanion where nitrogen gas is released by the two triple bonded nitrogen. Similar to the second step, water is formed from the proton that got ejected due to the basic nature of the environment.
Fifth Step
Just like in the third step of the reduction process, water protonates the carbon through which the desired hydrocarbon is formed. Hence, the alkane is obtained by the process of conversion of ketone or aldehyde.
The formation of a bond of hydrogen with the terminal carbon is the step that determines the rate of the reaction process. Substituents that mildly withdraw electron helps in the formation of the hydrogen bond. Substituents that highly withdraw electrons do not work because the process would lead to the terminal nitrogen having a negative charge which would result in the N-H bond being difficult to be broken down. The Wolff Kishnerreduction has been modified into several techniques, each with its own set of benefits and drawbacks. For example, the Huang Minion modification (which uses the carbonyl compound, 85 per cent hydrazine, and potassium hydroxide as the reagent) allows for a faster reaction time and higher temperatures, but it necessitates distillation.
Generally, reagents that are Cr03-based are used to convert primary alcohols into carboxylic acids and aldehydes and they also help in obtaining ketones by converting the secondary alcohols. The use of LiAIH4 can help to reduce the oxidation products to their alcohols. The interaction of hydrochloric acid with zinc releases hydrogen gas. In a sense, the mercury in zinc amalgam serves to “trap” the active hydrogen as it is generated, allowing it to strike the carbonyl molecule rather than releasing it as H2 gas. This is the very reason Clemmensen is reduced by zinc amalgam. Due to the considerations that will be discussed as follows, ketones are generally less reactive than aldehydes. The carbonyl carbon present in aldehydes generally possesses positive charges partially as compared to the ones in ketones. There is only one type of electron donor in aldehydes whereas there are two types in ketones. A chemical reagent is used to find out the identity of the functional groups of an alpha-hydroxy ketone, aldehyde and aromatic aldehyde and this reagent is called Tollens’ reagent and it is a composition of ammonia solution and silver nitrate.
Some of the applications of the Wolff-Kishner reduction is as described in the following: The synthesis of aspidospermine can be done with the method of the Wolff-Kishner reduction. The reduction mechanism was further developed and was called the Huang Minion modification and this modified reaction can be useful in the reduction of some tricyclic carbonyl compounds. In addition to this, the synthesis of functionalized imidazole substrates can be done using the Wolf-Kishner reduction mechanism.
Some of the limitations of the Wolff-Kishner reduction mechanism is that it requires a high temperature for the reduction reaction to take place, the mechanism is found to be useless for ketones that have been sterically hindered.
To learn more about topics like this, refer: Aldehydes,ketones And Carboxylic Acids from Class 12 Chemistry English Medium – Find All the Questions & Answers