12.22:
Aldehydes and Ketones to Alkanes: Wolff–Kishner Reduction
Wolff–Kishner reduction transforms a carbonyl group into a methylene group through deoxygenation by replacing the oxygen atom with two hydrogen atoms.
The reaction is carried out at elevated temperatures, using a strong base and hydrazine.
The reduction mechanism has two parts: Formation of hydrazone and elimination of nitrogen.
The first part involves a multi-step condensation reaction between hydrazine and the carbonyl compound to form a hydrazone.
In the second part, the base removes an N–H proton to generate the hydrazone anion, which resonance stabilizes to give a negative charge on the carbon atom.
This deprotonation step is not easy and requires heat, necessitating the use of high-boiling solvents.
The anion is reprotonated by water, followed by a second deprotonation of nitrogen, which drives the reaction forward by eliminating nitrogen gas and generating a carbanion.
A final reprotonation on the carbanion gives an alkane.
12.22:
Aldehydes and Ketones to Alkanes: Wolff–Kishner Reduction
Wolff–Kishner reduction involves converting aldehydes and ketones to alkanes using hydrazine and a base. The reaction converts a carbonyl group to a methylene group. The method was independently discovered by N. Kishner in 1911 and L. Wolff in 1912. The reduction is carried out in high-boiling solvents such as ethylene glycol and diethylene glycol because heat is required to deprotonate the N–H proton in one of the reaction steps.
Wolff–Kishner reduction involves two key stages, including the formation of an imine derivative, hydrazone, through a series of steps and the loss of N2. The mechanism involves multiple proton transfer reactions forming an N=N bond. The final steps include the transfer of a proton from nitrogen, a rearrangement reaction to form a carbanion with a subsequent loss of N2, and a proton transfer to the carbanion to give the final product—alkane.
