Aldehyde Reactions: Hydrazone Formation and Reduction to Alkane from Aldehyde, Ketone using NH2NH2 (Wolff Kishner Reaction)
The Wolff Kishner reaction takes place in 2 discrete steps:
- The ketone (or aldehyde) is treated with hydrazine (NH2NH2) in a weakly acidic environment (H+, H3O+) to form a hydrazone molecule
- Treatment of the new hydrazone molecule with a base (NaOH, KOH) in hot conditions (commonly referred to as ‘heat’ or ‘Δ’, will remove the NNH2 group and reduce the molecule down to an alkane
First Half of the Wolff Kishner Reaction
Full Wolff Kishner Reaction
The reaction mechanism is depicted in two parts below:
In the first step, the lone pair of electrons from the NH2NH2 attack the ketone-carbon, sending the double bond electrons to the ketone-oxygen.
In the second step, the lone pair electrons from the hydroxide molecule deprotonate the nitrogen on the NH2NH2 moiety while simultaneously the lone pair electrons on the ketone-oxygen attack the proton of a nearby water molecule .
In the third step, the lone pair electrons from the NH2NH2 nitrogen form a double bond with the ketone-carbon, kicking off the OH- group..
In the fourth step, the OH- group returns and deprotonates the nitrogen that just formed a double bond. This is now the hydrazone product, but the Wolff Kishner reaction continues to reduce the hydrazone to an alkane using a base (such as NaOH)
In the fifth step, the NaOH attacks a proton on the nitrogen of the NH2NH2 moiety, causing the lone pair of electrons to travel to form a double bond with the other nitrogen, causing it to break its bond and send the lone pair electrons to the ketone-carbon, forming a carbanion.
In the sixth step, the carbanion reacts with a nearby water molecule, deprotonating it.
In the seventh step, a new NaOH molecule deprotonates the last H from the NH2NH2 moiety, causing the electrons to form a triple bond between the two N’s, and breaking the bond connecting it to the alkane backbone, once again forming a carbanion.
In the eighth step, the carbanion attacks a proton from a nearby water molecule, completing the reaction.
This reaction is often presented as the first half, or the entire reaction. Therefore, it is important to remember the intermediate hydrazone product as well as the reduced alkane..