# How is the hydrazone formed in the Wolff-Kishner reduction?

In the Wolff-Kishner reduction the first step is to react a carbonyl with hydrazine to form a hydrazone. The later steps of the reaction requires a strong base to cause loss of nitrogen.

However, the mechanism for the formation of the imine/hydrazone is acid catalyzed:

Wikipedia mentions that the formation of the hydrazone is typically done in situ. So how does the formation of the hydrazone occur in basic conditions?

References:

• I think they prepare the imine in acidic conditions and then switch to base. 1) Hydrazine reaction, 2) KOH/OH- Ethylene glycol. – Aditya Roychowdhury Apr 15 at 8:15
• Thank you @AdityaRoychowdhury, that is what I wanted to know. I edited the title to reflect that answer. I would accept that answer. – Cody Aldaz Apr 15 at 8:21

Wolff–Kishner reduction is the method for the reduction of aldehydes and ketones to corresponding alkanes. First, the carbonyl compound condenses with hydrazine to form the corresponding hydrazone. The resulting hydrazone is treated with base to induce the reduction of the $$\mathrm{sp^2}$$-carbon to $$\mathrm{sp^3}$$-$$\ce{CH2}$$, and the oxidation of the hydrazine to gaseous nitrogen in the process to yield the corresponding alkane:

Source of the image

The following is the accepted mechanism of the reduction:

Source of the image

How does the formation of the hydrazone occur in basic conditions?

The formation of the hydrazone can be done in either acidic or basic conditions (recall imine formation of aldehydes and ketones with primary amine and 2,4-dinitrophenylhydrazone formation of them with 2,4-dinitrophenylhydrazine). And, yes, as Wikipedia mentioned, sometimes it is advantageous to use a pre-formed hydrazone as substrate for the reduction. Yet, remember, Wolff–Kishner reduction is better suited for substrates with acid sensitive groups. Therefore, use of acid catalyst to pre-form hydrazone for the reduction is meaningless in that situation.

• More on the mechanism of the Wolff-Kishner reduction: chemistry.stackexchange.com/questions/85995/… – user55119 Apr 15 at 16:32
• I can’t find any source for “hydrazine can be formed under either acidic or basic conditions“. On the contrary I find the opposite “ athigh pH there will not be enough acid to protonate the OH in the intermediate to allow for removal as H2O” – Cody Aldaz Apr 15 at 16:40
• Did you look at the sorces given in the answer? For example, look at "imine formation of aldehydes and ketones" in the text. – Mathew Mahindaratne Apr 15 at 17:29
• I did, that’s where the second quote comes from – Cody Aldaz Apr 15 at 19:02
• It'd be nice if the downvoter give some reasoning for their downvote. That's the war to improve the answer. – Mathew Mahindaratne Apr 16 at 6:24