O-Chem Question: Wolf Kishner

[oksusu]

Hi SDN,

I have a question about the Wolf Kishner reduction that fully reduces the carbonyl and replaces it with 2 hydrogens. So the first part of the mechanism is where you form a hydrazone (imine-like structure) in basic conditions. I know how to form imines with acid, but can't find the mechanism with the base.

Does the oxygen just fall from the carbonyl fall off as OH-? since we are in basic conditions, it would be okay for it to have an OH- fall off?

Thanks.

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[orgohacks]

I agree that many textbooks aren't very clear on this. In practice there's probably a proton transfer as you lose OH- so that it becomes H2O.
You're right that imine formation is catalyzed by acid. If I had to to this in the lab, I'd use neutral conditions for hydrazone formation, perhaps with catalytic acid, and then add an excess of strong base/heat for the second step.


Relevant: http://masterorganicchemistry.com/r...on-conversion-of-ketonesaldehydes-to-alkanes/

 

[solitarius]

Carbonyl gets converted into an intermediate imine. Typically, basic conditions are not enough for the full acetalization; they proceed up until hemi-acetal and stop. I'd imagine you have to start this reaction in acidic conditions. Once you get the hydrazone product, move it to basic conditions.

So something like this C=O + H2-N=N-H2 ---> C=N=N-H2. From this intermediate, then proceed..