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Wolff Kishner Reaction – conversion of ketones/aldehydes to alkanes

Description: The Wolff-Kishner is a reaction for converting carbonyls (such as ketones and aldehydes) into alkanes.
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Notes: NH2NH2 is called “hydrazine”. The imine of hydrazine is called a “hydrazone”.

Examples

Notes: Ethylene glycol is a high boiling alcohol solvent

Mechanism (hydrazone formation): The reaction begins by 1,2-addition of the nitrogen on the carbonyl carbon (Step 1, arrows A and B). This is followed by transfer of a proton from the nitrogen to the oxygen (Step 2, arrows C and D) and then 1,2-elimination of hydroxide (Step 3, arrows E and F). Finally the hydrogen is removed from the nitrogen in an acid-base reaction (Step 4, arrows G and H).

Notes: Here is an alternative (and probably more correct) mechanism for proton transfer

Water is a byproduct here. Acid can be used to facilitate this reaction. There are other reasonable bases that can be used to remove the proton in step 4.

Mechanism (Wolff-Kishner reaction)

The hydrazone is deprotonated by OH and the negative charge moves to the carbon (Step 1, arrows A, B, C) which is protonated (Step 2, arrows D and E), whereupon the nitrogen is deprotonated again, and nitrogen gas (N2) is lost (Step 3, arrows F, G, and H). This leaves an anion which is then protonated to give the alkane (Step 4, arrows I and J).

Notes: Ethylene glycol is also a reasonable acid to use in the last step.

Potassium (K) is not crucial here, it’s just a spectator ion.

Also note that KOH here is catalytic.

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