Wolff Kishner Reaction – conversion of ketones/aldehydes to alkanes

by James

Description: The Wolff-Kishner is a reaction for converting carbonyls (such as ketones and aldehydes) into alkanes.
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{ 15 comments… read them below or add one }

bazla siddiqui

i think it is a very good way of representing this method.i like it

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Nima Omid-Fard

does this reduce alcohols to alkanes too?

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james

no, just ketones and aldehydes

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Casey

Yes, but for a Wolff-Kishner reaction that does not imply the use of a strong base (i.e. hydroxide), would water be able to deprotonate that amine in step one to give the N-N double bond? I ask because in the synthesis of the drug Amiodarone, they do not specify basic conditions. Rather, they allude that they use hydrazine hydrate only.

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james

Water isn’t basic enough to deprotonate. The pKa of the hydrazone hydrogens is about 22, whereas that of H3O(+) is -2. The equilibrium would be 24 orders of magnitude in opposition to the desired reaction.
I found a publicly available reference to the synthesis here: : http://bit.ly/J2TCRA
Although they don’t specifiy basic conditions in the reaction scheme, it’s not uncommon to leave out details such as base, solvent and heat. The Wolff Kishner is generally a very unfavorable reaction as it is – it requires heating at 180 degrees C or more – so I am nearly 100% certain that they merely omitted writing base in the scheme here.

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Casey

Great! Thank you.

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Aleksandra

And what is a role of ethylene glycol in this reaction?
The site is really great.

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james

Ethylene glycol serves as a solvent here. It has a very high boiling point (~200 degrees C); the fact that the reaction needs to be heated so much is why this solven tis used.

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james

Good question. One reason I’ve read is the greater strength of the C-O (pi) bond as opposed to the C-C (pi) bond. I’m not sure this is the whole picture but it is at least a component.

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Aleksandra

Thanks for answer.

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shashikumar k.paknikar

nice presentation. Can we use potassium carbonate and ethylene glycol and temperature ~200 degree C?

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james

I suppose you could use potassium carbonate but it will not deprotonate ethylene glycol irreversibly; at 200 degrees C, however, the reaction should still proceed well.

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Gagandeep

In the Wolff kishner we first convert ketone or aldehyde to hydrazone. Hydrazone formation is done in acid medium. Now if we have a carbonyl compound with an Acetal and Ketone, will the acetal be also reduced in Wolff kishner due to equilibrium being established in during hydrazone formation (acid medium form ketone and ketone formed along with original one will react to form two Hydrazone sites, which can be reduced in proceeding step with strong base. Couldn’t find any answer to this.

Thanks in advance!

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James

Acetal hydrolysis requires aqueous acid. Hydrazone formation is done using anhydrous acid. So the acetal should be unaffected by these conditions.

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Gagandeep

It really makes sense, low H2O – complete acetal side of equilibrium. Thanks from saving me from a headache. And great site.

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