Clemmensen Reduction of Ketones/Aldehydes to Alkanes

by James

Description: Addition of zinc amalgam, Zn(Hg) and acid to a ketone results in an alkane. This is called the Clemmensen reduction.
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{ 6 comments… read them below or add one }

Layla February 16, 2014 at 3:30 pm

On an Orgo 2 HW question we had to propose the synthesis of 1-bromo-3-propylbenzene. This was a multiple choice question, so we had to figure out the order of using the clemmensen reduction, bromination of arenes, and friedel crafts. What kept throwing me off was what order to put them in. It’s tricky! Could you explain how to approach that?

Thanks so much for your help, this is a great site and is extremely useful!

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Karthik April 22, 2014 at 7:10 am

My org chem professor told me that clemmensen reduction of 1,3 dicarbonyl compounds gives a differently rearranged compound. Like 3,3-dimethyl-1,5-cyclohexanedione, gives 1,3,3-trimethyl-cyclopentanone (less membered ring). And 2,4-pentanedione gives 2-methyl-3-butanone. Could you explain to me the mechanism of these reactions and suggest ways to write the product for any such dicarbonyl clemmensen reduction?

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James April 25, 2014 at 11:05 pm

That is seriously funky. I wonder if it’s generating a ketyl radical and then going through a cyclopropane, much like some versions of the Dowd-Beckwith rearrangement? http://en.wikipedia.org/wiki/Dowd–Beckwith_ring-expansion_reaction

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Karthik October 3, 2014 at 12:46 am

I honestly have no idea. I’m still in high school so I have no idea about the Dowd-Beckwith expansion reaction, however the Wikipedia page did throw some light on it. However I think that shouldn’t be happening, since we aren’t using a alkyl halide side chain or AlBN.
This page seems to offer an explanation, stating 1,3 dicarbonyls form ring contraction products via cyclopropanediols, trapped as acetates: http://bit.ly/1CI3nhj
I still haven’t fully understood it though.

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Karthik October 3, 2014 at 12:49 am

Also, in my previous comment I seem to have gotten the IUPAC names wrong. It should have been 5,5-dimethyl-1,3-cyclohexanedione, 2,4,4-trimethyl cyclopentanone and 3-methyl-2-butanone.

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Karthik October 3, 2014 at 12:45 am

I honestly have no idea. I’m still in high school so I have no idea about the Dowd-Beckwith expansion reaction, however the Wikipedia page did throw some light on it. However I think that shouldn’t be happening, since we aren’t using a alkyl halide side chain or AlBN.
This page seems to offer an explanation, stating 1,3 dicarbonyls form ring contraction products via cyclopropanediols, trapped as acetates: http://bit.ly/1CI3nhj
I still haven’t fully understood it though.

Reply

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