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Topic: Decarboxylation in basic conditions  (Read 909 times)

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Offline RkRs21

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Decarboxylation in basic conditions
« on: January 18, 2022, 03:48:26 PM »
Hi all!

In my most recent lab assignment (bachelor's student) we were set to make 2-octanone via an Acetoacetic ester. With the mechanism figured out and written down, I went to work. The plan would be as follows:

  • Dissolve sodium in ethanol to deprotonate the C-α position between the ketones.
  • Add 1-bromopentane to make the main chain carbon-carbon link
  • Hydrolyse the ester in 4M NaOH to 2-acetylheptanoic acid (and ethanol)
  • Lastly decarboxylate in acidic conditions

Everything seemed to go as planned, right up until I checked a layer labelled as 'waste' after the hydrolysis step (I separated off what I presumed to be unreacted material). At the advice of my tutor, I made an infrared absorption spectrum of the waste material. It so turns out that it was a very close match to the spectrum of 2-octanone. This arises a crucial question I haven't been able to answer after asking around on the lab floor or doing thorough searches on the internet:

Is it possible to decarboxylate under (strong) basic conditions?

From what I found online the main problem would be that the acid will be deprotonated, making it impossible for the concerted mechanism to work. (Going off the image below).

Thanks a lot!

Offline phth

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Re: Decarboxylation in basic conditions
« Reply #1 on: January 26, 2022, 12:48:25 AM »
Yes,

If you dissolved the base too quickly there could be enough heat to decarboxylate the anion. This mechanism is known as the kraptcho decarboxylation but with sodium not lithium: https://en.wikipedia.org/wiki/Krapcho_decarboxylation

Offline rolnor

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Re: Decarboxylation in basic conditions
« Reply #2 on: January 26, 2022, 01:56:01 AM »
It requires 150° so I think not.

Offline rolnor

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Re: Decarboxylation in basic conditions
« Reply #3 on: January 26, 2022, 06:42:16 AM »
Also, you need the ester+halide ion, not the carboxylate so I think not.

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