[Rutherford]

What carboxylic acids will decarboxylate and not dehydrate when heated and why?

[discodermolide]

Decarboxylation usually requires a 6 membered "transition state" where the H of the acid is transferred to the O of a carbonyl group (ketone, ester etc) you get a cyclic movement of electrons to give CO₂ and the enol of the carbonyl which tautomerises.
So beta carbonyl acids like acetoacetic acid.
See drawing. R can be alkyl, ester, even H,  I think. Also aromatic, heteroaromatic etc.

[Rutherford]

Why does oxalic acid undergo this reaction? It's alpha carbonyl acid.

[discodermolide]

Heat it hard enough and it will decarboxylate, not by this process though.

[Babcock_Hall]

This comment might be judged slightly off-topic, but I am not intending to derail.  Biological catalysts such as thiamine pyrophosphate (which is a coenzyme) make alpha-ketoacids like pyruvic acid behave more like beta-ketoacids.  In other words, they decarboxylate much more easily than without this catalyst.  The nitrogen of the thiazolium ring acts as the electron sink for the decarboxylation.

[Rutherford]

I am a little confused now. Malonic acid is a beta carbonyl acid, but it can easily dehydrate. Can any acid decarboxylate if heated enough?

This comment might be judged slightly off-topic, but I am not intending to derail.  Biological catalysts such as thiamine pyrophosphate (which is a coenzyme) make alpha-ketoacids like pyruvic acid behave more like beta-ketoacids.  In other words, they decarboxylate much more easily than without this catalyst.  The nitrogen of the thiazolium ring acts as the electron sink for the decarboxylation.

I was thinking more about the lab experiments rather than the processes in the human body, but thanks for the info.

[discodermolide]

If you heat them hard enough, yes

[Rutherford]

Okay, one last question. You said here that beta carbonyl acids decarboxylate (on heating) easy and that oxalic acid (which is dicarboxylic) can decarboxylate, too. My question is: can the simple aliphatic acids like acetic acid decarboxylate only on heating, too?

[discodermolide]

As I said if you heat them hard enough.

[Rutherford]

I couldn't believe that acetic acid and similar can decarboxylate only on heating (without a base being present). Thanks for the help.