[miaou5]

according to my notes, carboxylic acids that have carbonyl groups beta to the carboxylic acid group undergo decarboxylation--from what i can see, this is because the carbonyl oxygen grabs the proton from the alcohol group in the carboxylic acid. but wouldn't it be able to grab the proton if it were alpha? why does it have to be beta? thank you so much!

[eRReR]

Anyone can shoot me down for being wrong (I may be), but with a beta-keto acid, when you draw the mechanism, you form an enol first, which can tautomerize into a ketone (stable). With an alpha-keto acid, I don't think you form an enol. In fact, before releasing CO2, the intermediate is a carbocation that, on releasing CO2, pushes electrons onto the carbon which looks even less stable.

[discodermolide]

according to my notes, carboxylic acids that have carbonyl groups beta to the carboxylic acid group undergo decarboxylation--from what i can see, this is because the carbonyl oxygen grabs the proton from the alcohol group in the carboxylic acid. but wouldn't it be able to grab the proton if it were alpha? why does it have to be beta? thank you so much!

Because in this system you can form a nice 6-membered "intermediate" which promotes the proton transfer from the acid OH to the beta carbonyl oxygen.Here is a drawing. (ChemDraw screwed up. That's a hydrogen between the two oxygen's. Which is hydrogen bonded to the ketone carbonyl, sorry).

[Babcock_Hall]

Delocalization of electrons is at least part of the story.  In enzymatic beta-ketoacid decarboxylations, a lysine forms a Schiff's base intermediate, which serves as the electron sink to facilitate decarboxylation (Bruice, p. 829, 4th ed).  In biochemical alpha-ketoacid decarboxylations, thiamine pyrophosphate is the cofactor.  I remember being taught by a well-known enzymologist that TPP makes alpha-ketoacids think that they are beta-ketoacids.  If you draw out resonance forms for the intermediate, you can see that TPP does this by resonance delocalization onto its nitrogen atom.  I can draw out chemdraw figure if anyone is interested.