Question about which oxygen acts as a nucleophile

JHUK

I posted in picture format to post on another website, but haven't found a reply yet:

sjb-2812

How long ago did you post the other question - seems to me that this was posted whilst a vast majority of PFers were asleep, perhaps this is the issue with your other question.

However, by analogy with other reactions of the carbonyl group I'd think this is likely to involve effectively internal attack on the C=O by the COH, and work from there. Proton transfer, however is fairly rapid so the two mechanisms are pretty identical. Whether a kinetic isotope effect may be present in pure deuterated acid, I don't know.

Yanick

You're question is a little unclear because the mechanism shows nucleophilic attack by a -COOH group. The oxygens in these groups are actually equivalent and are best represented by resonance structures where the hydroxyl and carbonyl oxygens exchange.

JHUK

I was able to find an answer - the protonated carbonyl is the correct mechanism for both (the second is a mistake). You can see why by considering resonance.

sjb-2812

No, they are equivalent in the carboxylate anion, but not in the protonated version. Resonance structures do not involve the movement of protons.

Yanick

You are correct sir. I shouldn't be answering questions when I'm dead tired.