Question about which oxygen acts as a nucleophile

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Discussion Overview

The discussion revolves around the mechanisms of nucleophilic attack involving oxygen atoms in carboxylic acid derivatives, particularly focusing on the roles of different oxygen atoms in the context of resonance structures and protonation states. The scope includes theoretical considerations and mechanistic insights.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that nucleophilic attack likely involves internal attack on the carbonyl by the hydroxyl group, with rapid proton transfer making the mechanisms effectively similar.
  • Others argue that the question is unclear due to the representation of nucleophilic attack by a -COOH group, noting that the oxygens are equivalent in resonance structures.
  • One participant claims to have found an answer indicating that the protonated carbonyl is the correct mechanism, while suggesting that the second mechanism presented is a mistake.
  • Further clarification is provided that while oxygens in the carboxylate anion are equivalent, this equivalence does not hold in the protonated version, emphasizing that resonance structures do not involve proton movement.

Areas of Agreement / Disagreement

Participants express differing views on the clarity of the question and the mechanisms involved, indicating that multiple competing interpretations remain without a consensus on the correct approach.

Contextual Notes

There are limitations in the clarity of the original question and the assumptions regarding the equivalence of oxygen atoms in different protonation states, which may affect the interpretations of the mechanisms discussed.

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

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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.
 
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.
 
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.
 
Yanick said:
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.

No, they are equivalent in the carboxylate anion, but not in the protonated version. Resonance structures do not involve the movement of protons.
 
sjb-2812 said:
No, they are equivalent in the carboxylate anion, but not in the protonated version. Resonance structures do not involve the movement of protons.

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

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