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When to know: will the carbonyl or the carboxy be protonated...
Josh_4080
#1 Posted : Sunday, July 05, 2020 3:27:44 PM
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From my understanding it is always more likely that the carbonyl will be protonated. It has a higher electron density. So if there is a carboxylic acid present, the carbonyl oxygen will be the one being protonated (like in the hemiacetal/ketal reaction or forming a triglyceride). But I can also see that after the hemi is formed, the hydroxyl will also get protonated, but not the ether. Why is it that the hydroxyl will be the thing protonated here? What makes it more likely than the ether, but less likely than the carbonyl?
INSTR_Katerina_102
#2 Posted : Sunday, July 05, 2020 4:36:17 PM
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Hi,

I wouldn't worry about this too much as the MCAT doesn't really pick at protonation steps.

Usually, if I'm drawing a mechanism, I just do a protonation that is reasonable and lets me continue in the mechanism, as protonation state can change fast.

For this particular case - yes, the carbonyl oxygen on a carboxylic acid is more likely to be protonated as it has more electron density around it.

However, what you do after this sounds like forming a hemiketal/hemiacetal.

This doesn't really happen on the MCAT as the orthoesters formed from carboxylic acids tend not to be particularly stable. I will talk about the rest in the case of hemiketals/acetals which are stable.


As for why OHs tend to be more likely than ethers, this relates to the productivity step - you are often adding alcohols to carbonyls in this type of reaction, meaning that you don't want them to leave again when you draw the mechanism, so you protonate the OH instead promoting it to leave.

In reality, there's probably about an equal propensity for OH and OR to be protonated, it's just you only want to show the forward reaction in your mechanism so you conveniently ignore the non productive pathway where the OR adds then leaves.

Please let me know if this is unclear.
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