[Cheshyre]

I'm reviewing for the MCAT and using a Kaplan 07-08 MCAT book. One of the mechanisms (Fig 9.12, if anyone has it) shows a carboxylic acid being attacked at the carbonyl carbon by a nucleophile anion. Their abbreviated mechanism shows the carbonyl double bond moving up into the oxygen atom which forms an oxygen anion, and then they show the protonation and spontaneous leaving of the adjacent hydroxyl group. I'm wondering if this mechanism is even possible. It seems implausible that a hydroxyl group would be protonated instead of an oxygen anion. At the very least, I would think that it'd be the mechanism for a minor product. If anyone could help me, I'd appreciate it very much.

Thanks!

[spirochete]

In short, it sounds like your book is sort of mixing up its carboxylic acid substitution mechanisms. 

Strong nucleophile anions are usually basic, which means they can't add directly to the carboyxylic acid because deprotonation is by far a faster reaction, and a deprotonated carboxylic acid is very deactivated to direct attack by strong nucleophiles even if you have excess reagent.  One important example of this is that you can't make an amide directly by amine substitution of a carboxylic acid.

Also halogens can't add directly because the rxn is so uphill thermodynamically, so that's not a plausible mechanism.

Substitution of a carboxylic acid requires acid.  For an example look up fischer esterification.

[Cheshyre]

Thanks much for your answer! This has been bothering me for two days, but with your answer in mind, I took a look through an Organic Chem textbook (Ege) and I think that the mechanism was trying to abbreviate the mechanism for the reduction of a carboxylic acid using Lithium Aluminum Hydride with that of ester formation.