Oxygen Charge & Electron Density in Simple Carboxylic Acids

[Karan Punjabi]

In any simple carboxylic acid there are two oxygen atoms then i have a confusion that which oxygen has more negative charge on it or which one has the most electron density on it?

 

[Borek]

They are equivalent due to resonance, plus the carboxylic group freely rotates around the single bond - so it doesn't matter.

 

[Karan Punjabi]

They are equivalent due to resonance, plus the carboxylic group freely rotates around the single bond - so it doesn't matter.

But in mechanism of esterification in presence of an acid...the proton attacks tje double bonded oxygen

 

[Borek]

Once the carboxylic group becomes protonated (in other words: is not dissociated) it is no longer symmetrical (no resonance). Which oxygen becomes protonated is in most cases completely random. But it still doesn't matter to which oxygen the H is attached, as the group can freely rotate around "carbon-rest of the molecule" bond, so the final product will be exactly the same.

 

[TeethWhitener]

I'm assuming you're talking about acidic Fischer esterification, but it's hard to tell from your question what step is tripping you up. Since the environment is acidic, the carboxyl is going to be overwhelmingly neutral. However, the key intermediate in the reaction is a doubly protonated carboxyl group. Both oxygens become protonated and the two OH groups attached to the carboxyl carbocation draw a lot of electron density away from this carbon. The carbocation therefore has a significant partial positive charge which is readily attacked by a nucleophile (in the case of esterification, the nucleophile is the oxygen on a hydroxyl group). The link below gives a good overview:

http://www.organic-chemistry.org/namedreactions/fischer-esterification.shtm

The carbocation intermediate that I referred to is given by the resonance structure in the first set of brackets.