Question

If any substance producing H+ is acid, but in esterification reaction carboxylic acid is an acid but it breaks OH bond and releases hydroxyl ion so it should be base, why it is breaking OH bond instead of giving hydronium ion?

Answer 1

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In order to understand why this happens, we need to look at the mechanism.

So fisher esterfication is when we react a carboxylic acid with an alcohol along with an acid catalyst(such as H2SO4).

The mechanism goes like this:

1. The acid protonates the O of the carbonyl because it's more basic than an alcohol. (Note it doesn't have an H to donate like the alcohol does)
2. This makes the site more electrophilic(with a positive charge) and allows the alcohol oxygen to do a nucleophilic attack at the carbonyl site.
3. After a series of proton shuffling, the carbonyl is reformed and a water molecule is kicked out because it's a great leaving group. The driving force for this reaction is the reformation of a C=O bond because it’s more stable.
4. Our acid catalyst gets reformed.

It’s important to note that every step in this reaction is reversible which is why it takes a long time. This reaction tends to be carried out under reflux(thermodynamic control). The product is then obtained by separation of the organic layer(ester) from the aqueous layer with a separatory funnel.