Esterification is when two reactants basically form an ester in the end

Esterification is when two reactants basically form an ester in the end. But you knew that.

A common one is called the Fischer esterification, which is when excess/xs alcohol reacts with a carboxylic acid in (other) acid.

Here is an example of a general carboxylic acid reacting with a general alcohol in ##"HCl"##:

The mechanism is just like other nucleophilic addition-elimination reactions in your book.

1. The ##"HCl"## catalyst has already protonated your alcohol and the alcohol has already transferred the proton to the electron-rich carbonyl oxygen. Under these conditions, the regular alcohol can act as a good nucleophile.
2. Proton transfer part 1.
3. Proton transfer part 2.
4. Now the water is a good leaving group, so the tetrahedral collapse occurs.
5. The alcohol takes the proton off of the carbonyl oxygen to regenerate the dominant form of your ##"HCl"## catalyst (the protonated alcohol) and finish the reaction. The pKa of ethanol is about ##15.9##, and water's is about ##15.7##, so ethanol is more likely to want a proton.

You should notice that the ##"OH"## on the original, general carboxylic acid was replaced with the alkoxide of your general alcohol.