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QUESTION

# What is esterification?

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.