ester hydrolysis

Ester hydrolysis is simply the reverse of esterification: water breaks the ester apart, returning a carboxylic acid and an alcohol. It is how your body digests fats, how an old aspirin tablet slowly degrades, and how a polyester garment can eventually break down — water is patiently undoing what synthesis put together.

Hydrolysis means "splitting with water," and for an ester R-CO-O-R' the water molecule's oxygen attacks the carbonyl carbon, and after the dust settles you get R-COOH (the acid) and R'-OH (the alcohol). Acidic conditions make this happen by the same acid-catalyzed nucleophilic acyl substitution as Fischer esterification, just run in reverse: protonate the carbonyl, water adds to form a tetrahedral intermediate, the -OR' leaves as the alcohol, and the acid is released. Because it is the reverse of esterification, acid hydrolysis is an equilibrium, and you drive it forward by flooding the system with water.

There are two main routes. Acid-catalyzed hydrolysis is reversible and needs that excess of water to go to completion. Base-promoted hydrolysis — saponification — is different and irreversible, because the base deprotonates the carboxylic acid product to a carboxylate that cannot go back. This is why making soap uses lye (a strong base): the reaction runs cleanly one way. Hydrolysis is central to biology too; digestive enzymes called esterases and lipases hydrolyze dietary fats so the body can absorb the fatty acids.