Saponification

Saponification is a process that produces soap, usually from fats and lye.

Vegetable oils and animal fats are the main materials that are saponified. These greasy materials, triesters called triglycerides, are mixtures derived from diverse fatty acids. Triglycerides can be converted to soap in either a one- or a two-step process. In the traditional one-step process, the triglyceride is treated with a strong base (e.g., lye), which accelerates cleavage of the ester bond and releases the fatty acid salt and glycerol. This process is the main industrial method for producing glycerol. If necessary, soaps may be precipitated by salting it out with saturated sodium chloride. The saponification value is the amount of base required to saponify a fat sample. For soap making, the triglycerides are highly purified, but saponification includes other base hydrolysis of unpurified triglycerides, for example, the conversion of the fat of a corpse into adipocere, often called "grave wax." This process is more common where the amount of fatty tissue is high, the agents of decomposition are absent or only minutely present.

The mechanism by which esters are cleaved by base involves a series of equilibria. The hydroxide anion adds to (or "attacks") the carbonyl group of the ester. The immediate product is called an orthoester:

Expulsion of the alkoxide generates a carboxylic acid:

The alkoxide ion is a strong base so that the proton is transferred from the carboxylic acid to the alkoxide ion creating an alcohol:

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