Saponification

Saponification is the reaction in which an oil or fat is heated with a hot concentrated alkali, such as sodium hydroxide or potassium hydroxide, to produce soap and glycerol. Malay: saponifikasi · Chinese: 皂化反应.

An oil or fat is a triglyceride: an ester built from one glycerol molecule joined to three fatty acid molecules. During saponification the alkali breaks these three ester bonds. Each fatty acid is set free and immediately neutralised by the alkali, so it leaves as the sodium salt of the fatty acid, which is the soap. The glycerol backbone is released at the same time as the second product. Because a strong base is doing the breaking, saponification is the alkaline hydrolysis of an ester, and unlike ordinary ester hydrolysis it is not easily reversed, because the acid is turned into a salt as soon as it forms.

Example. When palm oil is boiled with concentrated sodium hydroxide solution, the mixture thickens as sodium salts of the fatty acids form. Salt is then added to make the soap separate, and the layer that rises to the top is scooped off, washed and pressed into bars. The word equation is simply: oil or fat + sodium hydroxide gives soap + glycerol. On an industrial scale the same reaction is run in large vessels, and the glycerol is recovered and sold rather than thrown away.

Confusion to avoid. Do not confuse saponification with esterification, which is almost its reverse: esterification joins an acid and an alcohol to make an ester, while saponification splits an ester with alkali. Remember too that soap is only one of the two products; students often forget to name glycerol, and lose the second mark for the products.

Saponification is the reaction at the centre of soap manufacture in this chapter, and it links the ester chemistry you met in Carbon Compounds to the cleaning action you study next. Writing the reactants and both products correctly is a common SPM Chemistry requirement.