saponification
Summary
Saponification is base-promoted hydrolysis of an ester, classically a triglyceride with NaOH or KOH, giving glycerol and carboxylate salts (soap). The carboxylic acid is deprotonated as it forms, so the reaction is effectively irreversible, unlike acid-catalyzed ester hydrolysis.
Detail
Meaning: Saponification is base-promoted ester hydrolysis. Hydroxide attacks the carbonyl carbon, the tetrahedral intermediate collapses, and an alkoxide leaves (nucleophilic acyl substitution). The carboxylic acid formed is deprotonated at once, which makes the reaction effectively irreversible. With a triglyceride, three hydroxides yield glycerol and three fatty acid carboxylates, which are soap: amphipathic salts that form micelles around grease.
Equation: RCOOR′ + OH⁻ → RCOO⁻ + R′OH. RCOOR′ is the ester (R the acyl group, R′ the alkyl group), OH⁻ is hydroxide from NaOH or KOH, RCOO⁻ is the carboxylate, R′OH is the alcohol. Amounts are in mol; one OH⁻ is consumed per ester group.
On the MCAT: Expect product prediction (carboxylate salt plus alcohol, not the free acid), stoichiometry (3 mol NaOH per mol triglyceride), and mechanism arrows. Soap questions ask which part dissolves in grease: the nonpolar tail enters the grease and the charged head faces water. Adding strong acid afterward protonates the carboxylate to the carboxylic acid.
Common confusion: Students treat hydroxide as a catalyst, like H⁺ in acid-catalyzed hydrolysis. Here the base is a reagent and is consumed, and the product is the carboxylate, not the acid. Acid-catalyzed hydrolysis is reversible.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (McMurry)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.