Triglycerides and saponification template

Glycerol and three fatty acids give a triacylglycerol. Hydroxide adds to each ester C=O, the glyceryl alkoxide leaves and takes the acid’s proton, so three NaOH turn the fat into glycerol and three carboxylate soaps.

Mechanism step by step

Glycerol esterified by three fatty acids is a fat; hot NaOH hydrolyses the esters to glycerol and soap

1. Addition – OH⁻ attacks the ester carbonyl carbon
2. Elimination – C=O reforms and the glyceryl alkoxide leaves
3. Proton transfer – The alkoxide takes the acid’s proton: the carboxylate cannot react back
4. Twice more – The other two esters are cut the same way

Key points

• In the body, triacylglycerols are made from glycerol 3-phosphate and fatty acyl-CoA (acyltransferases), not from free acids.
• Saponification uses 3 mol NaOH per mol of fat and is irreversible: the final proton transfer leaves a carboxylate that no nucleophile attacks.
• Soaps are amphipathic. In water they form micelles, with the hydrocarbon tails inside (holding grease) and the COO⁻ heads facing the water.
• In hard water, Ca²⁺ and Mg²⁺ precipitate the soap as scum. The saponification value (mg KOH per g of fat) is higher for shorter chains.

Drawn with 154 atoms, 12 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 10 Textbook
• Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 21 Textbook
• Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019 Textbook