Triglyceride

A triglyceride is the triester formed from one glycerol molecule and three fatty acid molecules. It is the general structure of all oils and fats. Malay: trigliserida · Chinese: 三酸甘油酯.

Glycerol has three –OH groups, and each one can join to the –COOH group of a fatty acid, releasing water and forming an ester bond. Three such bonds turn one glycerol and three fatty acids into a single large molecule with three ester linkages, which is why it is called a triglyceride, or triester. Every oil and fat you meet in this chapter, whether from palm, coconut or animal sources, has this same backbone; only the fatty acid chains differ. When the fatty acids are unsaturated the substance tends to be a liquid oil, and when they are saturated it tends to be a solid fat.

Example. A molecule of a typical vegetable oil is a triglyceride in which glycerol is joined to three long fatty acid chains. When this triglyceride is boiled with concentrated alkali during saponification, the three ester bonds break, releasing soap and glycerol.

Confusion to avoid. Do not confuse a triglyceride with a single fatty acid. The fatty acid is just one of the three chains; the triglyceride is the whole ester made from glycerol and three of those chains. Nor is it the same as glycerol alone, which is only the three-carbon backbone before the acids are attached.

The triglyceride structure links the properties of oils and fats to saponification and to the bromine water test, so understanding it underpins several SPM Chemistry questions in this chapter. If you can picture the molecule as a glycerol backbone with three fatty acid chains hanging from it, you can see at once why saponification gives one glycerol and three soap molecules, and why an unsaturated triglyceride decolourises bromine water at the double bonds in its chains. That single mental picture ties several separate facts of the chapter together.