Fatty acid

A fatty acid is a long-chain carboxylic acid that combines with glycerol to form the oils and fats known as triglycerides. Malay: asid lemak · Chinese: 脂肪酸.

A fatty acid has a –COOH group at one end and a long hydrocarbon chain making up the rest of the molecule. The –COOH group is what lets it join to glycerol through an ester bond, and three fatty acids joined to one glycerol give a triglyceride, which is an oil or a fat. The chain part can be saturated, with only single carbon-to-carbon bonds, or unsaturated, with one or more C=C double bonds, and this is what decides whether the fat is solid or the oil is liquid. When soap is made, saponification releases the fatty acids again, this time as their sodium salts.

Example. Stearic acid and palmitic acid are saturated fatty acids common in animal and palm fats, while oleic acid is an unsaturated fatty acid common in vegetable oils. Their sodium salts, sodium stearate and sodium palmitate, are ordinary soaps.

Confusion to avoid. Do not confuse a fatty acid with the whole fat. The fatty acid is only one building block; the fat is the triglyceride made when three fatty acids join a glycerol. Also keep the fatty acid, which is an acid, separate from glycerol, which is an alcohol; the ester bond in the fat is formed between the two.

The fatty acid is the building block that links oils, fats, saponification and the saturated-unsaturated distinction, so it underlies much of the SPM Chemistry work in this chapter. Notice that the same –COOH group you met in carboxylic acids in Carbon Compounds reappears here, so a fatty acid behaves as an acid in its own right and forms an ester with glycerol in the fat. Seeing that connection back to the earlier chapter makes the whole idea of oils, fats and soaps feel like an extension of work you have already done, not a fresh topic to memorise from scratch.