Chemical properties

The most chemically reactive portion of fatty acids is the acidic carboxyl group (COOH). It reacts with alcohols (R′OH) to form products known as esters (RCOOR′) and releases water in the process. This ester bond is the principal covalent bond linking fatty acid moieties to other groups in the more-complex lipids discussed in other sections of this article. A second chemical bond, occurring much less frequently in biological lipids involving fatty acids, is the ether bond (R′―O―R). Ether bonds are chemically more stable than ester bonds.

The hydrocarbon part of a fatty acid molecule is quite resistant to chemical attack unless carbon-carbon double bonds are present. A number of different kinds of molecules react with such a double bond. For example, when a catalyst such as platinum is present, hydrogen gas adds to the double bond to give a saturated fatty acid. Halogens (chlorine, bromine, and iodine) and their derivatives such as hydroiodic acid (HI) also react with the double bond to form saturated fatty acids, but in these cases one or two atoms of the halogen replace one or two of the hydrogens normally found in the saturated acyl chain. Carbon-carbon double bonds can also react with oxygen in either nonenzymatic processes or enzymatically catalyzed oxidation reactions. This process generates a variety of products, some of which contribute to the rancid smell in spoiled meat and vegetable products. In general, the more highly unsaturated the fatty acid, the more easily it is oxidized.