fatty acid

A fatty acid is the simplest lipid: a long hydrocarbon tail with a carboxylic acid head — a carboxylic acid you already know, but with a chain of ten, sixteen, eighteen or more carbons hanging off it. They are the building blocks of fats and oils, the molecules your body burns for long-term energy, and the raw material from which soap is made. Stearic acid (in beef fat) and oleic acid (in olive oil) are everyday examples.

Structurally a fatty acid is two personalities glued together. One end is a -COOH group: polar, water-loving, and acidic, the same carboxyl chemistry you studied. The other end is a long zig-zag chain of -CH2- units: completely nonpolar, water-fearing, held to other chains only by weak dispersion forces. The single most important variable is whether that tail is saturated (all C-C single bonds, a straight chain that packs tightly and stacks into a solid) or unsaturated (one or more C=C double bonds, usually cis, which put a permanent kink in the chain so the molecules cannot pack and the substance is a liquid oil). This is exactly the same idea as melting points rising with chain length and falling with branching — saturated animal fats are solid, unsaturated plant oils are liquid, for precisely this reason.

Fatty acids matter because all the other lipids are built from them. Esterify three fatty acids onto glycerol and you get a triglyceride, the fat in your food and your body. Replace one fatty acid with a phosphate group and you get a phospholipid, the molecule that forms every cell membrane. Treat a fat with strong base and you cleave the ester back to fatty-acid salts — soap. The whole lipid world, structural and edible, hangs off this one carboxylic-acid-with-a-tail.