Fats and oils

Section 2 of 4

Butter, olive oil and the fat stored in your body are all the same kind of molecule: a big ester, built from one glycerol and three fatty acids.

Glycerol is a chain of three carbons with an -⁠OH group on every carbon, so it is an alcohol three times over.

Glycerol (propane-1,2,3-triol), C₃H₈O₃A chain of three carbon atoms with an -OH group on each carbon.HHHHCCCHOOOHHH
Glycerol, C₃H₈O₃: an -⁠OH group on each of its three carbons. Its chemical name, propane-⁠1,2,3-⁠triol, says the same.

A fatty acid is a long hydrocarbon chain with a carboxylic acid group, -⁠COOH, on the end. Palmitic acid, named after palm oil, has a chain of 16 carbons. Each -⁠OH on glycerol reacts with the -⁠COOH of a fatty acid, the way ethanol reacts with ethanoic acid, and makes an ester link. Three links give off three water molecules and make one fat molecule, called a triglyceride.

A fat molecule: glycerol joined to three fatty acid chains On the left, glycerol's three carbons, CH2, CH and CH2, in a column. Each one is joined through an oxygen atom to a carbon with a double-bonded oxygen, the ester link, highlighted. From each ester link a long zigzag chain of carbon atoms runs to the right: three palmitic acid chains of 16 carbons each. O O CH₂ O O CH O O CH₂ glycerol three fatty acid chains ester links
A fat molecule: glycerol (left) joined through three ester links (highlighted) to three palmitic acid chains. Making it gave off three water molecules.

Saturated and unsaturated

In palmitic acid every carbon-carbon bond is a single bond, so each carbon holds as many hydrogen atoms as it can. Chemists call it saturated: full up with hydrogen. Oleic acid, the main fatty acid in olive oil, has one C=C double bond in the middle of its 18-⁠carbon chain, so it is unsaturated. A C=C bond holds its atoms flat and fixed in place, and in oleic acid both halves of the chain sit on the same side of it, so the chain keeps a kink.

Palmitic acid as a skeletal formula A straight zigzag line of 16 carbon atoms joined by single bonds. At the left end, the first carbon carries a double-bonded oxygen and an -OH group. HO O
Palmitic acid: a straight chain of 16 carbons joined by single bonds. It melts at 63 °C.
Oleic acid as a skeletal formula A zigzag line of 18 carbon atoms. At the left end, the first carbon carries a double-bonded oxygen and an -OH group. Halfway along, a highlighted C=C double bond bends the chain downward. HO O
Oleic acid: 18 carbons with one C=C double bond (highlighted), where the chain bends. It melts at 13 °C.

Straight chains lie close together, like pencils in a box, and cling to each other along their whole length. Fats rich in saturated chains, such as butter and beef fat, are solid at room temperature. Kinked chains leave gaps between them, so oils rich in unsaturated chains, such as olive oil and sunflower oil, stay liquid. Adding hydrogen across some of the C=C bonds with a nickel catalyst, the addition reaction you met earlier, takes out kinks, and that is how a liquid vegetable oil becomes a solid spread.

Straight chains pack tightly; kinked chains stay apart Two panels of fatty acid chains drawn as zigzag lines. Left, a fat: eight straight chains lie side by side, close together. Right, an oil: two chains, each with a highlighted C=C double bond in the middle that bends the chain, so the chains cannot lie close together. fat straight chains pack tightly oil kinked chains stay apart
Straight saturated chains pack tightly, so a fat is solid. The kink at each C=C bond (highlighted) keeps unsaturated chains apart, so an oil is liquid.

Quick check

Butter is solid at room temperature and olive oil is liquid. What explains this?