Terephthalic acid

Terephthalic acid is a white, crystalline powder. Each molecule is a ring of six carbon atoms, like benzene, with an acid group made of carbon, oxygen and hydrogen attached at opposite ends. Having a reactive group at each end is what makes it useful: it can join a chain in two directions at once.

It's made from para-xylene, a ring molecule from oil refining that has a small methyl group at each end. Para-xylene is dissolved in acetic acid and blown with air in a hot, pressurized reactor, with a catalyst of cobalt, manganese and bromine salts. Oxygen from the air turns each methyl group into an acid group. The product crystallizes out and is purified, often by dissolving it in hot water under pressure and treating it with hydrogen over a palladium catalyst.

Paired with ethylene glycol, it links up into PET. Paired with butanediol, it makes PBT plastic, used for connector housings. In both, the stiff ring gives the plastic its strength and heat resistance. The industry calls the pure grade PTA, for purified terephthalic acid, and makes tens of millions of tonnes of it a year.