Ethylene glycol

Ethylene glycol is a colorless, syrupy liquid. Each molecule is two carbon atoms with an alcohol group, made of oxygen and hydrogen, on each end. It's best known as car antifreeze. It tastes sweet but is poisonous, so some antifreeze has a bitter additive to discourage drinking it.

It's made in two steps from ethylene. First, ethylene and oxygen are passed over a silver catalyst at about 200–300 °C. One oxygen atom bridges the two carbons, forming a tight three-atom ring called ethylene oxide. Then the ethylene oxide is mixed with water and heated. The water pops the ring open, leaving an alcohol group on each carbon. Distillation separates the glycol from the water and from the longer glycols that form on the side.

Its two alcohol groups let it link up with terephthalic acid, one at each end, to make PET. In an electrolytic capacitor, it's the main liquid of the electrolyte that soaks the paper between the aluminum foils, carrying current to the thin oxide layer that stores the charge. Its high boiling point, about 197 °C, helps keep the capacitor from drying out.