Propylene

Propylene is a colorless gas, the slightly bigger cousin of ethylene. Its molecule has three carbon atoms in a row, with a double bond between two of them, and six hydrogen atoms. The double bond makes it reactive, and the extra carbon on the end gives chemists a handle for building more complex molecules.

Most propylene comes out of the same steam crackers that make ethylene: oil or gas molecules are heated with steam until they break apart, and the propylene is separated from the other gases by distillation. Oil refineries also produce it in their catalytic crackers, and some plants make it on purpose by pulling two hydrogen atoms off propane.

Its biggest use is polypropylene plastic, but in this build it does other jobs:

• With benzene, it makes cumene, which splits into phenol and acetone.
• Reacted with oxygen, it becomes acrylic acid.
• With ammonia and oxygen, it becomes acrylonitrile.
• With chlorine, it leads to epichlorohydrin, a key ingredient of epoxy resin.

Through these routes, propylene ends up in a TV's circuit boards, chip packages, optical films and plastic case.