Vinyl chloride

Vinyl chloride is a small molecule: two carbon atoms joined by a double bond, three hydrogens and one chlorine (C₂H₃Cl). It is a sweet-smelling, flammable gas at room temperature. Nearly all of it is made for one purpose: to be turned into PVC.

It is made from ethylene and chlorine in two steps. First the two react to form ethylene dichloride, a liquid, with a chlorine atom added to each carbon. Then that liquid is heated to around 500 °C in a furnace, a step called cracking, which pulls off one hydrogen and one chlorine as hydrogen chloride and leaves vinyl chloride. Plants recycle the hydrogen chloride by reacting it with more ethylene and oxygen to make more ethylene dichloride, so very little chlorine is wasted.

The double bond is the useful part. In a reactor it can open and link to the next molecule, over and over, forming the long chains of PVC. Vinyl chloride causes a rare liver cancer, discovered among factory workers in the 1970s, so today it is handled in sealed systems with strict exposure limits.