Dimethyldichlorosilane

Dimethyldichlorosilane, (CH₃)₂SiCl₂, is a colorless liquid with a silicon atom at its center, bonded to two methyl groups and two chlorine atoms. It fumes in moist air, because water attacks the chlorines and releases hydrogen chloride. That same reaction makes it the starting point for nearly every silicone.

It's made by the direct process, discovered independently in the early 1940s by Eugene Rochow in the US and Richard Müller in Germany. Powdered metallurgical-grade silicon is mixed with a copper catalyst in a fluidized-bed reactor, where gas blows up through the powder so it churns like a boiling liquid. Chloromethane gas is fed in at about 300 °C. Its methyl groups and chlorine atoms attach to silicon atoms at the surface, and a mix of methylchlorosilanes leaves as vapor. Dimethyldichlorosilane is usually most of it, and is separated by careful distillation, since several of the products boil at very similar temperatures.

Its two chlorines are the key to silicone. With water, each becomes a link point, so each molecule joins two neighbors and builds a chain. Related molecules with three chlorines make branch points for cross-linked resins, and ones with one chlorine cap the chain ends.