Chloromethane

Chloromethane, CH₃Cl, is methane with one of its hydrogens swapped for chlorine. It's a colorless gas with a faint, sweet smell. It was once used as a refrigerant but was dropped because it's toxic and flammable. Today its main job is as a building block.

The molecule is one carbon atom bonded to three hydrogen atoms and one chlorine atom. Most is made from methanol and hydrogen chloride: they react over a catalyst, the -OH on methanol is replaced by chlorine, and water is released. Some is made by reacting methane directly with chlorine, which also gives more heavily chlorinated products. Silicone plants often run a loop, feeding the hydrogen chloride released when silicones are made back in to make more chloromethane.

In this build it reacts with silicon in the direct process to make dimethyldichlorosilane. Each chloromethane gives one methyl group, which ends up on the silicone chain, and one chlorine. Silicones are its biggest use. Nature makes it too: oceans, plants, fungi and wildfires release millions of tonnes a year, making it the largest natural source of chlorine in the atmosphere.