Acetone

Acetone is the simplest ketone: a carbon double-bonded to oxygen, with a methyl group on each side (CH₃COCH₃). It's a clear liquid that evaporates quickly, mixes with water and dissolves many plastics and glues. It's the main ingredient in many nail polish removers, and the human body makes small amounts too.

Most acetone is a co-product of the cumene process that makes phenol. Benzene and propylene are joined into cumene, a ring with a three-carbon branch. Oxygen from air attaches to the branch, and then acid splits the molecule in two. The ring keeps the oxygen and becomes phenol, while the three-carbon branch from propylene breaks off as acetone. So benzene carries the reaction, but the acetone itself comes from propylene. About 0.6 tonnes of acetone come out for every tonne of phenol, so the supply of one is tied to demand for the other.

In this build, acetone's job is to join. Its central carbon links two phenol molecules into bisphenol A, the core of epoxy resin. Acetone is also used to make methyl methacrylate, the building block of acrylic plastic, and as a cleaning solvent in electronics factories.