Chloral hydrate, also known as trichloroacetaldehyde monohydrate, 2,2,2-trichloro-1,1-ethanediol, and under the tradenames Aquachloral, Novo-Chlorhydrate, Somnos, Noctec, and Somnote, is a sedative and hypnotic drug as well as a chemical reagent and precursor. Its chemical formula is C2H3Cl3O2.
It was discovered through the chlorination of ethanol in 1832 by Justus von Liebig in Gießen. It was widely abused and misprescribed in the late 19th century. Chloral hydrate is soluble in both water and alcohol, readily forming concentrated solutions. A solution of chloral hydrate in alcohol called "knockout drops" was used to prepare a Mickey Finn.
It is a minor side-product of the chlorination of water, concentrations rarely exceeding 5 micrograms per litre (µg/l).