Chloral Hydrate

Chloral hydrate is an unapproved, Schedule IV sedative and hypnotic drug as well as a chemical reagent and precursor. The name chloral hydrate indicates that it is formed from chloral (trichloroacetaldehyde) by the addition of one molecule of water. Its chemical formula is C₂H₃Cl₃O₂.

It was discovered through the chlorination of ethanol in 1832 by Justus von Liebig in Gießen. Its sedative properties were first published in 1869 and subsequently, because of its easy synthesis, its use was widespread. It was widely used recreationally 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. More reputable uses of chloral hydrate include its use as a clearing agent for chitin and fibers and as a key ingredient in Hoyer's mounting medium, which is used to prepare permanent or semipermanent microscope slides of small organisms, histological sections, and chromosome squashes.

It is, together with chloroform, a minor side-product of the chlorination of water when organic residues are present in the water, though concentrations rarely exceed 5 micrograms per litre (µg/L).

Chloral hydrate has not been approved by the FDA in the United States or the EMA in the European Union for any medical indication and is on the list of unapproved drugs that is still prescribed by clinicians. Usage of this drug as a sedative or hypnotic may be very dangerous given the lack of clinical trials.