Chloral hydrate

Chloral hydrate is a 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).

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