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An aldehyde.

The first syllable of aldehyde or alcohol.

This is one of the systematic naming rules of the International Union of Pure and Applied Chemistry (IUPAC). The aldehyde group is —CHO, formed by the oxidation of alcohols. Some aldehydes were named before the rules were developed, so ethanal is better known as acetaldehyde and methanal as formaldehyde; other examples of systematic names are propanal and butanal; the generic term is alkanal. Chloral, a colourless, viscous liquid made by chlorinating acetaldehyde, is formally trichloroethanal; the name is usually an abbreviation for chloral hydrate, once used as a hypnotic and anaesthetic and as knock-out drops. See also aldo‑.

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