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Acetaldehyde, also known as ethanal, is a chemical compound, an aldehyde with formula CH3CHO and structure

It is a highly reactive flammable liquid with a strong fruity smell. It melts at -123.5° C and boils at 21° C.

It occurs naturally in ripe fruit and coffee, and is produced by plants as part of their normal metabolism.

In the liver, the enzyme alcohol dehydrogenase converts ethanol into acetaldehyde, which is then further converted into the harmless acetic acid by acetaldehyde dehydrogenase[?]. Acetaldehyde is more toxic than alcohol and is responsible for many hangover symptoms. Some persons from far-Eastern descent have a mutation in their acetaldehyde dehydrogenase gene, making this enzyme less effective. In these people, acetaldehyde accumulates after drinking, leading to severe and immediate hangover symptoms. These people are therefore less likely to become alcoholics. The drug antabuse (disulfiram) also prevents the oxidation of acetaldehyde to acetic acid, with the same unpleasant effects for drinkers. It is used in the treatment of certain alcoholics.

The last steps of alcoholic fermentation in bacteria, plants and yeast involves the conversion of pyruvate into acetaldehyde by the enzyme pyruvate decarboxylase[?], followed by the conversion of acetaldehyde into ethanol. The latter reaction is again catalyzed by alcohol dehydrogenase, now operating in the opposite direction.

In the chemical industry, acetaldehyde is mainly used as an intermediary to produce acetic acid, certain esters, and a number of other chemicals. In 1989, US production stood at 740 million pounds.

When exposed to acetaldehyde vapors, humans develop irritation of the eyes, skin and the respiratory tract. Acetaldehyde is an air pollutant, emitted by cars and certain production facilities. It is also contained in tobacco smoke. It is classified as a probable carcinogen.

Three molecules of acetaldehyde can form the cyclic paraldehyde[?], and four can form the cyclic acetaldehyde tetramer[?].

The CAS registry number of acetaldehyde is 75-07-0 and its UN number is UN 1089.

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