acetaldehyde

[as-i-tal-duh-hahyd]

acetaldehyde or ethanal, CH₃CHO, colorless liquid aldehyde, sometimes simply called aldehyde. It melts at -123°C;, boils at 20.8°C;, and is soluble in water and ethanol. It is formed by the partial oxidation of ethanol; oxidation of acetaldehyde forms acetic acid. Acetaldehyde is made commercially by the oxidation of ethylene with a palladium catalyst (see Wacker process). It is used as a reducing agent (e.g., for silvering mirrors), in the manufacture of synthetic resins and dyestuffs, and as a preservative. When treated with a small amount of sulfuric acid it forms paraldehyde, (CH₃CHO)₃, a trimer, which is used as a hypnotic drug.

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Acetaldehyde

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Acetaldehyde, sometimes known as ethanal, is an organic chemical compound with the formula C H₃CHO or MeCHO. It is a flammable liquid with a fruity smell. Acetaldehyde occurs naturally in ripe fruit, coffee, and fresh bread, and is produced by plants as part of their normal metabolism. It is popularly known as the chemical that causes hangovers.

In the chemical industry, acetaldehyde is used as an intermediate in the production of acetic acid, certain esters, and a number of other chemicals. In 1989, US production stood at 740 million pounds (336,000 tonnes). An important production method for acetaldehyde is the Wacker process.

Biological aspects

In the liver, the enzyme alcohol dehydrogenase converts ethanol into acetaldehyde, which is then further converted into harmless acetic acid by acetaldehyde dehydrogenase. The last steps of alcoholic fermentation in bacteria, plants and yeast involve 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 an alcohol dehydrogenase, now operating in the opposite direction.

Acetaldehyde and hangovers

Most people of East Asian descent have a mutation in their alcohol dehydrogenase gene that makes this enzyme unusually effective at converting ethanol to acetaldehyde, and about half of such people also have a form of acetaldehyde dehydrogenase that is less effective at converting acetaldehyde to acetic acid. This combination causes them to suffer from the alcohol flush reaction, in which acetaldehyde accumulates after drinking, leading to severe and immediate hangover symptoms. These people are therefore less likely to become alcoholics. The drug disulfiram (Antabuse) also prevents the oxidation of acetaldehyde to acetic acid, with the same unpleasant effects for drinkers. It is used in the treatment of alcoholism.

Applications in organic synthesis

Acetaldehyde is a common 2-carbon building block in organic synthesis. Because of its small size and its availability as the anhydrous monomer (unlike formaldehyde), it is a common electrophile. With respect to its condensation reactions, acetaldehyde is prochiral. It is mainly used as a source of the CH₃C⁺H(OH) synthon in aldol and related condensation reactions. Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives. In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH₂OH)₄.

In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine. Acetaldehyde can condense with amines to yield imines, such as the condensation with cyclohexylamine to give N-ethylidenecyclohexylamine. These imines can be used to direct subsequent reactions like an aldol condensation.

It is also an important building block for the synthesis of heterocyclic compounds. A remarkable example is its conversion upon treatment with ammonia to 5-ethyl-2-methylpyridine ("aldehyde-collidine”).

Acetal derivatives

Three molecules of acetaldehyde condense to form “paraldehyde,” a cyclic trimer containing C-O single bonds; four condense to form the cyclic molecule called metaldehyde.

Acetaldehyde forms a stable acetal upon reaction with ethanol under conditions that favor dehydration. The product, CH₃CH(OCH₂CH₃)₂, is in fact called "acetal, although acetal is used more widely to describe other compounds with the formula RCH(OR')₂.

Ethenol

Only a trace of acetaldehyde exists as the enol form, ethenol, with K_eq = 6 x 10^-5.

Environmental occurrence

Acetaldehyde is an air pollutant resulting from combustion, such as automotive exhaust and tobacco smoke. It is also created by thermal degradation of polymers in the plastics processing industry.