Acetaldehyde (systematically: ethanal) is an organic chemical compound with the formula CH₃CHO or MeCHO. It is a flammable liquid. 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 a chemical that causes hangovers.

Structure

Like other carbonyl compounds, acetaldehyde tautomerizes to give the enol, ethenol, with K_eq = 6 x 10^-5.

Production

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).Fact: date=February 2007 An important production method for acetaldehyde is the Wacker process.

The hydration of acetylene, catalyzed by mercury salts gives ethenol, which tautomerizes to acetaldehyde. This process was conducted on an industrial scale prior to the Wacker process.

Reactions

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.

Biochemistry

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.