|Molecular mass||44.05 g mol−1|
Pungent, fruity odor
|Density||0.788 g cm−3|
|Solubility in water||soluble in all proportions|
|Melting point||−123.5 °C|
|Boiling point||20.2 °C|
|Critical temperature||188 °C at 6.4 MPa|
|Viscosity||~0.215 at 20 °C|
|Molecular shape||trigonal planar (sp2) at C1
tetrahedral (sp3) at C2
|Dipole moment||2.7 D|
|EU classification||Very flammable (F+)
Carc. Cat. 3
|S-phrases||, , ,|
|Flash point||−39 °C|
|Autoignition temperature||185 °C|
|Supplementary data page|
|n, εr, etc.|
Solid, liquid, gas
|Spectral data||UV, IR, NMR, MS|
|Disclaimer and references|
Acetaldehyde, sometimes known as ethanal, is an organic chemical compound with the formula CH3CHO (or MeCHO). It is a flammable liquid with a fruity smell. It occurs naturally in ripe fruit, coffee, and fresh bread and is produced by plants as part of their normal metabolism. It is perhaps best 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.
Traces of acetaldehyde exist in the enol form, ethenol, with Keq = 6 x 10-5. Ethenol has been detected in the interstellar medium.
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”).
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.
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.
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 which 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 Antabuse (disulfiram) also prevents the oxidation of acetaldehyde to acetic acid, with the same unpleasant effects for drinkers. It has been used in the treatment of alcoholism.
All links retrieved February 10, 2016.
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