Acetaldehyde

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Acetaldehyde
Acetaldehyde-skeletal.svg
Acetaldehyde-3D-balls.png
Acetaldehyde-3D-vdW.png
IUPAC name acetaldehyde
Systematic name ethanal
Other names acetaldehyde
Identifiers
CAS number 75-07-0
RTECS number AB1925000
SMILES CC=O
Properties
Molecular formula C2H4O
Molar mass 44.05 g mol−1
Appearance Colorless liquid
Pungent, fruity odor
Density 0.788 g cm−3
Melting point

−123.5 °C

Boiling point

20.2 °C

Solubility in water soluble in all proportions
Viscosity ~0.215 at 20 °C
Structure
Molecular shape trigonal planar (sp²) at C1
tetrahedral (sp³) at C2
Dipole moment 2.7 D
Hazards
EU classification Very flammable (F+)
Harmful (Xn)
Carc. Cat. 3
NFPA 704

NFPA 704.svg

4
2
0
 
R-phrases R12, R36/37, R40
S-phrases (S2), S16, S33, S36/37
Flash point −39 °C
Autoignition
temperature
185 °C
Related Compounds
Related aldehydes Formaldehyde
Propionaldehyde
Ethylene oxide
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Overview

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. 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 t). An important production method for acetaldehyde is the Wacker process.

Ethenol

Only a trace of acetaldehyde exists as the enol form, ethenol, with Keq = 6 x 10-5.[1] Ethenol has been detected in the interstellar medium, specifically in between our Milky Way Galaxy and the Galaxy N-ID9[2]

Applications in organic synthesis

Acetaldehyde is a common 2-carbon building block in organic synthesis.[3] 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.[4] Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives.[5] In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH2OH)4.[6]

In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine.[7] 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.[8]

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”).[9]

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,"[10] although acetal is used more widely to describe other compounds with the formula RCH(OR')2.

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 which is less effective at converting acetaldehyde to acetic acid[11]. 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.

Other occurrences

Acetaldehyde is an air pollutant resulting from combustion, such as automotive exhaust and tobacco smoke, contributing to the addictive properties of tobacco.[12]

Safety

Acetaldehyde is toxic, an irritant, and a probable carcinogen.[13]

References

  1. March, J. “Organic Chemistry: Reactions, Mechanisms, and Structures” J. Wiley, New York: 1992. ISBN 0-471-58148-8.
  2. Spielfiedel, A.; Feautrier, N.; Balanca, C.; Dayou, F.; Lique, F.; Senent, M.-L. Molecules as diagnostic tools in the interstellar medium. Molecular Physics (2007), 105(9), 1263-1269..
  3. Sowin, T. J.; Melcher, L. M. ”Acetaldehyde” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289
  4. Behrens, C.; Paquette, L. A. “N-Benzyl-2,3-Azetidinedione” Organic Syntheses, Collected Volume 10, p.41 (2004)
  5. Walter, L. A. “1-(α-Pyridyl)-2-Propanol” Organic Syntheses, Collected Volume 3, p.757 (1955).
  6. Schurink, H. B. J. “Pentaerythritol” Organic Syntheses, Collected Volume 1, p.425 (1941). http://www.orgsyn.org/orgsyn/pdfs/CV1P0425.pdf
  7. Kendall, E. C. McKenzie, B. F. “dl-Alanine” Organic Syntheses, Collected Volume 1, p.21 (1941). http://www.orgsyn.org/orgsyn/pdfs/CV1P0021.pdf
  8. Wittig, G.; Hesse, A. “Directed Aldol Condensations: β-Phenylcinnamaldehyde” Organic Syntheses, Collected Volume 6, p.901 (1988).
  9. Frank, R. L.; Pilgrim, F. J.; Riener, E. F. “5-Ethyl-2-Methylpyridine” Organic Syntheses, Collected Volume 4, p. 451 (1963). http://www.orgsyn.org/orgsyn/pdfs/CV4P0451.pdf
  10. Adkins, H.; Nissen, B. H. “Acetal” Organic Syntheses, Collected Volume 1, p.1 (1941).http://www.orgsyn.org/orgsyn/pdfs/CV1P0001.pdf
  11. Xiao Q, Weiner H, Crabb DW (1996). "The mutation in the mitochondrial aldehyde dehydrogenase (ALDH2) gene responsible for alcohol-induced flushing increases turnover of the enzyme tetramers in a dominant fashion". J. Clin. Invest. 98 (9): 2027-32. PMID 8903321.
  12. Smoking. (2006). Encyclopædia Britannica. Accessed 27 Oct 2006.
  13. http://www.epa.gov/chemfact/s_acetal.txt

See also

External links


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