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Chemical structure of acetaldehydeThree-dimensional structure of acetaldehyde
Common name acetaldehyde
IUPAC name acetaldehyde
Systematic name ethanal
Chemical formula C2H4O
Molecular mass 44.05 g mol−1
Appearance Colorless liquid
Pungent, fruity odor
CAS number [75-07-0]
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
MSDS External MSDS
EU classification Very flammable (F+)
Harmful (Xn)
Carc. Cat. 3
NFPA 704
NFPA 704.svg

R-phrases R12

, R36/37 , R40

  1. REDIRECT Template:(S2)

, S16 , S33 , S36/37

Flash point −39 °C
Autoignition temperature 185 °C
RTECS number AB1925000
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Related aldehydes Formaldehyde
Ethylene oxide
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. Acetaldehyde occurs naturally in ripe fruit, coffee, and fresh bread and is produced by plants as part of their normal metabolism. It is probably best known as the chemical that causes "hangovers".

Biochemistry and health effects

In the liver, the enzyme alcohol dehydrogenase oxidizes ethanol into acetaldehyde, which is then further oxidized into harmless acetic acid by acetaldehyde dehydrogenase. These two oxidation reactions are coupled with the reduction of NAD+ to NADH.[1] In the brain, alcohol dehydrogenase has a minor role in the oxidation of ethanol to acetaldehyde. Instead, the enzyme catalase primarily oxidizes ethanol to acetaldehyde.[1] 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 [2]. 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.

Tobacco addiction

Acetaldehyde is a significant constituent of tobacco smoke. It has been demonstrated to have a synergistic effect with nicotine, increasing the onset and tenacity of addiction to cigarette smoking, particularly in adolescents.[3][4]

Alzheimer's disease

People who have a genetic deficiency for the enzyme responsible for the conversion of acetaldehyde into acetic acid may have a greater risk of Alzheimer's disease. "These results indicate that the ALDH2 deficiency is a risk factor for LOAD [late-onset Alzheimer's disease] …"[5]

Alcohol problems

Acetaldehyde derived from the consumption of ethanol binds to proteins to form adducts that are linked to organ disease.[6]

The drug disulfiram (Antabuse) prevents the oxidation of acetaldehyde to acetic acid, and it has the same unpleasant effect on drinkers. Antabuse is sometimes used as a deterrent for alcoholics who wish to stay sober.


Acetaldehyde is a probable carcinogen in humans.[7] In the year 1988 the International Agency for Research on Cancer stated, "There is sufficient evidence for the carcinogenicity of acetaldehyde (the major metabolite of ethanol) in experimental animals."[8] In October 2009 the International Agency for Research on Cancer updated the classification of acetaldehyde stating that acetaldehyde included in and generated endogenously from alcoholic beverages is a Group I human carcinogen.[9] In addition, acetaldehyde is damaging to DNA[10] and causes abnormal muscle development as it binds to proteins.[11]

A study of 818 heavy drinkers found that those who are exposed to more acetaldehyde than normal through a defect in the gene for acetaldehyde dehydrogenase are at greater risk of developing cancers of the upper gastrointestinal tract and liver.[12]


Acetaldehyde is toxic when applied externally for prolonged periods, an irritant, and a probable carcinogen.[7] It 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.[13] Acetaldehyde naturally breaks down in the human body[14] but has been shown to excrete in urine of rats.[15]


Only a trace of acetaldehyde exists as the enol form, ethenol, with Keq = 6 x 10-5.[16]

Applications in organic synthesis

Acetaldehyde is a common 2-carbon building block in organic synthesis.[17] 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.[18] Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives.[19] In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH2OH)4.[20]

In a Strecker amino acid synthesis, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine.[21] 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.[22]

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”).[23]

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

See also


  1. 1.0 1.1 Hipolito, L.; Sanchez, M. J.; Polache, A.; Granero, L. Brain metabolism of ethanol and alcoholism: An update. Curr. Drug Metab. 2007, 8, 716-727.
  2. 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.
  3. Study Points to Acetaldehyde-Nicotine Combination in Adolescent Addiction
  4. Nicotine's addictive hold increases when combined with other tobacco smoke chemicals, UCI study finds
  5. , S, Ohsawa I, Kamino K, Ando F, Shimokata H. (April 2004)Mitochondrial ALDH2 Deficiency as an Oxidative Stress. Annals of the New York Academy of Sciences 1011: 36–44.
  6. Nakamura, K.; Iwahashi, K.; Furukawa, A.; Ameno, K.; Kinoshita, H.; Ijiri, I.; Sekine, Y.; Suzuki, K.; Iwata, Y.; Minabe, Y.; Mori, N. Acetaldehyde adducts in the brain of alcoholics. Arch. Toxicol. 2003, 77, 591.
  7. 7.0 7.1 Chemical Summary For Acetaldehyde, US Environmental Protection Agency
  8. International Agency for Rescarch on Cancer, World Health Organization. (1988). Alcohol drinking, Lyon: World Health Organization, International Agency for Research on Cancer. p3
  9. International Agency for Research on Cancer Monograph Working Group, Special Report: Policy A review of human carcinogens—Part E: tobacco, areca nut, alcohol, coal smoke, and salted fish. The Lancet 2009 10, 1033–1034
  11. Nicholas S. Aberle, II, Larry Burd, Bonnie H. Zhao and Jun Ren (2004). Acetaldehyde-induced cardiac contractile dysfunction may be alleviated by vitamin B1 but not by vitamins B6 or B12. Alcohol & Alcoholism 39 (5): 450–454.
  12. Nils Homann, Felix Stickel, Inke R. König, Arne Jacobs, Klaus Junghanns, Monika Benesova, Detlef Schuppan, Susanne Himsel, Ina Zuber-Jerger, Claus Hellerbrand, Dieter Ludwig, Wolfgang H. Caselmann, Helmut K. Seitz Alcohol dehydrogenase 1C*1 allele is a genetic marker for alcohol-associated cancer in heavy drinkers International Journal of Cancer Volume 118, Issue 8, Pages 1998-2002
  13. Smoking. (2006). Encyclopædia Britannica. Accessed 27 October 2006.
  14. [1]
  15. Determinations of ethanol, acetaldehyde, and acetate in blood and urine during alcohol oxidation in man
  16. March, J. “Organic Chemistry: Reactions, Mechanisms, and Structures” J. Wiley, New York: 1992. ISBN 0-471-58148-8.
  17. Sowin, T. J.; Melcher, L. M. ”Acetaldehyde” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York.
    1. REDIRECT Template:Doi
  18. Behrens, C.; Paquette, L. A. “N-Benzyl-2,3-Azetidinedione” Organic Syntheses, Collected Volume 10, p.41 (2004)
  19. Walter, L. A. “1-(α-Pyridyl)-2-Propanol” Organic Syntheses, Collected Volume 3, p.757 (1955).
  20. Schurink, H. B. J. “Pentaerythritol” Organic Syntheses, Collected Volume 1, p.425 (1941).
  21. Kendall, E. C. McKenzie, B. F. “dl-Alanine” Organic Syntheses, Collected Volume 1, p.21 (1941).
  22. Wittig, G.; Hesse, A. “Directed Aldol Condensations: β-Phenylcinnamaldehyde” Organic Syntheses, Collected Volume 6, p.901 (1988).
  23. Frank, R. L.; Pilgrim, F. J.; Riener, E. F. “5-Ethyl-2-Methylpyridine” Organic Syntheses, Collected Volume 4, p. 451 (1963).
  24. Adkins, H.; Nissen, B. H. “Acetal” Organic Syntheses, Collected Volume 1, p.1 (1941).

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