Acetaldehyde

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Acetaldehyde
Image:Acetaldehyde-skeletal.svg Image:Acetaldehyde-3D-vdW.png
IUPAC recommended name	Acetaldehyde
Systematic name	Ethanal
Chemical formula	C_{2_{H_{4_O}}}
SMILES	CC=O
Molecular mass	44.05 g mol⁻¹
Appearance	Colorless liquid Pungent, fruity odor
CAS number	[75-07-0]
Properties
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
Structure
Molecular shape	trigonal planar (sp²) at C₁ tetrahedral (sp³) at C₂
Dipole moment	2.7 D
Hazards
MSDS	External MSDS
EU classification	Very flammable (F+) Harmful (Xn) Carc. Cat. 3
NFPA 704	Image:NFPA 704.svg 4 2 2
R-phrases	R12, R36/37, R40
S-phrases	S2, S16, S33, S36/37
Flash point	−39 °C
Autoignition temperature	185 °C
RTECS number	AB1925000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related aldehydes	Formaldehyde Propionaldehyde
Disclaimer and references

Acetaldehyde, sometimes known as ethanal, is an organic chemical compound with the formula C H₃CHO or MeCHO. It is 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".

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).^{[citation needed]}

Only a trace of acetaldehyde exists as the enol form, ethenol, with K_eq = 6 x 10^-5.<ref>March, J. “Organic Chemistry: Reactions, Mechanisms, and Structures” J. Wiley, New York: 1992. ISBN 0-471-6010-2.</ref>

1 Applications in organic synthesis
- 1.1 Acetal derivatives
2 Biological aspects
- 2.1 Acetaldehyde and hangovers
3 Other occurrences
4 Safety
5 See also
6 References
7 External links

[edit] Applications in organic synthesis

Acetaldehyde is a common 2-carbon building block in organic synthesis.<ref>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</ref> Because of its small size and its availability in anhydrous form (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.<ref>Behrens, C.; Paquette, L. A. “N-Benzyl-2,3-Azetidinedione” Organic Syntheses, Collected Volume 10, p.41 (2004)</ref> Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives.<ref>Walter, L. A. “1-(α-Pyridyl)-2-Propanol” Organic Syntheses, Collected Volume 3, p.757 (1955).</ref> In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH₂OH)₄.<ref>Schurink, H. B. J. “Pentaerythritol” Organic Syntheses, Collected Volume 1, p.425 (1941)</ref>

In a Strecker reaction, acetaldehyde condenses with cyanide to give, after hydrolysis, the amino acid alanine.<ref>Kendall, E. C. McKenzie, B. F. “dl-Alanine” Organic Syntheses, Collected Volume 1, p.21 (1941)</ref>

Amines condense to give imines, an example being the imine derived from cyclohexylamine, N-ethylidenecyclohexylamine.<ref>Wittig, G.; Hesse, A. “Directed Aldol Condensations: β-Phenylcinnamaldehyde” Organic Syntheses, Collected Volume 6, p.901 (1988).</ref>

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”).<ref>Frank, R. L.; Pilgrim, F. J.; Riener, E. F. “5-Ethyl-2-Methylpyridine” Organic Syntheses, Collected Volume 4, p. 451 (1963).</ref>

[edit] Acetal derivatives

Three molecules of acetaldehyde condense to form “paraldehyde,” a cyclic trimer containing C-O single bonds; four condense to form the cyclic 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."<ref>Adkins, H.; Nissen, B. H. “Acetal” Organic Syntheses, Collected Volume 1, p.1 (1941)</ref>

[edit] 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 alcohol dehydrogenase, now operating in the opposite direction.

[edit] Acetaldehyde and hangovers

Acetaldehyde is more toxic than ethanol and is responsible for many hangover symptoms. N-Acetylcysteine (NAC) is known to assist in processing acetaldehyde in the body and therefore can help to relieve hangover symptoms. Cysteine, which contains a thiol group, can force conversion back into ethanol, to similar effect.

Most people of East Asian descent have a mutation in their acetaldehyde dehydrogenase gene that makes this enzyme unusually effective at converting ethanol to acetaldehyde, but also less effective at converting acetaldehyde to acetic acid. Known as the alcohol flush reaction, 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 is used in the treatment of certain cases of alcoholism.

[edit] Other occurrences

Acetaldehyde is an air pollutant, emitted by cars and certain production facilities. It is also contained in tobacco smoke, contributing to the smoke's addictive properties. <ref>Smoking. (2006). Encyclopædia Britannica. Accessed 27 Oct 2006.</ref>