What is Acetaldehyde?

Acetaldehyde
IUPAC name	acetaldehyde
Systematic name	ethanal
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
MSDS	External MSDS
EU classification	Very flammable (F+) Harmful (Xn) Carc. Cat. 3
NFPA 704	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
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 C, 100 kPa)

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 K_eq = 6 x 10^-5.^[1]

Applications in organic synthesis

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

In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine.^[6] 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.^[7]

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”).^[8]

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,"^[9] 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^[10]. 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.^[11]

Safety

Acetaldehyde is toxic, an irritant, and a probable carcinogen.^[12]

References

See also

• Wine fault

External links

• International Chemical Safety Card 0009
• National Pollutant Inventory - Acetaldehde
• NIOSH Pocket Guide to Chemical Hazards
• IARC Monograph: "Acetaldehyde"
• EPA factsheet about acetaldehyde
• Hal Kibbey, Genetic Influences on Alcohol Drinking and Alcoholism, Indiana University Research and Creative Activity, Vol. 17 no. 3.
• United States Food and Drug Administration (FDA) information for acetaldehyde