acetaldehyde

Acetaldehyde
IUPAC name	Acetaldehyde
Systematic name	Ethanal
Other names	Acetic Aldehyde Ethyl Aldehyde[1]
Identifiers
CAS number	75-07-0 Y
PubChem	177
EC number	200-836-8
RTECS number	AB1925000
SMILES	O=CC
InChI	1/C2H4O/c1-2-3/h2H,1H3
InChI key	IKHGUXGNUITLKF-UHFFFAOYAB
ChemSpider ID	172
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, 150 K, -190 °F
Boiling point	20.2 °C, 293 K, 68 °F
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
R-phrases	R12 R36/37 R40
S-phrases	(S2) S16 S33 S36/37
NFPA 704	4 3 2
Flash point	−39 °C
Autoignition temperature	185 °C
Related compounds
Related aldehydes	Formaldehyde Propionaldehyde
Related compounds	Ethylene oxide
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Acetaldehyde (systematically: ethanal) is an organic chemical compound with the formula CH₃CHO or MeCHO. It is a flammable liquid. Acetaldehyde occurs naturally in ripe fruit, coffee, and bread, and is produced by plants as part of their normal metabolism. It is popularly known as a chemical that causes hangovers.^[2]

Structure

Like other carbonyl compounds, acetaldehyde tautomerizes to give the enol, ethenol, with K_eq = 6 x 10⁻⁵.^[3]

Production

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 tonnes).^{[citation needed]} An important production method for acetaldehyde is the Wacker process.

The hydration of acetylene, catalyzed by mercury salts gives ethenol, which tautomerizes to acetaldehyde. This process was conducted on an industrial scale prior to the Wacker process.^[4]

Reactions

Acetaldehyde is a common 2-carbon building block in organic synthesis.^[5] 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.^[6] Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives.^[7] In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH₂OH)₄.^[8]

In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine.^[9] 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.^[10]

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”).^[11]

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,"^[12] although acetal is used more widely to describe other compounds with the formula RCH(OR')₂.

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^[13]. 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^[14]. 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.

Hangover

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 that is less effective at converting acetaldehyde to acetic acid.^[15] 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^[16][17]. The drug disulfiram (Antabuse) also prevents the oxidation of acetaldehyde to acetic acid, with the same unpleasant effects for drinkers. Antabuse is used as a deterrent for alcoholics who wish to stay sober.

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.^[18][19]

Alzheimer's disease

People who have a genetic deficiency 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] …"^[20]

Organ Disease

Acetaldehyde has the capability to bind to certain proteins. After ethanol is consumed, acetaldehyde binds to proteins to form adducts. These adducts are linked to organ disease^[21].

Neuronal Damage

When acetaldehyde is present in the brain, it binds to proteins in the brain to form adducts. Acetaldehyde adducts are found in the frontal cortex and midbrain of alcoholics. These adducts cause neuronal damage. They can cause neurons and their related synaptic receptors to disappear^[22].

Carcinogen

Acetaldehyde is a probable carcinogen in humans.^[23] The International Agency for Research on Cancer states, "There is sufficient evidence for the carcinogenicity of acetaldehyde (the major metabolite of ethanol) in experimental animals."^[24] In addition, acetaldehyde is damaging to DNA^[25] and causes abnormal muscle development as it binds to proteins.^[26]

A study of 818 heavy drinkers found that those who are exposed to more acetaldehyde than normal through a defect in the gene for alcohol dehydrogenase are at greater risk of developing cancers of the upper gastrointestinal tract and liver.^[27]

Safety

Acetaldehyde is toxic when applied externally for prolonged periods, an irritant, and a probable carcinogen.^[28]

Acetaldehyde 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. ^[29]

See also

Wikimedia Commons has media related to: Acetaldehyde

• Wine fault

References

External links

• International Chemical Safety Card 0009
• National Pollutant Inventory - Acetaldehde
• NIOSH Pocket Guide to Chemical Hazards
• Methods for sampling and analysis
• IARC Monograph: "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