Definição e significado de Acetaldehyde

Acetaldehyde
IUPAC name ethanal
Other names Acetic Aldehyde Ethyl Aldehyde[1]
Identifiers
CAS number	75-07-0 Y
PubChem	177
ChemSpider	172 Y
UNII	GO1N1ZPR3B Y
EC number	200-836-8
KEGG	C00084 Y
ChEBI	CHEBI:15343 Y
ChEMBL	CHEMBL170365 Y
RTECS number	AB1925000
Jmol-3D images	Image 1 Image 2
SMILES O=CC CC=O
InChI InChI=1S/C2H4O/c1-2-3/h2H,1H3 Y Key: IKHGUXGNUITLKF-UHFFFAOYSA-N Y InChI=1/C2H4O/c1-2-3/h2H,1H3 Key: IKHGUXGNUITLKF-UHFFFAOYAB
Properties
Molecular formula	C2H4O
Molar mass	44.05 g mol−1
Appearance	Colourless 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
Thermochemistry
Std enthalpy of formation ΔfHo298	−166 kJ·mol−1
Standard molar entropy So298	250 J·mol−1·K−1
Hazards
MSDS	External MSDS
GHS pictograms	[2]
GHS hazard statements	H224, H319, H335, H351[2]
GHS precautionary statements	P210, P261, P281, P305+351+338[2]
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 2 2
Flash point	234,15 K (−39 °C)
Autoignition temperature	458,15 K (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 (verify) (what is: Y/N?) 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 CH₃ CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part of their normal metabolism. It is also produced by oxidation of ethylene and is popularly believed to be a cause of hangovers from alcohol consumption through drinking spirits.^[3] Pathways of exposure include air, water, land or groundwater as well as drink and smoke.^[4]

  Production

In 2003, global production was about 1 million tonnes.^[5] The main production method is the oxidation of ethylene via the Wacker process:

2 CH₂=CH₂ + O₂ → 2 CH₃CHO

Alternatively, hydration of acetylene, catalyzed by mercury salts gives ethenol, which tautomerizes to acetaldehyde. This industrial route was dominant prior to the Wacker process^[6] When smaller capacities are required it can also be prepared via dehydrogenation or partial oxidation combined with dehydrogenation. Some acetaldehyde forms upon hydrogenation of CO, but this method is not used commercially.^[5]

  Reactions

Like many other carbonyl compounds, acetaldehyde tautomerizes to give the enol. The enol of acetaldehyde is vinyl alcohol (IUPAC name: ethenol):

CH₃CH=O CH₂=CHOH

The equilibrium constant is only 6 x 10⁻⁷ at room temperature, so that the amount of the enol in a sample of acetaldehyde is very small.^[7]

  Ethanal-ethenol tautomerism

  Condensation reactions

Because of its small size and its availability as the anhydrous monomer (unlike formaldehyde), it is a common electrophile in organic synthesis.^[8] 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.^[9] Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives.^[10] In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH₂OH)₄.^[11]

In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine.^[12] 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.^[13]

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”).^[14]

  Acetal derivatives

Three molecules of acetaldehyde condense to form “paraldehyde,” a cyclic trimer containing C-O single bonds. The condensation of four molecules of acetaldehyde give 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".^[15] although acetal is used more widely to describe other compounds with the formula RCH(OR')₂

  Biochemistry

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.^[16] 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.^[16] 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.

  Uses

Traditionally, acetaldehyde was mainly used as a precursor to acetic acid. This application has declined because acetic acid is made more efficiently from methanol by the Monsanto and Cativa processes. In terms of condensation reactions, acetaldehyde is an important precursor to pyridine derivatives, pentaerythritol, and crotonaldehyde. Urea and acetaldehyde combine to give a useful resin. Acetic anhydride reacts with acetaldehyde to give ethylidene diacetate, a precursor to vinyl acetate, which is used to produce polyvinyl acetate.

  Safety

Acetaldehyde is toxic when applied externally for prolonged periods, an irritant, and a probable carcinogen.^[17] 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.^[18] Acetaldehyde naturally breaks down in the human body^[19] but has been shown to excrete in urine of rats.^[20]

Acetaldehyde is an irritant of the skin, eyes, mucous membranes, throat and respiratory tract. Symptoms of exposure to this compound include nausea, vomiting, headache. These symptoms may not happen immediately. It has a general narcotic action and large doses can even cause death by respiratory paralysis. It may also cause drowsiness, delirium, hallucinations and loss of intelligence. Exposure may also cause severe damage to the mouth, throat and stomach; accumulation of fluid in the lungs, chronic respiratory disease, kidney and liver damage, throat irritation, dizziness, reddening and swelling of the skin.

  Indoor air exposure

Although a relatively weak sensory irritant, acetaldehyde is a common contaminant in workplace, indoor, and ambient environments. It is also a potential carcinogen. Moreover, we spend more than 90% of our time in indoor environments, hence increasing any exposure and consequently the risk to human health.^[21]

In a study in France the mean indoor concentration of acetaldehydes measured in 16 homes was approximately seven times higher than the outside acetaldehyde concentration. The living room had a mean of 18.1±17.5 μg m⁻³ and the bedroom was 18.2±16.9 μg m⁻³, whereas the outdoor air had a mean concentration of 2.3±2.6 μg m⁻³. The main sources of acetaldehyde in homes include building materials, laminate, linoleum, wooden varnished, and cork/pine flooring, also found in plastic water based and matt emulsion paints, in wood ceilings, and wooden, particleboard, plywood, pine wood, and chipboard furniture.^[22]

It has been concluded that VOCs such as benzene, formaldehyde, acetaldehyde, toluene and xylenes have to be considered as priority pollutants with respect to their health effects. It has been pointed that in renovated or completely new buildings, the VOCs concentration levels are often several orders of magnitude higher. The main sources of acetaldehydes in homes include building materials, laminate, linoleum, wooden varnished, and cork/pine flooring. It is also found in plastic water-based and matt emulsion paints, in wood ceilings, and wooden, particle-board, plywood, pine wood, and chipboard furniture.^[22]

  Outdoor air exposure

The use of acetaldehyde is widespread in different industries, and it may be released into waste water or the air during production, use, transportation and storage. Sources of acetaldehyde include fuel combustion emissions from stationary internal combustion engines and power plants that burn fossil fuels, wood, or trash, oil and gas extraction, refineries, cement kilns, lumber and wood mills and paper mills. Acetaldehyde is also present in automobile and diesel exhaust.^[23]

  Tobacco smoke

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.^[24][25]

  Marijuana smoke

Acetaldehyde has been found in Marijuana smoke. This finding emerged through the use of new chemical techniques that demonstrated the acetaldehyde present was causing DNA damage in laboratory settings.^[26]

  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] …"^[27]

  Alcohol

Acetaldehyde derived from the consumption of ethanol binds to proteins to form adducts that are linked to organ disease.^[28]

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.

  Carcinogenicity

Acetaldehyde is a carcinogen in humans.^[17] In 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."^[29] 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.^[30] In addition, acetaldehyde is damaging to DNA^[31] and causes abnormal muscle development as it binds to proteins.^[32]

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.^[33]

Many microbes produce acetaldehyde from ethanol, but they have a lower capacity to eliminate the acetaldehyde which will then lead to the accumulation of acetaldehyde in saliva during heavy alcohol drinking. Acetaldehyde is also the most abundant carcinogen in tobacco smoke; it is dissolved into the saliva while smoking. Fermented food and many alcoholic beverages will also contain significant amounts of acetaldehyde. Acetaldehyde, derived from mucosal or microbial oxidation of ethanol, tobacco smoke, and diet, appears to act as a cumulative carcinogen in the upper digestive tract of humans.^[34]

  Stomach cancer

The normal human stomach is free of harmful microbes because of its low pH. Because of bacteria and yeasts that represent the normal oral flora in your mouth they can survive and even proliferate in increasing intragastric pH when adding ethanol. When this happens the bacteria overgrowth results in the form of glucose in the formation of minor concentrations of acetaldehyde in gastric juice. Moreover, poor oral hygiene, heavy drinking and chronic smoking modify oral flora to produce more acetaldehyde from ingested alcohol. Acetaldehyde production can increase 6.5 fold to the stomach when there is alcohol involved. Tobacco smoking can also accelerate this process by being an active smoker there are considerable amounts of salivary acetaldehyde that can be expected to reach the stomach through swallowing. ^[34]

  Colon cancer

Chronic alcohol consumption is an established risk factor for colorectal cancer. We also know that most of the acetaldehyde produced in the large intestine is generated from bacterial oxidation of ethanol. In experiments with animals, microbially mediated ethanol oxidation results were high acetaldehyde concentrations in the colon after alcohol administration. ^[34]

  See also

Wikimedia Commons has media related to: Acetaldehyde

• Wine fault
• Alcohol dehydrogenase

  References

  External links

• SEKAB, a producer of green Acetaldehyde [2]
• 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