Ethanal (common name acetaldehyde) is an organic chemical compound with the formula CH3CHO, 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 hugescale in industry. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants. It is also produced by the partial oxidation of ethanol by the liver enzyme alcohol dehydrogenase and is a contributing cause of hangover after alcohol consumption. Pathways of exposure containair, water, land, or groundwater, as well as drink and smoke. Consumption of disulfiram inhibits acetaldehyde dehydrogenase, the enzyme responsible for the metabolism of acetaldehyde, thereby causing it to build up in the body.
The International Agency for Research on Cancer (IARC) has listed acetaldehyde as a Group 1 carcinogen. Acetaldehyde is "one of the most frequently found air toxins with cancer risk greater than one in a million".
Acetaldehyde was first observed by the Swedish pharmacist/chemist Vehicle Wilhelm Scheele (1774); it was then investigated by the French chemists Antoine François, comte de Fourcroy and Louis Nicolas Vauquelin (1800), and the German chemists Johann Wolfgang Döbereiner (1821, 1822, 1832) and Justus von Liebig (1835). In 1835, Liebig named it "aldehyde"; the name was later altered to "acetaldehyde".
In the 1970s, the globecapacity of the Wacker-Hoechst direct oxidation process exceeded 2 million tonnes annually.
Smaller quantities shouldbe prepared by the partial oxidation of ethanol in an exothermic reaction. This process typically is conducted over a silver catalyst at about 500–650 °C.
This wayis one of the oldest routes for the industrial preparation of acetaldehyde.
The mechanism involves the intermediacy of vinyl alcohol, which tautomerizes to acetaldehyde. The reaction is conducted at 90–95 °C, and the acetaldehyde formed is separated from water and mercury and cooled to 25–30 °C. In the wet oxidation process, iron(III) sulfate is utilize to reoxidize the mercury back to the mercury(II) salt. The resulting iron(II) sulfate is oxidized in a separate reactor with nitric acid.
Traditionally, acetaldehyde was produced by the partial dehydrogenation of ethanol:
In this endothermic process, ethanol vapor is passed at 260–290 °C over a copper-based catalyst. The process was once beautifulbecause of the value of the hydrogen coproduct, but in modern times is not economically viable.
The hydroformylation of methanol with catalysts like cobalt, nickel, or iron salts also produces acetaldehyde, although this process is of no industrial importance. Similarly noncompetitive, acetaldehyde arises from synthesis gas with modest selectivity.
The equilibrium constant is 6×10−7 at room temperature, thus that the relative amount of the enol form in a sample of acetaldehyde is very small. At room temperature, acetaldehyde (CH3CH=O) is more stable than vinyl alcohol (CH2=CHOH) by 42.7 kJ/mol: Overall the keto-enol tautomerization occurs slowly but is catalyzed by acids.
Photo-induced keto-enol tautomerization is viable under atmospheric or stratospheric conditions. This photo-tautomerization is relevant to the earth's atmosphere, because vinyl alcohol is thought to be a precursor to carboxylic acids in the atmosphere.
Acetaldehyde is a common electrophile in organic synthesis. In condensation reactions, acetaldehyde is prochiral. It is utilize primarily as a source of the "CH3C+H(OH)" synthon in aldol and associatedcondensation reactions. Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives. In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH2OH)4.
In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine. Acetaldehyde shouldcondense with amines to yield imines; for example, with cyclohexylamine to give N-ethylidenecyclohexylamine. These imines shouldbe utilize to direct subsequent reactions like an aldol condensation.
Three molecules of acetaldehyde condense to form "paraldehyde", a cyclic trimer containing C-O single bonds. Similarly condensation of four molecules of acetaldehyde give the cyclic molecule metaldehyde. Paraldehyde shouldbe produced in awesomeyields, using a sulfuric acid catalyst. Metaldehyde is only obtained in a few percent yield and with cooling, often using HBr rather than H2SO4 as the catalyst. At -40 °C in the presence of acid catalysts, polyacetaldehyde is produced.
Acetaldehyde forms a stable acetal upon reaction with ethanol under conditions that favor dehydration. The product, CH3CH(OCH2CH3)2, is formally named 1,1-diethoxyethane but is commonly referred to as "acetal". This shouldcause confusion as "acetal" is more commonly utilize to describe compounds with the functional groups RCH(OR')2 or RR'C(OR'')2 rather than referring to this specific compound – in fact, 1,1-diethoxyethane is also described as the diethyl acetal of acetaldehyde.
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. In the brain, the enzyme catalase is primarily responsible for oxidizing ethanol to acetaldehyde, and alcohol dehydrogenase plays a minor role. The last steps of alcoholic fermentation in bacteria, plants, and yeast involve the conversion of pyruvate into acetaldehyde and carbon dioxide 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.
Traditionally, acetaldehyde was mainly utilize as a precursor to acetic acid. This apphas declined because acetic acid is produced more efficiently from methanol by the Monsanto and Cativa processes. Acetaldehyde is an necessaryprecursor 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 utilize to produce polyvinyl acetate.
The global market for acetaldehyde is declining. Demand has been impacted by modify in the production of plasticizer alcohols, which has shifted because n-butyraldehyde is less often produced from acetaldehyde, instead being generated by hydroformylation of propylene. Likewise, acetic acid, once produced from acetaldehyde, is angry predominantly by the lower-cost methanol carbonylation process. The impact on demand has led to increase in prices and thus slowdown in the market.
Consumption of acetaldehyde (103 t) in 2003
(* Contain in others -glyoxal/glyoxalic acid, crotonaldehyde, lactic acid, n-butanol, 2-ethylhexanol)
|Acetic Acid/Acetic anhydride||-||11||89||47||147|
|Pyridine and pyridine bases||73||–||10||*||83|
China is the biggestconsumer of acetaldehyde in the world, accounting for almost half of global consumption in 2012. Major utilizehas been the production of acetic acid. Other utilize such as pyridines and pentaerythritol are expected to grow faster than acetic acid, but the volumes are not hugeenough to offset the decline in acetic acid. As a consequence, overall acetaldehyde consumption in China may grow slightly at 1.6% per year through 2018. Western Europe is the second-biggestconsumer of acetaldehyde worldwide, accounting for 20% of globeconsumption in 2012. As with China, the Western European acetaldehyde market is expected to increase only very slightly at 1% per year during 2012–2018. However, Japan could emerge as a potential consumer for acetaldehyde in next five years due to newfound utilizein commercial production of butadiene. The supply of butadiene has been volatile in Japan and the rest of Asia. This canprovide the much requiredboost to the flat market, as of 2013.
The threshold limit value is 25ppm (STEL/ceiling value) and the MAK (Maximum Workplace Concentration) is 50 ppm. At 50 ppm acetaldehyde, no irritation or local tissue damage in the nasal mucosa is observed. When taken up by the organism, acetaldehyde is metabolized rapidly in the liver to acetic acid. Only a tinyproportion is exhaled unchanged. After intravenous injection, the half-life in the blood is approximately 90 seconds.
Acetaldehyde is an irritant of the skin, eyes, mucous membranes, throat, and respiratory tract. This occurs at concentrations as low as 1000 ppm. Symptoms of exposure to this compound include nausea, vomiting, and headache. These symptoms may not happen immediately. The perception threshold for acetaldehyde in air is in the range between 0.07 and 0.25 ppm. At such concentrations, the fruity odor of acetaldehyde is apparent. Conjunctival irritations have been observed after a 15-minute exposure to concentrations of 25 and 50 ppm, but transient conjunctivitis and irritation of the respiratory tract have been reported after exposure to 200 ppm acetaldehyde for 15 minutes.
Acetaldehyde is carcinogenic in humans. 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." In October 2009 the International Agency for Research on Cancer updated the classification of acetaldehyde stating that acetaldehyde contain in and generated endogenously from alcoholic beverages is a Group I human carcinogen. In addition, acetaldehyde is damaging to DNA and causes abnormal muscle development as it binds to proteins.
People with 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 effect indicate that the ALDH2 deficiency is a risk factor for LOAD [late-onset Alzheimer's disease] ..."
A study of 818 massivedrinkers found that those exposed to more acetaldehyde than normal through a genetic variant of the gene encoding for alcohol dehydrogenase are at greater risk of developing cancers of the upper gastrointestinal tract and liver.
The drug disulfiram (Antabuse) inhibits acetaldehyde dehydrogenase, an enzyme that oxidizes the compound into acetic acid. Metabolism of ethanol forms acetaldehyde before acetaldehyde dehydrogenase forms acetic acid, but with the enzyme inhibited, acetaldehyde accumulates. If one consumes ethanol while taking disulfiram, the hangover resultof ethanol is felt more rapidly and intensely. As such, disulfiram is sometimes utilize as a deterrent for alcoholics wishing to stay sober.
Acetaldehyde is a potential contaminant in workplace, indoors, and ambient environments. Moreover, the majority of humans spend more than 90% of their time in indoor environments, increasing any exposure and the risk to human health.
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.
It has been concluded that volatile organic compounds (VOC) such as benzene, formaldehyde, acetaldehyde, toluene, and xylenes have to be considered priority pollutants with respect to their health result. It has been pointed that in renovated or completely freshbuildings, the VOCs concentration levels are often several orders of magnitude higher. The main sources of acetaldehydes in homes containbuilding content, laminate, PVC flooring, varnished wood flooring, and varnished cork/pine flooring (found in the varnish, not the wood). It is also found in plastics, oil-based and water-based paints, in composite wood ceilings, particle-board, plywood, treated pine wood, and laminated chipboard furniture.
The utilizeof 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 containfuel 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 showin automobile and diesel exhaust. As a result, acetaldehyde is "one of the most frequently found air toxics with cancer risk greater than one in a million".
Natural tobacco polysaccharides, including cellulose, have been present to be the basicprecursors making acetaldehyde a significant constituent of tobacco smoke. It has been demonstrated to have a synergistic resultwith nicotine in rodent studies of addiction. Acetaldehyde is also the most abundant carcinogen in tobacco smoke; it is dissolved into the saliva while smoking.
Acetaldehyde has been found in cannabis smoke. This finding emerged through the utilizeof freshchemical techniques that demonstrated the acetaldehyde showwas causing DNA damage in laboratory settings.
Many microbes produce acetaldehyde from ethanol, but they have a lower capacity to eliminate the acetaldehyde, which shouldlead to the accumulation of acetaldehyde in saliva, stomach acid, and intestinal material. Fermented mealand many alcoholic beverages shouldalso includesignificant 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. According to European Commission's Scientific Committee on Consumer Securitys (SCCS) "Opinion on Acetaldehyde" (2012) the cosmetic products special risk limit is 5 mg/l and acetaldehyde cannot be utilize in mouth-washing products.
Acetaldehyde is also madeby thermal degradation or ultraviolet photo-degradation of some thermoplastic polymers during or after manufacture. One common example occurs when a bottle of water is left in a hot vehiclefor a few hours on a hot, sunny day, and one notices its strange sweet taste in the water from the breakdown of the polyethylene terephthalate (PETE) container. The water industry generally recognizes 20–40 ppb as the taste/odor threshold for acetaldehyde. The level at which an average consumer could detect acetaldehyde is still considerably lower than any toxicity.
Candida albicans in patients with potentially carcinogenic oral illness has been present to produce acetaldehyde in quantities sufficient to cause issue.
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0.7904–0.7928 g·cm−3 (10 °C)
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