Tuesday, 28 April 2015

Acetaldehyde

Acetaldehyde (systematic name ethanal) 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 large scale 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 may be a contributing factor to hangovers from alcohol consumption. Pathways of exposure include air, water, land, or groundwater, as well as drink and smoke.[4] Consumption of disulfiram inhibits acetaldehyde dehydrogenase, the enzyme responsible for the metabolism of acetaldehyde, thereby causing it to build up in the body.


History[edit]

Acetaldehyde was first observed by the Swedish pharmacist/chemist Carl Wilhelm Scheele (1774);[5] 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)[6] and Justus von Liebig (1835).[7] In 1835, Liebig named it "aldehyde";[8] the name was later altered to "acetaldehyde".[9]

Production[edit]

In 2003, global production was about 1 million tonnes. Before 1962, ethanol and acetylene were the major sources of acetaldehyde. Since then, ethylene is the dominant feedstock.[10]
The main production method is the oxidation of ethylene via the Wacker process. The process involves oxidation of ethylene using a homogeneous palladium/copper system:
2 CH2=CH2 + O2 → 2 CH3CHO
In the 1970s, the world capacity of this process, the Wacker-Hoechst direct oxidation, increased to over 2×10^6 t/a.
Also, when smaller capacities are required, it can also be 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.[10]
CH3CH2OH + 1/2O2 → CH3CHO + H2O
This method is one of the oldest routes for the industrial of preparation of acetaldehyde.
Acetaldehyde is produced by the hydration of acetylene, catalyzed by mercury salts. The reaction produces ethenol, which tautomerizes to acetaldehyde. This industrial route was dominant prior to the Wacker process.[11] One drawback of this process is the formation of polymerization and condensation products of acetaldehyde. Furthermore, the production of acetylene is costly and environmentally problematic. The wet oxidation process was used before the Wacker process was commercially established. Iron(III) sulfate is added to reoxidize the mercury metal to the mercury(II) salt to maintain required concentration of catalyst and, thus, avoiding direct handling of mercury. Acetylene reacts at 90-95oC and the acetaldehyde formed is separated from water and mercury and cooled to 25-30oC. Iron(II) sulfate is formed in the reaction and is oxidized in a separate reactor with 30% nitric acid at 95oC. Pure acetaldehyde is obtained by fractional distillation of the aqueous solution at about 200 kPa.

Other methods[edit]

Traditionally but no longer viable economically, acetaldehyde was produced by the partial dehydrogenation of ethanol:
CH3CH2OH → CH3CHO + H2
In this endothermic process, ethanol vapor is passed at 260–290 °C over a copper-based catalyst. The process was once attractive because of the value of the hydrogen coproduct.[10]
The hydroformylation of methanol catalysts like cobalt, nickel, or iron salts produces acetaldehyde. This process is of no industrial importance. Similarly noncompetitive, acetaldehyde arises from synthesis gas with modest selectivity.[10]

Reactions[edit]

Like many other carbonyl compounds, acetaldehyde tautomerizes to give the enol. The enol of acetaldehyde is vinyl alcohol (IUPAC name: ethenol):
CH3CH=O \overrightarrow{\leftarrow} 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.[12]
Ethanal-ethenol tautomerism

Tautomerization of acetaldehyde to vinyl alcohol[edit]

At room temperature, acetaldehyde (CH3CH=O) is more stable than vinyl alcohol (CH2=CHOH) by 42.7 kJ/mol:[13]
CH2=CHOH → CH3CH=O                 ∆H298,g = –42.7 kJ/mol
This keto-enol tautomerization has a very high reaction barrier height and will therefore not occur at room temperature. Nevertheless, it was recently found that this keto-enol tautomerization can be efficiently catalyzed via photochemical process[14] or by inorganic acid catalysts (e.g. HClO4 and H2SO4).[15] These findings suggest that the keto-enol tautomerization is a viable route under atmospheric and stratospheric conditions, and have received considerable attention due to their potential atmospheric implications, since vinyl alcohol is believed to be a key intermediate in the production of organic acids in the atmosphere.[16][17]

Condensation reactions[edit]

Because of its small size and its availability as the anhydrous monomer (unlike formaldehyde), it is a common electrophile in organic synthesis.[18] 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.[19] Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives.[20] In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C(CH2OH)4.[21]
In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine.[22] 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.[23]
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”).[24]

Acetal derivatives[edit]

Three molecules of acetaldehyde condense to form “paraldehyde,” a cyclic trimer containing C-O single bonds.[how?] 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".[25] although acetal is used more widely to describe other compounds with the formula RCH(OR')2

Biochemistry[edit]

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.[26] In the brain, alcohol dehydrogenase has a minor role in the oxidation of ethanol to acetaldehyde. Instead, primarily the enzyme catalase oxidizes ethanol to acetaldehyde.[26] 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.

Uses[edit]

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.
Global market for acetaldehyde is declining. Demand has been impacted by changes in the production of plasticizer alcohols, which has shifted from n-butyraldehyde based on acetaldehyde to hydroformylation of propylene. Likewise, acetic acid, once produced from acetaldehyde, is made predominantly by the lower-cost methanol carbonylation process.[27] The impact on demand has led to increase in prices and thus slowdown in the market.
Production Of Acetaldehyde
Consumption of acetaldehyde (103 t) in 2003[10]
(* Included in others -glyoxal/glyoxalic acid, crotonaldehyde, lactic acid, n-butanol, 2-ethylhexanol)
ProductUSAMexicoW. EuropeJapanTotal
Acetic Acid/Acetic anhydride-118947147
Acetate esters35854224321
Pentaerythritol26-431180
Pyridine and pyridine bases73-10*83
Peracetic acid23--*23
1,3-Butylene glycol14--*14
Others53108098
Total17622206362766
China is the largest consumer of acetaldehyde in the world, accounting for almost half of global consumption in 2012. Major use has been the production of acetic acid. Other uses such as pyridines and pentaerythritol are expected to grow faster than acetic acid, but the volumes are not large enough 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-largest consumer of acetaldehyde worldwide, accounting for 20% of world consumption 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 use in commercial production of butadiene. The supply of butadiene has been volatile in Japan and the rest of Asia. This should provide the much needed boost to the flat market, as of 2013.[28]

Safety[edit]

Exposure Limits[edit]

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 small proportion is exhaled unchanged. After intravenous injection, the half-life in the blood is approximately 90 seconds.[10]

Dangers[edit]

Toxicity[edit]

Acetaldehyde is toxic when applied externally for prolonged periods, an irritant, and a probable carcinogen.[29] 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.[30] Acetaldehyde naturally breaks down in the human body[31] but has been shown to excrete in urine of rats.[32]

Irritation[edit]

Acetaldehyde is an irritant of the skin, eyes, mucous membranes, throat, and respiratory tract. This occurs at concentrations up to 1000 ppm. Symptoms of exposure to this compound include nausea, vomiting, headache. These symptoms may not happen immediately. The perception limit of acetaldehyde in air is in the range between 0.07 and 0.25 ppm.[10] 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. 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.

Carcinogenicity[edit]

Acetaldehyde is a probable or possible carcinogen in humans.[29][33] 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."[34] 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.[35] In addition, acetaldehyde is damaging to DNA[36] and causes abnormal muscle development as it binds to proteins.[37]

Aggravating factors[edit]

Alzheimer's disease[edit]

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 results indicate that the ALDH2 deficiency is a risk factor for LOAD [late-onset Alzheimer's disease] …"[38]

Genetic conditions[edit]

A study of 818 heavy drinkers found that those 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.[39]

Disulfiram[edit]

The drug disulfiram (Antabuse) prevents the oxidation of acetaldehyde to acetic acid, and it has the same[clarification needed] unpleasant effect on drinkers. Antabuse is sometimes used as a deterrent for alcoholics wishing to stay sober.

Sources of exposure[edit]

Indoor air[edit]

Acetaldehyde is common contaminant in workplace, indoors, and ambient environments. It is also a potential carcinogen. Moreover, humans spend more than 90% of their time in indoor environments, hence increasing any exposure and, as a consequence, the risk to human health.[40]
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 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.[41]

Outdoor air[edit]

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.[42]

Tobacco smoke[edit]

Acetaldehyde is a significant constituent of tobacco smoke. It has been demonstrated to have a synergistic effect with nicotine in rodent models of addiction.[43][44] Acetaldehyde is also the most abundant carcinogen in tobacco smoke; it is dissolved into the saliva while smoking.
Acetaldehyde can also be found in e-cigarette vapour, though this is reported to be trace amounts.[45]

Cannabis smoke[edit]

Acetaldehyde has been found in cannabis smoke. This finding emerged through the use of new chemical techniques that demonstrated the acetaldehyde present was causing DNA damage in laboratory settings.[46]

Alcohol consumption[edit]

Many microbes produce acetaldehyde from ethanol, but they have a lower capacity to eliminate the acetaldehyde, which can lead to the accumulation of acetaldehyde in saliva, stomach acid, and intestinal contents. Fermented food and many alcoholic beverages can 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.[47]

See also[edit]

Wikimedia Commons has media related to Acetaldehyde.
Some major acetaldehyde producers include Celanese Chemicals Europe GmbH (Germany), Eastman Chemical Company (USA), ECROS (SA, Spain), Japan Aldehyde Company Ltd. (Japan), Jilin Chemical Industrial Company (China), Kyowa Yuka Company Ltd. (Japan), Showa Denko K.K. (Japan), Sinopec Yangzi Petrochemical Co. (China), and Wacker Chemie AG (Germany).

References[edit]

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  6. ^ See:
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External links[edit]


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