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Acetaldehyde: Carcinogenic Potency Database potency.berkeley.edu |
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]
[edit] StructureLike other carbonyl compounds, acetaldehyde tautomerizes to give the enol, ethenol, with Keq = 6 x 10−5.[3] [edit] ProductionIn 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] [edit] ReactionsAcetaldehyde 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] [edit] Acetal derivativesThree 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. [edit] Biochemistry and health effectsIn 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. [edit] HangoverMost 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. [edit] Tobacco addictionAcetaldehyde 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] [edit] Alzheimer's diseasePeople 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] [edit] Organ DiseaseAcetaldehyde 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]. [edit] Neuronal DamageWhen 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]. [edit] CarcinogenAcetaldehyde 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] [edit] SafetyAcetaldehyde 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] [edit] See also
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