Acetaldehyde

Acetaldehyde, systematically named ethan-1-al, is a volatile organic compound with the molecular formula CH₃CHO. Colourless and possessing a sharp, fruity odour, it exists as a liquid or gas near room temperature. One of the most widely occurring aldehydes in nature, it is present in ripe fruits, bread, and coffee, and is generated in plants as part of normal metabolic processes. Acetaldehyde is also produced in the human body as the primary intermediate of ethanol metabolism and contributes significantly to the physiological effects associated with alcohol consumption. Industrially, it is manufactured on a large scale and serves as an important feedstock in chemical synthesis.
Acetaldehyde is classified by the International Agency for Research on Cancer as a Group 1 carcinogen, and environmental monitoring frequently identifies it as an air toxic capable of posing measurable cancer risk in urban settings.

Historical Background

The compound was first observed in 1774 by the Swedish chemist Carl Wilhelm Scheele, who noted a distinctive odour when ethanol was heated with manganese dioxide and mineral acids. Subsequent work by Fourcroy and Vauquelin in 1800 clarified aspects of its formation, although they believed incorrectly that sulphuric acid was consumed in the reaction. Johann Wolfgang Döbereiner later produced the substance by passing ethanol vapour over platinum black exposed to air and referred to it as “oxygen ether”. In 1835, Justus von Liebig introduced the name aldehyde, derived from “alcohol dehydrogenatum”, which later evolved into the modern term acetaldehyde.
Throughout the nineteenth century the compound’s reactions and derivatives were extensively studied, and its significance in organic chemistry and industrial synthesis became widely recognised.

Industrial Production

Acetaldehyde is manufactured by several chemical routes, although the Wacker process—the catalytic oxidation of ethene—is the dominant modern method. This process employs a homogeneous palladium–copper catalyst and is conducted at industrial scale, reaching a global production of several hundred thousand tonnes.
Other methods include:

• Partial oxidation of ethanol, carried out over a silver catalyst at elevated temperatures; historically important but now of secondary industrial value.
• Hydration of acetylene, an older industrial route catalysed by mercury salts; largely discontinued due to cost and toxicity concerns, though of biochemical interest because certain anaerobic bacteria can catalyse an analogous mercury-free hydration.
• Dehydrogenation of ethanol, in which ethanol vapour passes over copper catalysts at 260–290 °C, producing acetaldehyde and hydrogen.
• Hydroformylation of methanol, yielding acetaldehyde as a by-product of limited commercial importance.

Chemical Properties and Reactions

Acetaldehyde is a reactive electrophile and occupies a central place in organic synthesis.
TautomerismIt exists in equilibrium with its enol tautomer, vinyl alcohol (ethenol), although the keto form dominates overwhelmingly. Acid catalysis accelerates tautomerisation, and photochemical tautomerisation is relevant under atmospheric conditions, where vinyl alcohol participates in pathways leading to carboxylic acids.
Addition and condensation reactionsAs a prochiral aldehyde, acetaldehyde undergoes numerous nucleophilic additions:

• Reaction with Grignard or organolithium reagents yields secondary alcohols.
• Condensation with formaldehyde in basic media produces pentaerythritol.
• Participation in Strecker synthesis produces amino acids such as alanine.
• Combination with amines forms imines, useful intermediates for further transformations.
• Reaction with ammonia can yield heterocycles such as 5-ethyl-2-methylpyridine.

Polymeric and cyclic derivativesAcetaldehyde readily forms polymeric species:

• Paraldehyde, a cyclic trimer, produced under acidic conditions and historically used as a sedative.
• Metaldehyde, a cyclic tetramer, obtained in low yield and used in formulations such as solid fuels and molluscicides.
• Polyacetaldehyde, formed in strongly acidic environments at low temperatures.Although vinyl alcohol is structurally a polymeric representation, true polyvinyl alcohol cannot be prepared directly from acetaldehyde.

Acetal formationWith ethanol under dehydrating conditions, acetaldehyde forms 1,1-diethoxyethane, commonly referred to simply as acetal. This compound illustrates the broader class of acetals but also lends its name to the functional group, a source of terminological ambiguity.
Precursor to vinylphosphonic acidAcetaldehyde reacts with phosphorus trichloride in the initial step of synthesising vinylphosphonic acid, a precursor for ion-conducting membranes and specialty adhesives.

Biological and Environmental Roles

In human metabolism acetaldehyde arises chiefly from the oxidation of ethanol by alcohol dehydrogenase and is subsequently converted to acetic acid by acetaldehyde dehydrogenase. Accumulation occurs when this enzyme is inhibited, as in the pharmacological action of disulfiram. Within the brain, catalase contributes significantly to ethanol oxidation, forming acetaldehyde locally.
In microbial and plant biology acetaldehyde appears at the terminal stages of alcoholic fermentation, where pyruvate decarboxylation produces acetaldehyde prior to its reduction to ethanol.
Environmental exposure to acetaldehyde occurs through air, water, soil, and tobacco smoke. Its volatility and reactivity allow it to participate in atmospheric chemistry, contributing to the formation of secondary pollutants.