Acetaldehyde is also known under the systematic name ‘ethanal’. The CAS identification number assigned to the substance is 75-07-0. Its molecular formula is C2H4O, but the equally correct form CH3CHO is more commonly used in notation.
Characteristics of acetaldehyde
As the name suggests, this compound is derived from a group of organic substances – aldehydes. They are characterised by the presence of an aldehyde group -CHO in their structure. In practice, acetaldehyde is a relatively small chemical compound containing only two carbon atoms, one of which is part of the functional group.
The aldehyde group -CHO is monovalent, with one oxygen atom attached directly to the carbon atom by a double bond and one hydrogen atom attached by a single bond. The presence of such a strong polar group, in which the charges are distributed over two atoms – a negative charge on the oxygen atom and a positive charge on the carbon atom – makes acetaldehyde highly reactive in the nucleophilic addition reaction model.
The structure of acetaldehyde is similar to the ethanol molecule, and the aldehyde itself is involved in both alcohol production and metabolism processes. Ethanal, present in tobacco and alcohol, is among the most common carcinogens and its toxicity is higher than that of ethanol alone. Research is still ongoing into its effects, particularly neurotoxic effects.
Physicochemical properties of acetaldehyde
Acetaldehyde is most often found as a colourless liquid that is soluble in both water and typical organic solvents, such as ethanol. In water, it dissolves very well in any ratio, unlimitedly. The n-octanol/water partition coefficient for acetaldehyde is 0.63. It is a highly volatile liquid and can therefore be a colourless gas at room temperature, which is characterised by flammability and low stability in air. In any form, it has a strong, characteristic fruity green apple aroma.
Its molar mass is 44.05 g/mol and its density at 20oC is 0.78 g/cm3. Due to its characteristic melting and boiling temperatures of, respectively: -123oC and 21oC, it is necessary to store it below room temperature. Spontaneous combustion occurs when the substance is heated to 140oC under atmospheric pressure.
A 10 g/l aqueous solution of acetaldehyde at 20oC has a slightly acidic pH of approximately 5. Its vapour pressure at 25oC is 1.202 hPa and its viscosity at 20oC is at the level of 0.21 mPa·s.
The substance available as a commercial product with a content of approximately 99% acetaldehyde shows no tendency to bioaccumulate and degrades by approximately 80% after 14 days.
Identification of hazards
The safety data sheet for acetaldehyde indicates its allocation to several hazard classes. The exact data can be seen in Table 1.
| Section | Hazard class | Category | Hazard class and category | Phrase indicating the type of hazard |
| 2.6 | Flammable liquid | 1 | Flam. Liq. 1 | H224 |
| 3.1O | Acute toxicity (oral route) | 4 | Acute Tox. 4 | H302 |
| 3.3 | Serious eye damage/irritation | 2 | Eye Irrit. 2 | H319 |
| 3.5 | Mutagenic effects on reproductive cells | 2 | Muta. 2 | H341 |
| 3.6 | Carcinogenicity | 1B | Carc. 1B | H350 |
| 3.8R | Specific target organ toxicity – single exposure (respiratory tract irritation) | 3 | STOT SE 3 | H335 |
Table 1 Hazard classification of acetaldehyde according to Regulation (EC) No. 1272/2008 (CLP) source: safety data sheet available at www.carlroth.de
Based on the data from the table above, work with acetaldehyde should be performed with extreme caution. Both the digestive and respiratory tracts are at high risk, making it necessary to take special safety precautions.
Occurrence and formation of acetaldehyde
In the environment, acetaldehyde is found in coffee and ripe fruit. It is mainly obtained in three ways: by catalytic oxidation of ethanol, hydration of acetylene and by oxidation of ethylene with oxygen in the presence of an aqueous solution of palladium(II) chloride and copper(II) salts.
On an industrial scale, it is produced by combining water with acetylene. The reaction leads to the formation of an intermediate product, the non-permanent enol form, which then tautomerises to the aldehyde form.
The occurrence of acetaldehyde in food products is widespread. The largest group of products rich in this compound are those resulting from fermentation processes, i.e. alcoholic beverages, vinegar, dairy products, beer and mead, as well as tobacco. This is because acetaldehyde is an intermediate product of alcoholic fermentation, produced by yeast. It is also often used in confectionery, fruit juices, flavoured alcoholic and non-alcoholic drinks, dairy products and sweet desserts due to its prominent apple-like aroma and flavour.
Nevertheless, the most important and greatest risk is the consumption of alcoholic beverages and tobacco, which contain the most of it. Acetaldehyde produced by fermentation is mostly converted into ethyl alcohol, but some part of the intermediate product remains in the body, which has a visible effect in the form of ‘hangover’ symptoms after alcohol consumption. In addition, acetaldehyde is also formed during hepatic metabolism of ingested alcohol, where, in the first stage, the ethanol molecule undergoes alcohol dehydrogenase and, losing two hydrogen atoms, is converted to acetaldehyde. It is also further transformed up to the end products of water and carbon dioxide.
Another exogenous source of acetaldehyde is the use of alternative vehicle fuels, which increase overall aldehyde emissions. This is because of the addition of ethanol to fuels as an oxidiser, which consequently increases the amount of, among other things, acetaldehyde in the exhaust gas. Unfortunately, this toxic chemical can also be found in many areas of industry. It is also present in products such as furniture, carpets, textiles and paints. Kitchen fumes also contain some acetaldehyde, and it is also emitted from wood-burning cookers and fireplaces. It is used, among other things, during the production of alkyd resins, and is released from the reactor as off-gas during reaction processes to produce acetic acid.
Characteristic reaction for the presence of aldehydes
In the laboratory, a method for demonstrating the presence of acetaldehyde using the Schiff’s reagent is used, which allows for the qualitative determination of the presence or absence of acetaldehyde (or indeed all aldehydes) in a sample. The test should begin with the preparation of Schiff’s reagent, i.e. dissolving a small amount of rosaniline hydrochloride (fuchsin) in a few millilitres of distilled water. Dilute sulphuric acid (VI) should be added to the prepared solution drop by drop until discoloured. A few drops of reagent thus prepared should be added to the sample to be analysed. If a violet colour appears – the presence of an aldehyde has been detected.
Symptoms of poisoning by high concentrations of acetaldehyde
Excessive amounts of acetaldehyde in the body result in the occurrence of homeostatic disorders. A sharp rise in the levels of this metabolite can consequently lead not only to danger and damage to health, but even to death. This is because it is particularly dangerous for the cardiovascular system. Symptoms of the onset of poisoning with this chemical include intense flushing and redness of the face, hot flashes, increased sweating, general malaise due to nausea and vomiting, intense headaches and dizziness, a drop in blood pressure, accelerated heart rate and a feeling of palpitations, as well as a sense of inner fear and constant anxiety.
Effects of acetaldehyde on the human body
Although acetaldehyde occurs mainly in the form of intermediates, there is always a certain amount that does not undergo further transformation. Exposure pathways also include air, water, land and groundwater, which means that we can be exposed to it virtually all the time. Chronic and prolonged exposure to large amounts of acetaldehyde can cause serious impairment of body function. This is because this toxin damages many key tissues and organs and is a carcinogen. The oesophagus, large intestine, pancreas and liver are particularly susceptible to carcinogenic effects. The liver is where alcohol is metabolised, and acetaldehyde has proven hepatoxic properties, i.e. damaging liver cells.
As a consequence of excessive amounts of this compound, we may see the onset of alcoholic liver disease and therefore cirrhosis. In a healthy liver environment, the hydrochloric acid present kills microbes, such as yeast and bacteria, responsible for producing acetaldehyde. In some people, as a result of atrophy of the mucous membrane covering the stomach, the mucic acid-producing cells also disappear and microorganisms proliferate. This results in the accumulation of excessive amounts of acetaldehyde in saliva, gastric acid and the lower intestine. This also happens when taking medicines that neutralise acid over a long period of time. The affliction commonly referred to as ‘beer belly’ is also caused by the accumulation of acetaldehyde, or rather its transformation into characteristic fat in this part of the body.
Hangover cure
As mentioned earlier, acetaldehyde is the main reason for feeling bad after consuming alcoholic beverages. However, there is another chemical that has the ability to accelerate the metabolism of acetaldehyde in the body, which is why it is used in ‘hangover’ pills. This substance is N-acetyl cysteine, an L-cysteine derivative that has an acetyl group attached at the N-end.