Acetaldehyde

The organic chemical compound acetaldehyde (IUPAC systematic name ethanal) has the formula CH3CHO and is commonly abbreviated by scientists as MeCHO (Me = methyl). It is a colourless liquid or gas that boils at a temperature close to room temperature. It’s one among the most important aldehydes, with widespread natural occurrence and widespread industrial production.

Introduction:

Acetaldehyde is also created when the liver enzyme alcohol dehydrogenase partially oxidises ethanol, and it’s a contributing cause of hangovers after drinking. Acetaldehyde is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). Acetaldehyde is “one of the most commonly identified air pollutants with a cancer risk greater than one in a million,” according to the National Cancer Institute.

Structural formula of acetaldehyde:

Two carbon atoms are seen covalently connecting with other atoms in the structure. One carbon atom is linked to three hydrogens and a carbon atom, whereas the other is linked to the terminal carbon, a hydrogen, and an oxygen atom. By sharing two electron pairs, the core C atom forms a double bond with the oxygen atom ( C=O).The formyl group -CHO is the functional group of acetaldehyde and all aldehydes.A carbonyl group -CO, whose carbon is linked to a hydrogen atom, plus a side chain makes up the formyl group. Any alkyl or aryl group can be used as a side chain.

In acetaldehyde, the formyl group is connected to a normal aliphatic alkyl side chain, the methyl group. The chemical compound acetaldehyde is classed as an alkanol. It’s an aliphatic aldehyde, in other words. A nonaromatic chemical molecule with a formyl group connected to a normal aliphatic chain or ring is known as an aliphatic aldehyde. If the formyl group of an aldehyde is linked to an aromatic ring, it is classified as an aromatic aldehyde.

Production:

Global production was at 1 million tonnes in 2003. Until 1962, the main sources of acetaldehyde were ethanol and acetylene. Since then, ethylene has become the most common feedstock.

The Wacker process, which involves oxidation of ethylene using a homogenous palladium/copper system, is the most common method of production:

2 CH2=CH2 + O2 → 2 CH3CHO

The Wacker-Hoechst direct oxidation process had a global capacity of over 2 million tonnes per year in the 1970s.

The partial oxidation of ethanol in an exothermic reaction can be used to make smaller volumes. At 500–650 °C, this process is usually carried out over a silver catalyst.

CH3CH2OH + 1⁄2 O2 → CH3CHO + H2O

 This is one of the oldest methods for producing acetaldehyde in the industrial setting.          

               Other methods of production:

• Hydration of acetylene

Acetaldehyde was made by hydration of acetylene prior to the Wacker method and the availability of cheap ethylene. Mercury(II) salts catalyse this reaction:

C2H2 + Hg2+ + H2O → CH3CHO + Hg

 The intermediacy of vinyl alcohol, which tautomerizes to acetaldehyde, is involved in the mechanism. The reaction is carried out at 90–95 degrees Celsius, and the acetaldehyde produced is separated from the water and mercury before being cooled to 25–30 degrees Celsius. Iron(III) sulphate is employed in the wet oxidation process to re oxidise mercury back to mercury(II) salt. In a separate reactor, the iron(II) sulphate is oxidised with nitric acid.

• Dehydrogenation of ethanol

Acetaldehyde was traditionally made by partially dehydrogenating ethanol:

CH3CH2OH → CH3CHO + H2

Ethanol vapour is pushed through a copper-based catalyst at 260–290 °C in this endothermic process. The technique was originally appealing due to the value of the hydrogen coproduct, but it is no longer economically viable in today’s world.

• Hydroformylation of methanol

Methanol can be hydroformylated with catalysts such as cobalt, nickel, or iron salts to create acetaldehyde, albeit this is a non-industrial process. Acetaldehyde, which is also noncompetitive, is produced with moderate selectivity from synthesis gas.

Uses of acetaldehyde:

Acetaldehyde was traditionally employed as a precursor to acetic acid. Acetaldehyde is a precursor to pyridine derivatives, pentaerythritol, and crotonaldehyde, among others. When urea and acetaldehyde are combined, a useful resin is formed. When acetic anhydride is combined with acetaldehyde, ethylidene diacetate is formed, which is a precursor to vinyl acetate, which is used to make polyvinyl acetate. Acetaldehyde’s global market is shrinking. Changes in the manufacture of plasticizer alcohols have had an impact on demand, as n-butyraldehyde is now produced more frequently from propylene hydroformylation rather than acetaldehyde. Similarly, acetic acid, which was previously manufactured from acetaldehyde, is now primarily produced through the lower-cost methanol carbonylation process.

As a result of the impact on demand, prices have risen and the market has slowed. In 2012, China was the world’s largest user of acetaldehyde, accounting for over half of global consumption. The manufacture of acetic acid has been a major application. Other uses, such as pyridines and pentaerythritol, are predicted to rise faster than acetic acid, but the volumes will not be great enough to compensate for acetic acid’s fall. 

Physical properties :

This organic compound is very flammable and toxic. Acute short-term contact to this organic molecule can cause significant irritation to the skin, eyes, and respiratory system. The characteristics of acetaldehyde are summarised in the table below :-

Because of the uneven electron distribution, acetaldehyde is a polar molecule; the end with the formyl group is more electronegative than the end with the terminal carbon atom. CH3CHO has a relatively high pKa value. Because of its strong attachment to its proton, a high pKa value indicates that the molecule is a very weak acid. Although acetaldehyde is not a strong acid, it is used to make acetic acid, which is a common weak acid. It’s also a component of acetic anhydride synthesis.

 Key facts about acetaldehyde:

• Acetaldehyde is generated when ethanol is oxidised, and acetaldehyde is then transformed to acetate.

•  Acetaldehyde is an exceedingly reactive molecule that boils at room temperature in its pure form.

• Acetaldehyde circulation levels in the body are very low, and it is attached to plasma proteins or red blood cells, decreasing its toxicity.

• Acetaldehyde binds to proteins, lipids, and nucleic acids covalently.

• Cells are toxic to acetaldehyde-protein adducts.

• Acetaldehyde-protein adducts start the neo-antigen production process.

Conclusion:

Acetaldehyde (ethanal) is a highly reactive and poisonous aldehyde. Acetaldehyde is a carcinogen that damages cells and genomes. The large number of enzymes involved in the metabolism and detoxification of acetaldehyde and other aldehyde forms demonstrates the reactivity of this compound. Endogenous acetaldehyde is formed through metabolic or autooxidation pathways, and a significant number of genes are involved in the metabolic detoxification of acetaldehyde. Acetaldehyde is classified as a Class 1 poison by the World Health Organization (human carcinogen). Alcohol drinking is the main source of acetaldehyde. Ethanol is primarily converted to acetaldehyde in the body. There are, however, many other natural and synthetic sources of acetaldehyde.