Aldehydes, Ketones and Carboxylic Acids

Acetaldehyde

Acetaldehyde is a common name of ethanal. Acetaldehyde is an organic chemical compound with the chemical formula \(CH_3CHO\). It is also abbreviated by chemists as MeCHO where ‘Me’ means methyl. Acetaldehyde is one of the most important aldehydes. It is being produced on a large scale in many industries. Acetaldehyde occurs widely in nature as in coffee, bread, and ripe fruit and is produced by plants. It is also contributing to the cause of hangover after alcohol consumption. Pathways of exposure to acetaldehyde include air, water, land, or groundwater, as well as drink and smoke. Consumption of disulfiram inhibits acetaldehyde dehydrogenase. It is the enzyme that is responsible for the metabolism of acetaldehyde.

Acetaldehyde is easily miscible with naptha, gasoline, xylene, ether, turpentine, alcohol and benzene. It is a colourless, flammable liquid and has a suffocating smell. Acetaldehyde is non-corrosive to many metals but when it has a narcotic action, it can cause mucous irritation. It was observed by the Swedish pharmacist/chemist Carl Wilhelm Scheele in the year 1774.

Acetaldehyde 

Production of Acetaldehyde

  • The main method of production of acetaldehyde is the oxidation of ethylene. It is done by the Wacker process. This process involves the oxidation of ethylene by homogeneous palladium or copper system.

\(2 CH_2=CH_2 + O2 \rightarrow 2 CH_3CHO\)

A small quantity of acetaldehyde can be prepared by the partial oxidation of ethanol. It is an exothermic reaction and is conducted over a silver catalyst at about \(500^oC\) to \(650^oC\).

\(CH_3CH_2OH + 1/2 O_2 \rightarrow CH_3CHO + H_2O\)

It is the oldest method for the preparation of acetaldehyde.

  • Prior to the Wacker process and the availability of ethylene, acetaldehyde is also produced by the hydration of acetylene and is catalyzed by mercury (II) salts.

\(C_2H_2 + Hg^{2+} + H_2O \rightarrow CH_3CHO + Hg\)

The mechanism involves the intermediacy of vinyl alcohol that is tautomerized to acetaldehyde. The reaction is conducted at \(90^oC\) to \(95^oC\). Acetaldehyde formed here is separated from water and mercury and cooled to \(25^oC\) to \(30^oC\). In the wet oxidation process, iron (III) sulfate is in use to reoxidize the mercury to the mercury (II) salt. The resulting iron (II) sulfate is then oxidized in a separate reactor with nitric acid.

  • Traditionally, it was also produced by the partial dehydrogenation of ethanol.

\(CH_3CH_2OH \rightarrow CH_3CHO + H_2\)

This is an endothermic process. Ethanol vapour is passed by a copper-based catalyst at \(260^oC\) to \(290^oC\).

Physical Properties of Acetaldehyde

  • The molecular weight/ molar mass of acetaldehyde is 44.05 gram per mole.
  • The density of acetaldehyde is 0.784 gram per centimetre cube.
  • Also, the boiling of acetaldehyde is \(20.2^oC\).
  • The melting of acetaldehyde is \(-123.5^oC\).

Chemical Properties of Acetaldehyde

  • The chemical properties of acetaldehyde are similar to formaldehyde. It is a precursor in organic synthesis, especially as an electrophile.
  • By condensation reaction, one can gain intermediates like pentaerythritol that we can be used in organic synthesis.
  • Also, can be useful to produce hydroxyethyl derivatives by a reaction with a Grignard reagent. Acetaldehyde is a building block that is in use in the synthesis of heterocycles, such as imines and pyridines.

Uses of Acetaldehyde

The uses of Acetaldehyde are:

  • As a precursor to acetic acid.
  • As a precursor to pyridine derivatives, crotonaldehyde, and pentaerythritol.
  • In the manufacturing of resin.
  • To produce polyvinyl acetate.
  • In the manufacturing of disinfectants, perfumes, and drugs.
  • In the production of chemicals such as acetic acid.

FAQs on Acetaldehyde

Question 1: How does acetaldehyde affect the liver?

Answer: It is a major toxic metabolite. Acetaldehyde is one of the main compounds that is mediating the fibrogenic and mutagenic effects of alcohol in the liver. Basically, it promotes the formation of adducts that are leading to functional impairments of key proteins, including enzymes, and DNA damage and this promotes mutagenesis.

Question 2: What is the difference between formaldehyde and acetaldehyde?

Answer: The iodoform test is in use to differentiate between formaldehyde and acetaldehyde. Methyl ketones react to yellow precipitate by giving iodine and potassium hydroxide whereas, acetaldehyde binds to the carboxylic acid with iodine and KOH to give sodium salt. It is noteworthy that formaldehyde does not screen for iodoformity.

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