The simplest unsaturated aldehyde is acrolein (propenal in scientific terms). It's a colourless liquid with an acrid, piercing odour. The scent of burnt fat is caused by glycerol in the burning fats breaking down into acroleins (similar to how the cooking oil is heated to its smoke point). It's made in a factory from propylene and is mainly used as a biocide and a building block for other chemicals.
Production of Acrolein Sigma Aldrich
Industrially, acrolein is made by oxidising propene. Air is used as an oxygen source, and metal oxides are used as heterogeneous catalysts.
CH2CHCH3+O2 → CH2CHCHO + H2O
This method produces around 500,000 tonnes of acrolein per year in North America, Europe, and Japan. Furthermore, the transient formation of acrolein is the source of all acrylic acid and helps in acrylic acid reaction. The primary difficulty is the competing overoxidation of this acid. Propane is a promising but challenging feedstock for the production of acrolein (and then acrolein to acrylic acid).
Here is the formation of Acrolein Sigma Aldrich:
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When heated to 280°C, glycerol (also known as glycerin) decomposes into acrolein:
(CH2OH)2CHOH → CH2 = CHCHO + 2H2O
When glycerol is produced to make biodiesel from vegetable oils or animal fats, this route is attractive. While glycerol dehydration has been demonstrated, it is not competitive with the petrochemical path.
Laboratory Methods for Producing Acroleins
Degussa developed the first industrial route to acrolein, which includes condensation of formaldehyde and acetaldehyde:
HCHO + CHCHO → CH2 = CHCHO+H2O
On a lab scale, acrolein can be made by reacting potassium bisulfate with glycerol (glycerine).
Reactions Related to Acrolein
Since acrolein is a relatively electrophilic and reactive substance, it has high toxicity. It has a strong Michael acceptor, which explains why it reacts well with thiols. It readily forms acetals, one of which is the alkylidene pentaerythritol spirocycle derived from pentaerythritol. Also itself, acrolein participates in several Diels-Alder reactions. It is a precursor to commercial fragrances, such as lyral, norbornene-2-carboxaldehyde, and myrac aldehyde, through Diels-Alder reactions. Via the intermediacy of tetrahydro benzaldehyde, the monomer 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate is also generated from acrolein.
Furthermore, it helps produce critical chemical compounds, including methyl acrolein, poly acrolein, etc. Moreover, the production method and the reaction cost helps in determining the acrolein price.
Acrolein is primarily used as a contact herbicide in irrigation canals to combat submerged and floating weeds, as well as algae. It is used in irrigation and recirculating waters at a concentration of 10 ppm. It is used as a biocide in fracking waters and as a scavenger for hydrogen sulphide and mercaptans in the oil and gas industry.
Acrolein's bifunctionality allows it to be used to make a variety of valuable compounds. The amino acid methionine is synthesised by combining methanethiol and Strecker synthesis. Methyl Pyridines are formed when acrolein condenses with acetaldehyde and amines. It is also considered an intermediate in the Skraup synthesis of quinolines, but it is rarely used due to its instability.
In the presence of oxygen and at concentrations greater than 22% in water, acrolein polymerises. The colour and texture of the polymer are affected by the environment. It will polymerise with itself over time, forming a transparent, yellow solid. It can turn into a rigid, brittle plastic when exposed to water.
In the preparation of biological specimens for electron microscopy, acrolein is often used as a fixative.
Health Risks Associated With Acrolein
Acrolein is a powerful irritant to the skin, eyes, and nasal passages and is harmful. The alkylation of glutathione is the primary metabolic pathway for acrolein. The WHO recommends a daily intake of 7.5 grammes of acrolein per kilogramme of body weight as a "tolerable oral acrolein intake." At the same time, acrolein can be found in French fries (and other fried foods), only a few grammes per kilogramme. The Health Administration and US Occupational Safety have set an acceptable exposure limit of 0.1 ppm (0.25 mg/m3) at an eight-hour time-weighted average in response to occupational acrolein exposures.
Acrolein suppresses the immune system and may stimulate regulatory cells, preventing allergy development on the one hand but raising the risk of cancer on the other. One of the chemicals implicated in the toxic contamination of the Kim Kim River in 2019 has been identified as acrolein.
The "acrolein test" is used to determine if glycerin or fats are present. If the test is positive, a sample is heated with potassium bisulfate, and acrolein is released. When fat is heated to high temperatures in the presence of a dehydrating agent like potassium bisulfate (KHSO4), the glycerol part of the molecule dehydrates, forming the unsaturated aldehyde acrolein (CH2=CH-CHO), which has the odour of burnt cooking grease. There are more modern methods available.