Acrolein is the simplest unsaturatedaldehyde. It is a colourless liquid with a piercing, acrid smell. The smell of burnt fat is caused by glycerol in the burning fat breaking down into acrolein. It is produced industrially from propylene and mainly used as a biocide and a building block to other chemical compounds, such as the amino acidmethionine.
Production
Acrolein is prepared industrially by oxidation of propene. The process uses air as the source of oxygen and requires metal oxides as heterogeneous catalysts: About 500,000 tons of acrolein are produced in this way annually in North America, Europe, and Japan. Additionally, all acrylic acid is produced via the transient formation of acrolein. The main challenge is in fact the competing overoxidation to this acid. Propane represents a promising but challenging feedstock for the synthesis of acrolein. When glycerol is heated to 280 °C, it decomposes into acrolein: This route is attractive when glycerol is co-generated in the production of biodiesel from vegetable oils or animal fats. The dehydration of glycerol has been demonstrated but has not proven competitive with the route from petrochemicals.
Niche or laboratory methods
The original industrial route to acrolein, developed by Degussa, involves condensation of formaldehyde and acetaldehyde: Acrolein may also be produced on lab scale by the reaction of potassium bisulfate on glycerol.
Acrolein is mainly used as a contact herbicide to control submersed and floating weeds, as well as algae, in irrigation canals. It is used at a level of 10 ppm in irrigation and recirculating waters. In the oil and gas industry, it is used as a biocide in drilling waters, as well as a scavenger for hydrogen sulfide and mercaptans.
Chemical precursor
A number of useful compounds are made from acrolein, exploiting its bifunctionality. The amino acid methionine is produced by addition of methanethiol followed by the Strecker synthesis. Acrolein condenses with acetaldehyde and amines to give methylpyridines. It is also thought to be an intermediate in the Skraup synthesis of quinolines, but is rarely used as such due to its instability. Acrolein will polymerize in the presence of oxygen and in water at concentrations above 22%. The color and texture of the polymer depends on the conditions. Over time, it will polymerize with itself to form a clear, yellow solid. In water, it will form a hard, porous plastic. Acrolein is sometimes used as a fixative in preparation of biological specimens for electron microscopy.
Health risks
Acrolein is toxic and is a strong irritant for the skin, eyes, and nasal passages. The main metabolic pathway for acrolein is the alkylation of glutathione. The WHO suggests a "tolerable oral acrolein intake" of 7.5 μg per day per kg of body weight. Although acrolein occurs in French fries, the levels are only a few μg per kg. In response to occupational exposures to acrolein, the US Occupational Safety and Health Administration has set a permissible exposure limit at 0.1 ppm at an eight-hour time-weighted average. Acrolein acts in an immunosuppressive manner and may promote regulatory cells, thereby preventing the generation of allergy on the one hand, but also increasing the risk of cancer.
Cigarette smoke
Connections exist between acrolein gas in the smoke from tobacco cigarettes and the risk of lung cancer. In terms of the "noncarcinogenic health quotient" for components in cigarette smoke, acrolein dominates, contributing 40 times more than the next component, hydrogen cyanide. The acrolein content in cigarette smoke depends on the type of cigarette and added glycerin, making up to 220 µg acrolein per cigarette. Importantly, while the concentration of the constituents in mainstream smoke can be reduced by filters, this has no significant effect on the composition of the side-stream smoke where acrolein usually resides, and which is inhaled by passive smoking. E-cigarettes, used normally, only generate "negligible" levels of acrolein.
Chemotherapy metabolite
and ifosfamide treatment results in the production of acrolein. Acrolein produced during cyclophosphamide treatment collects in the urinary bladder and if untreated can cause hemorrhagic cystitis.
Analytical methods
The "acrolein test" is for the presence of glycerin or fats. A sample is heated with potassium bisulfate, and acrolein is released if the test is positive. When a fat is heated strongly in the presence of a dehydrating agent such as potassium bisulfate, the glycerol portion of the molecule is dehydrated to form the unsaturated aldehyde, acrolein, which has the odor peculiar to burnt cooking grease. More modern methods exist. In the US, EPA methods 603 and 624.1 are designed to measure acrolein in industrial and municipal wastewater streams.
