Acrolein is the simplest unsaturated aldehyde. It is a colourless liquid with a piercing,
disagreeable, 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 acid
methionine.
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: CH2=CHCH3 + O2 → CH2=CHCHO + H2O
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 over oxidation 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:
(CH2OH)2CHOH → CH2=CHCHO + 2 H2O This route is attractive when glycerol is
cogenerated in the production of biodiesel from fatty acids. 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:
HCHO + CH3CHO → CH2=CHCHO + H2O Acrolein may also be produced on lab scale
by the reaction of potassium bisulfate on glycerol.
Reactions Acrolein is a relatively electrophilic compound
and a reactive one, hence its high toxicity. It is a good Michael acceptor, hence its useful
reaction with thiols. It forms acetals readily, a prominent one being the spirocycle derived
from pentaerythritol, diallylidene pentaerythritol. Acrolein participates in many Diels-Alder
reactions, even with itself. Via Diels-Alder reactions, it is a precursor to some commercial
fragrances, including lyral, norbornene-2-carboxaldehyde, and myrac aldehyde.
Uses Biocide
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/day
per kilogram of body weight. Although acrolein occurs in French fries, the levels are only
a few micrograms per kilogram. 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. 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. Chemotherapy metabolite
Cyclophosphomide 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 (KHSO
4), 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 method 603 is designed to measure acrolein in industrial and municipal wastewater
streams. References