1,4-Dioxane, often simply called dioxane because the 1,2 and 1,3 isomers of dioxane are rare,
is a heterocyclic organic compound. It is a colorless liquid with a faint sweet odor
similar to that of diethyl ether. It is classified as an ether. Dioxane is used mainly as a stabilizer
for the solvent trichloroethane. It is an occasionally used solvent for a variety of
practical applications as well as in the laboratory.
Synthesis and structure Dioxane is produced by the acid-catalysed
dehydration of diethylene glycol, which in turn arises from the hydrolysis of ethylene
oxide. The molecule is centrosymmetric, meaning that it adopts a chair conformation, typical
of relatives of cyclohexane. The molecule is conformationally flexible, and the boat
conformation is easily adopted, as required for chelation to metal cations. In 1985, the
global production capacity for dioxane was between 11,000 and 14,000 tons. In 1990, the
total U.S. production volume of dioxane was between 10,500,000 and 18,300,000 pounds.
Dioxane has three isomers: in addition to 1,4-dioxane, 1,2-dioxane and 1,3-dioxane are
also known.
Uses Dioxane is primarily used as a stabilizer
for 1,1,1-trichloroethane for storage and transport in aluminium containers. Normally
aluminium is protected by a passivating oxide layer, but when these layers are disturbed,
highly reactive metallic aluminium is exposed to the chlorocarbon. This aluminium reacts
with 1,1,1-trichloroethane to give aluminium trichloride, which in turn catalyses the dehydrohalogenation
of the remaining 1,1,1-trichloroethane to vinylidene chloride and hydrogen chloride.
Reflecting its properties as a ligand, dioxane "poisons" the aluminum trichloride catalyst,
by formation of an adduct. Apart from its use as a stabilizer, dioxane is used in a
variety of applications as a solvent, e.g. in inks and adhesives.
Solvent properties Diethyl ether is rather insoluble in water,
whereas dioxane is miscible and in fact is hygroscopic. At standard pressure, the mixture
of water and dioxane in the ratio 17.9:82.1 by mass is a positive azeotrope that boils
at 87.6 °C. Dioxane is a versatile aprotic solvent. The oxygen atom is Lewis basic, so
it is able to solvate many inorganic compounds. Because of its lower toxicity, it is substituted
for tetrahydrofuran in some processes. However, it has a higher boiling point, which is important
when reactions are to be conducted at a higher temperature.
The oxygen centres are Lewis basic and so dioxane serves as a chelating diether ligand.
It reacts with Grignard reagents to precipitate the magnesium dihalide. In this way, dioxane
is used to drive the Schlenk equilibrium. Dimethylmagnesium is prepared in this manner:
2 CH3MgBr +2 → MgBr2(C2H4O)2 +2Mg It is used as an internal standard for proton
NMR spectroscopy in D2O.
Toxicology Safety
Dioxane has an LD50 of 5170 mg/kg. This compound is irritating to the eyes and respiratory
tract. Exposure may cause damage to the central nervous system, liver and kidneys. Accidental
worker exposure to 1,4-dioxane has resulted in several deaths. Dioxane is classified by
the National Toxicology Program as "reasonably anticipated to be a human carcinogen". It
is also classified by the IARC as a Group 2B carcinogen: possibly carcinogenic to humans
because it is a known carcinogen in other animals. The U.S. Environmental Protection
Agency classifies dioxane as a probable human carcinogen, and a known irritant at concentrations
significantly higher than those found in commercial products. Under Proposition 65, dioxane is
classified in the U.S. state of California to cause cancer. Animal studies in rats suggest
that the greatest health risk is associated with inhalation of vapors in the pure form.
Like some other ethers, dioxane combines with atmospheric oxygen upon prolonged exposure
to air to form potentially explosive peroxides. Distillation of dioxanes concentrates these
peroxides increasing the danger. Environment
Dioxane has affected groundwater supplies in several areas. Dioxane at the level of
1 μg/L has been detected in many locations in the US. In the State of New Hampshire alone
in 2010 it had been found at 67 sites, ranging in concentration from 2 ppb to over 11,000
ppb. Thirty of these sites are solid waste landfills, most of which have been closed
for years. It also has low toxicity to aquatic life and can be biodegraded via a number of
pathways. The problems are exacerbated since dioxane is highly soluble in water, does not
readily bind to soils, and readily leaches to groundwater. It is also resistant to naturally
occurring biodegradation processes. Due to these properties, a dioxane plume is often
much larger than the associated solvent plume. Cosmetics
As a byproduct of the ethoxylation process, a route to some ingredients found in cleansing
and moisturizing products, dioxane can contaminate cosmetics and personal care products such
as deodorants, shampoos, toothpastes and mouthwashes. The ethoxylation process makes the cleansing
agents, such as sodium lauryl sulfate, less abrasive and offers enhanced foaming characteristics.
1,4-Dioxane is found in small amounts in some cosmetics, a yet unregulated substance used
in cosmetics in both China and the U.S. In 2008, testing sponsored by the U.S. Organic
Consumers Association found dioxane in almost half of tested organic personal-care products.
Since 1979 the U.S. Food and Drug Administration have conducted tests on cosmetic raw materials
and finished products for the levels of 1,4-dioxane. 1,4-Dioxane was present in ethoxylated raw
ingredients at levels up to 1410 ppm, and at levels up to 279 ppm in off the shelf cosmetic
products. Levels of 1,4-dioxane exceeding 85 ppm in children's shampoos indicate that
close monitoring of raw materials and finished products is warranted. While the FDA encourages
manufacturers to remove 1,4-dioxane, it is not required by federal law.
See also 1,2-dioxane
1,3-dioxane Crown ether
Sodium laureth sulfate Oxalic anhydride
Dioxane tetraketone References