Ethylene oxide

Ethylene oxide
IUPAC name	oxirane [1]
Other names	epoxyethane, ethylene oxide, dimethylene oxide,oxacyclopropane
Identifiers
Abbreviations	EO, EtO
CAS number	[75-21-8]
PubChem	6354
EC number	200-849-9
KEGG	C06548
MeSH	Ethylene+Oxide
ChEBI	27561
RTECS number	KX2450000
SMILES	C1CO1
InChI	1/C2H4O/c1-2-3-1/h1-2H2
Properties
Molecular formula	C2H4O
Molar mass	44.05 g mol−1
Appearance	colorless gas
Density	0.882 g/mL, 7.360 lbs/gallon
Melting point	−111.3 °C
Boiling point	10.7 °C
Solubility in water	miscible
Thermochemistry
Std enthalpy of formation ΔfHo298	−52.6 kJ mol−1
Standard molar entropy So298	243 J mol−1 K−1
Hazards
Main hazards	carcinogen
NFPA 704	4 3 3
Flash point	−20 °C
Explosive limits	3 to 100%
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Ethylene oxide, also called oxirane, is the organic compound with the formula C₂H₄O. This colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered ring consisting of two carbon and one oxygen atom. It is commonly handled and shipped as a refrigerated liquid.^[1] It is the chief precursor to ethylene glycol and other high-volume chemicals but is also used for medical sterilization.

History

Ethylene oxide was first reported in 1859 by the French chemist Charles-Adolphe Wurtz,^[2] who prepared it by treating 2-chloroethanol with a base. It achieved industrial importance during World War I as a precursor to both the coolant ethylene glycol and the chemical weapon mustard gas. In 1931, Theodore Lefort, another French chemist, discovered a means to prepare ethylene oxide directly from ethylene and oxygen, using silver as a catalyst. Since 1940, almost all ethylene oxide produced industrially has been made using this method.^[3] Ethylene oxide sterilization for the preservation of spices was patented in 1938 by the American chemist Lloyd Hall, and it is still used in that role.

Production

Ethylene oxide is produced by oxidation of ethylene with oxygen at circa 250 °C over a catalyst comprising metallic silver supported on alumina^[4]. Typically, promoters such as chloride are also included. Pressures used are in the region of 1-2MPa. The overall chemical equation is:

7 H₂C=CH₂ + 6 O₂ → 6 C₂H₄O + 2 CO₂ + 2 H₂O

The mechanism involves binding of ethylene and oxygen (O2) to the catalyst. Per molecule of oxygen, one atom is inserted into ethylene affording ethylene oxide. The other oxygen atom remains strongly absorbed on the catalyst and must be burnt off before the next productive catalytic cycle can take place. This consumes one extra molecule of ethylene for every six molecules of ethylene oxide produced. The extra molecule of ethylene undergoes combustion to carbon dioxide and water presumably via acetaldehyde:

6 H₂C=CH₂ + 6 O₂ → 6 C₂H₄O + 6 O (bound to catalyst)

O (bound to catalyst) + H₂C=CH₂ → CH₃CHO (bound to catalyst)

CH₃CHO (bound to catalyst) + 5 O (bound to catalyst) → 2 CO₂ + 2 H₂O

Hence, the yield of the process is limited to 6/7 (85.7%). The yield under industrial conditions stands at 83-84%. The high yield is due to extensive research by big producers, driven by enormous cost savings potential: With approximately 15 million tonnes of ethylene oxide being produced annually^[1] every percent of yield increase saves 95,500 tonnes of ethylene worth $67 million.

Ethylene oxide can be produced in the laboratory by the action of an alkali hydroxide on ethylene chlorohydrin.^[5]

HOH₂CCH₂Cl + NaOH → C₂H₄O + NaCl + H₂O

Uses

Most ethylene oxide is consumed as the precursor to ethylene glycol as well as a variety of other chemicals. Ethylene glycol is more commonly known for its use as an automotive coolant and antifreeze. Other chemical applications include ethanolamine, diverse surfactants (see reactions section below), and glycol ethers such as ethoxyethanol.^[1]

Ethylene oxide is also used as sterilant, although the amount used for this purpose is minor compared to the applications in the chemical industry. The gas kills bacteria (and their endospores), mold, and fungi. It is used to sterilize substances that would be damaged by high temperature techniques such as pasteurization. Ethylene oxide is widely used to sterilize the majority of medical supplies such as bandages, sutures, and surgical implements in a traditional chamber sterilization method, where a chamber is flooded with a mixture of ethylene oxide and other gases that are later aerated. The more recent gas diffusion method developed in 1967 relies on a bag that wraps the elements to be sterilized and serves as a mini-chamber in order to minimize gas consumption and make the process economically feasible for small loads. Other names for this alternative method for small loads are the Anprolene method, bag sterilization method or micro-dose sterilization method.

Reactions

Most reactions are ring openings by nucleophiles. In industry, ethylene oxide is hydrolyzed (reacted with water) in the presence of a sulfuric acid catalyst. A tenfold molar excess of water is used to obtain ethylene glycol:

C₂H₄O + H₂O → HOCH₂CH₂OH

Despite the large excess of water, various types of polyethylene glycol (PEG) or polyethylene oxide (PEO) are still formed as secondary products. The degree of polymerization increases as a smaller proportion of water is used:

n(CH₂CH₂O) + H₂O → HO(CH₂CH₂O)_nH

For example, under the right conditions it can give diethylene glycol (HOCH₂CH₂OCH₂CH₂OH), triethylene glycol, etc.

Similarly, reaction with ammonia yields ethanolamine as well as diethanolamine and triethanolamine.

Ethylene oxide is also important in the manufacture of surfactants and other detergents, in a process called ethoxylation.

One class of ethylene oxide derivatives that has attracted much scientific attention is the crown ethers, which are cyclic oligomers of ethylene oxide. These compounds have the ability to make ionic compounds such as salts soluble in nonpolar solvents which they otherwise could not dissolve in. However, the high cost of these compounds has largely confined their use to the laboratory rather than industrial practice.

Health effects

Ethylene oxide is toxic by inhalation with an LD₅₀ of 330 mg/kg. Symptoms of overexposure include headache and dizziness, progressing with increasing exposure to convulsions, seizure and coma. It is also an irritant to skin and the respiratory tract, and inhaling the vapors may cause the lungs to fill with fluid several hours after exposure.^[6]

Ethylene oxide is usually stored as a pressurized or refrigerated liquid. At room temperature and pressure, it rapidly evaporates, potentially causing frostbite in cases of skin exposure.

Laboratory animals exposed to ethylene oxide for their entire lives have had a higher incidence of liver cancer. However, studies on human beings who have worked with ethylene oxide for extended periods and may have experienced low doses during that time have found no increase in cancer risk. Chronic ethylene oxide exposure may increase the risk of cataracts in humans.

In animals, ethylene-oxide can cause numerous reproductive effects, including mutations and a higher rate of miscarriages. Its reproductive effects on humans have not been well studied, but it is considered probable that ethylene oxide exposure has similar effects on human reproduction.

Ethylene oxide is classified as carcinogenic to humans by the International Agency for Research on Cancer (IARC).^[7]

See also

• Acetaldehyde
• Ethenol

External links

• EOSA Promoting the safe use of Ethylene Oxide for Sterilization
• WebBook page for C2H4O
• National Pollutant Inventory - Ethylene oxide fact sheet
• Ethylene Oxide User's Guide
• Ethylene Oxide MSDS (Material Safety Data Sheet).
• National Institute for Occupational Safety and Health - Ethylene Oxide Topic Page
• EOSA memo about Ethylene Oxide (EtO) facts

References

1. ^ ^{a b c} Siegfried Rebsdat, Dieter Mayer "Ethylene Oxide" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2005.doi:10.1002/14356007.a10_117 Article Online Posting Date: March 15, 2001.
2. ^ Wurtz, A. (1859). Compt. rend. 48: 101–104. 
3. ^ P. P. McClellan (1950). "Manufacture and Uses of Ethylene Oxide and Ethylene Glycol". Ind. Eng. Chem. 42: 2402–2407. doi:10.1021/ie50492a013. 
4. ^ http://www.freepatentsonline.com/6846774.html
5. ^ Streitwiser, Andrew; Heathcock, Clayton H. (1976). Introduction to Organic Chemistry. Macmillan. ISBN 0-02-418010-6. 
6. ^ http://www.anpro.com/support/MSDS.pdf
7. ^ IARC Vol 60