Ethylene Oxide

Ethylene Oxide Gas

Ethylene oxide gas (EtO) is a cyclic ether or simple organic epoxide compound with the chemical formula C₂H₄O. French chemist Charles-Adolphe Wurtz first prepared ethylene oxide by treating 2-chloroethanol with potassium hydroxide in 1859. In ethylene oxide structure, an oxygen atom is linked to the two carbon atoms with a three-member cyclic ring. At room temperature, ethylene oxide is a colourless gas that transforms into liquid at 0 °C. EtO is isomeric with acetaldehyde (CH₃CHO) and vinyl alcohol (CH₂=CH−OH). The viscosity of liquid ethylene oxide is much lower than that of water. It is readily miscible in water, ethanol, diethyl ether, and many organic solvents. Industrially, it has been produced by passing ethylene and air under pressure over a silver catalyst at 200 to 400 °C.

Due to the hazardous nature of gaseous ethylene oxide, we commonly handle and ship it as a refrigerated liquid. A large volume of EtO can be used for making many consumer products and non-consumer chemicals such as detergents, thickeners, solvents, plastics, and various organic chemicals (ethylene glycol, ethanolamines, polyglycol ethers, cellosolve, etc). It can also be used for the sterilization of medical equipment.

Properties of Ethylene Oxide

At room temperature, EtO is a highly flammable, extremely explosive, carcinogenic, irritating, and anesthetic gas. The chemical formula of the ethylene oxide molecule is C₂H₄O which is isomeric with acetaldehyde (CH₃CHO) and vinyl alcohol (CH₂=CH−OH).

According to the IUPAC system of nomenclature, EtO is denoted by the prefix epoxy. Therefore, EtO is called epoxyethane. It is named oxiran due to the presence of the oxiran ring. Oxiran compounds are also referred to as cyclic ethers or alkenes oxides.

The C−O group stretching in epoxides absorbs in the regions 1260−1240 cm⁻¹ but in acyclic ethers containing two alkyl groups absorbs in the regions 1150−1060 cm⁻¹. Therefore, such data can easily distinguish epoxy compounds from ethers.

Ethylene Oxide Production

French chemist Charles-Adolphe Wurtz first prepared it by treating 2-chloroethanol (chlorohydrin) with potassium hydroxide in 1859.

Dehydrochlorination of 2-Chloroethanol

Ethylene oxide was produced in the 19th century by the chlorohydrin process where ethylene chlorohydrin is prepared by reacting ethylene with hypochlorous acid (chlorine in water).

Cl₂ + H₂O → HOCl (hypochlorous acid) + HCl
CH₂=CH₂ + HOCl → HO–CH₂CH₂–Cl (chlorohydrin)

Ethylene chlorohydrin can be converted to ethylene oxide by treating it with sodium hydroxide or potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, or carbonates of alkali metals or alkaline earth metals.

2 HOCH₂CH₂Cl + Ca(OH)₂ → 2 (CH₂CH₂)O + CaCl₂ + 2H₂O

Such a method is no longer used in industry because most of the chlorine that was used was lost and unwanted organochlorine by-products were generated.

Therefore, this process has been gradually replaced by various other processes which can be described below:

Ethylene Oxide from Ethylene

Alkenes like ethylene can be converted to ethylene oxide by epoxidation with peroxy acids such as peroxybenzoic acid (C₆H₅COO₂H), meta-chloro-peroxybenzoic acid, monoperoxyphthalic acid, and p-nitro-peroxybezoic acid.

CH₂=CH₂ + C₆H₅COO₂NH → (CH₂CH₂)₂O + C₆H₅COONH

Peroxy trifluoroacetic acid (CF₃COO₂H) is a very good reagent for epoxidation and hydroxylation which converts the double bond of ethylene into an epoxide.

Industrial production

Ethylene oxide is manufactured by passing ethylene and air under pressure over a chemical catalyst like silver at 200 to 400 °C.

CH₂=CH₂ + O₂ → 2 (CH₂CH₂)O

Chemical Reaction of Ethylene Oxide

Ethylene oxide can readily react with various inorganic and organic compounds by opening its ring. It commonly participates in chemical reactions with nucleophiles via the SN² mechanism in acidic and alkaline media.

Molecular Rearrangement

Ethylene oxide undergoes molecular rearrangement on heating at about 400 °C in the presence of a catalyst (Al₂O₃, H₃PO₄) to form its isomer acetaldehyde (CH₃CHO).

(CH₂CH₂)O + heat → CH₃CHO

Addition of Water and Alcohol

An aqueous solution of ethylene oxide is stable and does not participate in any noticeable chemical changes. However, at room temperature when adding a small amount of strongly diluted sulfuric acid or any other acids, it immediately transforms into ethylene glycol.

(CH₂CH₂)O + H₂O → HO–CH₂CH₂–OH

Such type of chemical reaction can also occur in the gas phase in the presence of a catalyst (salt of phosphoric acid). When we use an alkaline catalyst, it can converted to polyethylene glycol.

n (CH₂CH₂)O + H₂O → HO–(–CH₂CH₂–O–)_n–H (polyethylene glycol)

Ethylene oxide can produce mono-ethers when reacts with alcohol in the presence of a small amount of acid as a catalyst. For example, it is produced ethyl cellosolve (2-ethoxyethanol) when reacts with ethanol.

(CH₂CH₂)O + C₂H₅OH → HO–CH₂CH₂–OC₂H₅

It can also react with alcohol under the influence of a basic catalyst. The reaction with fatty alcohols (lauryl, stearyl, and oleyl alcohols) can proceed in the presence of sodium metal, sodium hydroxide, or boron trifluoride.

Halide Addition Reaction

Gaseous ethylene oxide readily reacts with aqueous solutions of haloacids (HCl, HBr, HI) to form halohydrins. Such reactions always produce by-product ethylene glycol with a mixture of diethylene glycol. For pure products, the reaction can be conducted in the gas phase or an organic solvent.

(CH₂CH₂)O + HCl → HO–CH₂CH₂–Cl

Similarly, the addition of hydrogen cyanide produced ethylene cyanohydrin.

(CH₂CH₂)O + HCN → HO–CH₂CH₂–CN

Reaction with Ammonia

Ethylene oxide reacts with ammonia to form a mixture of three amino alcohols. They are usually called ethanolamines.

C₂H₄O + NH₃ → HOCH₂CH₂NH₂

HOCH₂CH₂NH₂ + C₂H₄O → (HOCH₂CH₂−)₂NH

(HOCH₂CH₂−)₂NH + C₂H₄O → (HOCH₂CH₂−)3N

A similar reaction can proceed when reacting with primary or secondary amines. C₂H₄O forms choline when reacts with trimethylamine in the presence of water.

C₂H₄O + (CH₃)₃N + H₂O → [HOCH₂CH₂N (CH₃)₃]⁺OH⁻

Reduction Reaction

Ethylene oxide is a symmetrical epoxide that forms ethyl alcohol when hydrogenated or reduced in the presence of nickel, platinum, palladium, boranes, lithium aluminum hydride, etc.

(CH₂CH₂)O + H₂ → 2 CH₃CH₂OH

Other unsymmetrical epoxides can give more highly substituted alcohol.

Propylene oxide → CH₃CHOHCH₃ + CH₃CH₂CH₂OH

Epoxides are readily reduced to the corresponding alkenes by t-phosphines or zinc dust and acetic acid. For example, ethylene oxide can give ethene when reduced by zinc dust and acetic acid.

(CH₂CH₂)O + H₂ → CH₂=CH₂ + H₂O

Dimerization

In the presence of dilute sulfuric acid, it dimerizes to form dioxan. Dioxan is a useful solvent for cryoscopic and ebullioscopy work.

The dimerization reaction of ethylene oxide is unselective and it produces acetaldehyde due to isomerization. Therefore, various catalysts such as platinum, platinum-palladium, or iodine with sulfolane can be used to increase selectivity and the speed of dimerization.

Methyl Cellosolve

Methyl cellosolve or monomethyl ether is prepared by heating EtO with methanol under pressure.

C₂H₄O + CH₃OH → HO–CH₂CH₂–OCH₃

Cellosolves are very useful solvents since they contain both alcohol and ether functional groups.

Ethylene oxide to Ethylene Glycol

Aqueous solutions of ethylene oxide are stable and exist for a long time without any chemical reaction but it is converted into ethylene glycol when added small amount of dilute sulfuric acid.

(CH₂CH₂)O + H₃O⁺ → HO−CH₂CH₂−OH + H⁺

The mechanism of the reaction in acid media follows the steps given in the picture,

Ethylene Oxide Uses

The direct use of ethylene oxide gas is very low due to health hazards and reactivity. It is readily flammable in the air. Therefore, EtO is used for making many consumer products and non-consumer chemicals such as detergents, thickeners, solvents, plastics, and various organic chemicals (ethylene glycol, ethanolamines, polyglycol ethers, etc).

• Ethylene oxide is the most impotent intermediate used for the production of different types of industrial organic compounds.
• It is used for the production of ethylene glycol.
• When EtO is heated with methanol under pressure, the monomethyl ether of glycol or methyl cellosolve is formed. The corresponding ethyl ether is known as ethyl cellosolve. Cellosolves are very useful solvents due to the presence of alcohol and ether functional groups.
• Directly, EtO is used as a sterilizing agent, disinfecting agent, and fumigant. Therefore, it is used for gas-phase sterilization of medical equipment and instruments, packaging materials and clothing, and surgical and scientific equipment.
• EtO gas reacts with ammonia to form a mixture of three amino alcohols which are usually referred to as ethanolamines. It is widely used for the preparation of emulsion.
• It is used as a fungicide. Ethylene oxide gas can also be used to accelerate the maturation of tobacco leaves.