Ethylene oxide

Ethylene oxide is an organic compound with the formula. It is a cyclic ether and the simplest epoxide: a three-membered ring consisting of one oxygen atom and two carbon atoms. Ethylene oxide is a colorless and flammable gas with a faintly sweet odor. Because it is a strained ring, ethylene oxide easily participates in a number of addition reactions that result in ring-opening. Ethylene oxide is isomeric with acetaldehyde and with vinyl alcohol. Ethylene oxide is industrially produced by oxidation of ethylene in the presence of a silver catalyst.
The reactivity that is responsible for many of ethylene oxide's hazards also makes it useful. Although too dangerous for direct household use and generally unfamiliar to consumers, ethylene oxide is used for making many consumer products as well as non-consumer chemicals and intermediates. These products include detergents, thickeners, solvents, plastics, and various organic chemicals such as ethylene glycol, ethanolamines, simple and complex glycols, polyglycol ethers, and other compounds. Although it is a vital raw material with diverse applications, including the manufacture of products like polysorbate 20 and polyethylene glycol that are often more effective and less toxic than alternative materials, ethylene oxide itself is a very hazardous substance. At room temperature it is a very flammable, carcinogenic, mutagenic, irritating; and anaesthetic gas.
Ethylene oxide is a surface disinfectant that is widely used in hospitals and the medical equipment industry to replace steam in the sterilization of heat-sensitive tools and equipment, such as disposable plastic syringes. It is so flammable and extremely explosive that it is used as a main component of thermobaric weapons; therefore, it is commonly handled and shipped as a refrigerated liquid to control its hazardous nature.

History

Ethylene oxide was first reported in 1859 by the French chemist Charles-Adolphe Wurtz, who prepared it by treating 2-chloroethanol with potassium hydroxide:
Wurtz measured the boiling point of ethylene oxide as, slightly higher than the present value, and discovered the ability of ethylene oxide to react with acids and salts of metals. Wurtz mistakenly assumed that ethylene oxide has the properties of an organic base. This misconception persisted until 1896, when Georg Bredig found that ethylene oxide is not an electrolyte. That it differed from other ethers — particularly by its propensity to engage in the addition reactions typical of unsaturated compounds — had long been a matter of debate. The heterocyclic triangular structure of ethylene oxide was proposed by 1868 or earlier.
Wurtz's 1859 synthesis long remained the only method of preparing ethylene oxide, despite numerous attempts, including by Wurtz himself, to produce ethylene oxide directly from ethylene. Only in 1931 did French chemist Theodore Lefort develop a method of direct oxidation of ethylene in the presence of silver catalyst. Since 1940, almost all industrial production of ethylene oxide has relied on this process. Sterilization by ethylene oxide for the preservation of spices was patented in 1938 by the American chemist Lloyd Hall. Ethylene oxide achieved industrial importance during World War I as a precursor to both the coolant ethylene glycol and the chemical weapon mustard gas.

Molecular structure and properties

The epoxy cycle of ethylene oxide is an almost regular triangle with bond angles of about 60° and a significant angular strain corresponding to the energy of 105 kJ/mol. For comparison, in alcohols the C–O–H angle is about 110°; in ethers, the C–O–C angle is 120°. The moment of inertia about each of the principal axes are I_A=, I_B= and I_C=.
The relative instability of the carbon-oxygen bonds in the molecule is revealed by the comparison in the table of the energy required to break two C–O bonds in the ethylene oxide or one C–O bond in ethanol and dimethyl ether:
This instability correlates with its high reactivity, explaining the ease of its ring-opening reactions.

Physical properties

Ethylene oxide is a colorless gas at and is a mobile liquid at – viscosity of liquid ethylene oxide at 0 °C is about 5.5 times lower than that of water. The gas has a characteristic sweet odor of ether, noticeable when its concentration in air exceeds 500ppm. Ethylene oxide is readily soluble in water, ethanol, diethyl ether, and many organic solvents.
Main thermodynamical constants are:

The surface tension of liquid ethylene oxide, at the interface with its own vapor, is at and at.
The boiling point increases with the vapor pressure as follows: , , and .
Viscosity decreases with temperature with the values of 0.577kPa·s at, 0.488 kPa·s at, 0.394kPa·s at, and 0.320kPa·s at.

Between, vapor pressure p varies with temperature as
*N/A – data not available.
*N/A – data not available.

Chemical properties

Ethylene oxide readily reacts with diverse compounds with opening of the ring. Its typical reactions are with nucleophiles which proceed via the S_N2 mechanism both in acidic and alkaline media. The general reaction scheme is
and more specific reactions are described below.

Addition of water and alcohols

Aqueous solutions of ethylene oxide are rather stable and can exist for a long time without any noticeable chemical reaction. However adding a small amount of acid, such as strongly diluted sulfuric acid, immediately leads to the formation of ethylene glycol, even at room temperature:
The reaction also occurs in the gas phase, in the presence of a phosphoric acid salt as a catalyst.
The reaction is usually carried out at about with a large excess of water, in order to prevent the reaction of the formed ethylene glycol with ethylene oxide that would form di- and triethylene glycol:
The use of alkaline catalysts may lead to the formation of polyethylene glycol:
Reactions with alcohols proceed similarly yielding ethylene glycol ethers:
Reactions with lower alcohols occur less actively than with water and require more severe conditions, such as heating to and pressurizing to and adding an acid or alkali catalyst.
Reactions of ethylene oxide with fatty alcohols proceed in the presence of sodium metal, sodium hydroxide, or boron trifluoride and are used for the synthesis of surfactants.

Addition of carboxylic acids and their derivatives

Reactions of ethylene oxide with carboxylic acids in the presence of a catalyst results in glycol mono- and diesters:
The addition of acid amides proceeds similarly:
Addition of ethylene oxide to higher carboxylic acids is carried out at elevated temperatures and pressure in an inert atmosphere, in presence of an alkaline catalyst, such as hydroxide or carbonate of sodium or potassium. The carboxylate ion acts as nucleophile in the reaction:

Adding ammonia and amines

Ethylene oxide reacts with ammonia forming a mixture of mono-, di-, and tri- ethanolamines. The reaction is stimulated by adding a small amount of water.
Similarly proceed the reactions with primary and secondary amines:
Dialkylamino ethanols can further react with ethylene oxide, forming amino polyethylene glycols:
Trimethylamine reacts with ethylene oxide in the presence of water, forming choline:
Aromatic primary and secondary amines also react with ethylene oxide, forming the corresponding arylamino alcohols.

Halide addition

Ethylene oxide readily reacts with aqueous solutions of hydrochloric, hydrobromic, and hydroiodic acids to form halohydrins. The reaction occurs easier with the last two acids:
The reaction with these acids competes with the acid-catalyzed hydration of ethylene oxide; therefore, there is always a by-product of ethylene glycol with an admixture of diethylene glycol. For a cleaner product, the reaction is conducted in the gas phase or in an organic solvent.
Ethylene fluorohydrin is obtained differently, by boiling hydrogen fluoride with a 5–6% solution of ethylene oxide in diethyl ether. The ether normally has a water content of 1.5–2%; in absence of water, ethylene oxide polymerizes.
Halohydrins can also be obtained by passing ethylene oxide through aqueous solutions of metal halides:

Metalorganic addition

Interaction of ethylene oxide with organomagnesium compounds, which are Grignard reagents, can be regarded as nucleophilic substitution influenced by carbanion organometallic compounds. The final product of the reaction is a primary alcohol:
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Similar mechanism is valid for other organometallic compounds, such as alkyl lithium:

Other addition reactions

Addition of hydrogen cyanide

Ethylene oxide easily reacts with hydrogen cyanide forming ethylene cyanohydrin:
A slightly chilled aqueous solution of calcium cyanide can be used instead of HCN:
Ethylene cyanohydrin easily loses water, producing acrylonitrile:

Addition of hydrogen sulfide and mercaptans

When reacting with the hydrogen sulfide, ethylene oxide forms 2-mercaptoethanol and thiodiglycol, and with alkylmercaptans it produces 2-alkyl mercaptoetanol:
The excess of ethylene oxide with an aqueous solution of hydrogen sulfide leads to the tris- sulfonyl hydroxide:

Addition of nitrous and nitric acids

Reaction of ethylene oxide with aqueous solutions of barium nitrite, calcium nitrite, magnesium nitrite, zinc nitrite, or sodium nitrite leads to the formation of 2-nitroethanol:
With nitric acid, ethylene oxide forms mono- and dinitroglycols:

Reaction with compounds containing active methylene groups

In the presence of alkoxides, reactions of ethylene oxide with compounds containing active methylene group leads to the formation of butyrolactones:

Alkylation of aromatic compounds

Ethylene oxide enters into the Friedel–Crafts reaction with benzene to form phenethyl alcohol:
Styrene can be obtained in one stage if this reaction is conducted at elevated temperatures and pressures, in presence of an aluminosilicate catalyst.