Ethylene oxide

Ethylene oxide

Ethylene oxide is a colorless and flammable gas with a faintly sweet odor, b.p. 10.7^oC . According to the A.U.P.A.C. system of nomenclature, an oxygen atom linked to two of the carbon atoms in a carbon chain is denoted by the prefix epoxy in all cases other than those in which a substance is named as a cyclic compound; thus ethylene oxide is epoxyethane. Epoxy-compounds contain the oxiran(oxirane) ring, and ethylene oxide is also known as oxiran (oxiraane). Oxiran compounds are also referred to as cyclic ethers or alkene oxides.

Epoxides may, in general, be prepared by epoxidation of alkenes with peroxy acids. Ethylene oxide may be prepared as follows:

The mechanism is possibly:

This is an example of neighboring group participation.

Ethylene oxide is manufactured by passing ethylene and air under pressure over a silver catalyst at 200-400^oC:

Ethylene oxide undergoes molecular rearrangement on heating to form acetaldehyde:

Epoxides are reduced by lithium aluminium hydride to alcohols, unsymmetrical epoxides giving as main product the more highly substituted alcohol; thus (I) is the main product:

On the other hand, (II) is the main product if LAH-AlCl₃ is used. Brown et al. have shown that (II) is the major product (^90%) when BH₃-BF₃ is the reducing regent.

Epoxides are readily reduced to the corresponding aalkenes by t-phosphines, e.g.,

They are also oxidized by dimethyl sulphoxide to a-hydroxyketones (acyloins).

Ethylene oxide is converted into ethylene glycol in dilute acid solution and into ethylene halogenohydrins with concentrated halogen acid solutions. Ethylene oxide also forms mono-ethers with alcohols in the presence of a smoall amount of acid as catalyst. The mechanisms of these reactions in acid media probably follow the same pattern, e.g., with dilute acid (with alcohols, replace H₂O by ROH):

Attack occurs from the position remote from the ⁺OH , and the  product is the trans diol. Thus cannot be demonstrated with ethylene oxide itself but has been, e.g., with 1,2-epoxycyclohexane. with concentrated halogen acid:

Ethylene oxide also reacts with alcohols under the influence of basis catalysts. A possible mechanism is :

Of particular interest is the addition of hydrogen cyanide to form ethylene cyanohydrins:

The structure of ethylene oxide is uncertain; it may contain ‘bent’ bonds.

When ethylene oxide is heated with methanol under pressure, the monomethyl ether of glycol is formed:

This is known as methyl cellosolve; the corresponding ethyl ether is known as ethyl cellosolve. Cellosolves are very useful as solvent since they contain both the alcohol and ether functional groups (cf. polyethylene glycols)

The further action of ethylene oxide on cellosolves produces carbitols, e.g.,

When glycol is treated with ethylene oxide, diethylene glycol is formed:

Diethylene glycol dimethyl ether (diglyme), b.p. 160^oC is used as a solvent.

Ethylene oxide reacts with ammonia to form a mixture of three aminoalcohols which are usually referred to as the ethanolamines:

The ethanolamines are widely used as emulsifying agents.

The mechanism of these reactions is possibly:

In the presence of excess of ethylene oxide, the reaction proceeds with the ethanolamine now being the amino regent, etc.

The C-O group (stretch) in epoxides absorbs in the region I 260-I 240 cm^-1 (s), and so these compounds may be readily distinguished from ethers.

The NMR spectra of epoxy compounds show a t-value -7.2-7.6 p.p.m. for methylene protons and 7.0 for methane protons. These ranges are sufficiently different from those of CH₂ and CH groups in other environments that epoxides may usually be recognized in this way.