Inorganic chemistry, anepoxide is a cyclicether, where the ether forms a three-atomring: two atoms ofcarbon and one atom ofoxygen. This triangular structure has substantialring strain, making epoxides highlyreactive, more so than other ethers. They are produced on a large scale for many applications. In general, low molecular weight epoxides are colourless andnonpolar, and oftenvolatile.[1]
A compound containing the epoxidefunctional group can be called an epoxy, epoxide, oxirane, and ethoxyline. Simple epoxides are often referred to as oxides. Thus, the epoxide ofethylene (C2H4) isethylene oxide (C2H4O). Many compounds have trivial names; for instance, ethylene oxide is called "oxirane". Some names emphasize the presence of the epoxidefunctional group, as in the compound1,2-epoxyheptane, which can also be called1,2-heptene oxide.
Apolymer formed from epoxide precursors is called anepoxy. However, few if any of the epoxy groups in theresin survive thecuring process.
The dominant epoxides industrially areethylene oxide andpropylene oxide, which are produced respectively on the scales of approximately 15 and 3 million tonnes/year.[2]
Aside from ethylene oxide, most epoxides are generated whenperoxidized reagents donate a single oxygen atom to analkene. Safety considerations weigh on these reactions because organic peroxides are prone to spontaneous decomposition or even combustion.
Botht-butyl hydroperoxide andethylbenzene hydroperoxide can be used as oxygen sources during propylene oxidation (although a catalyst is required as well, and most industrial producers use dehydrochlorination instead).[3]
Theethylene oxide industry generates its product from reaction ofethylene andoxygen. Modifiedheterogeneoussilver catalysts are typically employed.[4] According to a reaction mechanism suggested in 1974[5] at least one ethylene molecule is totally oxidized for every six that are converted to ethylene oxide:
Only ethylene produces an epoxide duringincomplete combustion. Other alkenes fail to react usefully, evenpropylene, though TS-1 supportedAu catalysts can selectively epoxidize propylene.[6]
Metal complexes are useful catalysts for epoxidations involvinghydrogen peroxide and alkyl hydroperoxides. Metal-catalyzed epoxidations were first explored usingtert-butyl hydroperoxide (TBHP).[7] Association of TBHP with the metal (M) generates the active metal peroxy complex containing the MOOR group, which then transfers an O center to the alkene.[8]
Vanadium(II) oxide catalyzes the epoxidation at specifically less-substituted alkenes.[9]
Electron-deficient olefins, such asenones andacryl derivatives can be epoxidized using nucleophilic oxygen compounds such as peroxides. The reaction is a two-step mechanism. First the oxygen performs anucleophilic conjugate addition to give a stabilized carbanion. This carbanion then attacks the same oxygen atom, displacing a leaving group from it, to close the epoxide ring.
Peroxycarboxylic acids, which are more electrophilic than other peroxides, convert alkenes to epoxides without the intervention of metal catalysts. In specialized applications,dioxirane reagents (e.g.dimethyldioxirane)perform similarly, but are more explosive.
Typical laboratory operations employ thePrilezhaev reaction.[10][11] This approach involves the oxidation of the alkene with aperoxyacid such asmCPBA. Illustrative is the epoxidation ofstyrene withperbenzoic acid tostyrene oxide:[12]
The stereochemistry of the reaction is quite sensitive. Depending on the mechanism of the reaction and the geometry of the alkene starting material,cis and/ortrans epoxidediastereomers may be formed. In addition, if there are other stereocenters present in the starting material, they can influence the stereochemistry of the epoxidation.
The reaction proceeds via what is commonly known as the "Butterfly Mechanism".[13] The peroxide is viewed as anelectrophile, and the alkene anucleophile. The reaction is considered to be concerted. The butterfly mechanism allows ideal positioning of theO−Osigma star orbital forC−C π electrons to attack.[14] Because two bonds are broken and formed to the epoxide oxygen, this is formally an example of acoarctate transition state.
Chiral epoxides are produced by epoxidation of prochiral alkenes. When the catalyst is chiral or the alkene is chiral, thenasymmetric epoxidation becomes possible. Prominent methodologies are theSharpless epoxidation, theJacobsen epoxidation, and theShi epoxidation.[15]
Halohydrins react with base to give epoxides.[17] The reaction is spontaneous because the energetic cost of introducing the ring strain (13 kcal/mol) is offset by the larger bond enthalpy of the newly introduced C-O bond (when compared to that of the cleaved C-halogen bond).
Formation of epoxides from secondary halohydrins is predicted to occur faster than from primary halohydrins due to increased entropic effects in the secondary halohydrin, and tertiary halohydrins react (if at all) extremely slowly due to steric crowding.[18]
Starting withpropylene chlorohydrin, most of the world's supply ofpropylene oxide arises via this route.[3]
An intramolecular epoxide formation reaction is one of the key steps in theDarzens reaction.
In theJohnson–Corey–Chaykovsky reaction epoxides are generated fromcarbonyl groups andsulfonium ylides. In this reaction, a sulfonium is the leaving group instead of chloride.
Epoxides are uncommon in nature. They arise usually via oxygenation of alkenes by the action ofcytochrome P450.[19] (but see also the short-livedepoxyeicosatrienoic acids which act as signalling molecules.[20] and similarepoxydocosapentaenoic acids, andepoxyeicosatetraenoic acids.)
Arene oxides are intermediates in the oxidation of arenes bycytochrome P450. For prochiral arenes (naphthalene,toluene,benzoates,benzopyrene), the epoxides are often obtained in high enantioselectivity.
Ring-opening reactions dominate the reactivity of epoxides.
Epoxides react with a broad range of nucleophiles, for example, alcohols, water, amines, thiols, and even halides. With two often-nearly-equivalent sites of attack, epoxides exemplify "ambident substrates".[21] Ring-openingregioselectivity in asymmetric epoxides generally follows the SN2 pattern of attack at the least-substituted carbon,[22] but can be affected by carbocation stability under acidic conditions.[23] This class of reactions is the basis ofepoxy glues and the production of glycols.[16]
Lithium aluminium hydride oraluminium hydride bothreduce epoxides through a simple nucleophilic addition of hydride (H−); they produce the correspondingalcohol.[24]
Polymerization of epoxides givespolyethers. For exampleethylene oxide polymerizes to givepolyethylene glycol, also known as polyethylene oxide. The reaction of an alcohol or a phenol with ethylene oxide,ethoxylation, is widely used to produce surfactants:[25]
With anhydrides, epoxides give polyesters.[26]
Lithiation cleaves the ring to β-lithioalkoxides.[27]
Epoxides can be deoxygenated usingoxophilic reagents, with loss or retention of configuration.[28] The combination oftungsten hexachloride andn-butyllithium gives thealkene.[29][30]
When treated withthiourea, epoxides convert to theepisulfide (thiiranes).
Ethylene oxide is widely used to generate detergents and surfactants byethoxylation. Its hydrolysis affordsethylene glycol. It is also used forsterilisation of medical instruments and materials.
The reaction of epoxides with amines is the basis for the formation ofepoxy glues and structural materials. A typical amine-hardener istriethylenetetramine (TETA).
Epoxides arealkylating agents, making many of them highly toxic.[32]
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