In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ethers. They are produced on a large scale for many applications. In general, low molecular weight epoxides are colourless and nonpolar, and often volatile.<ref name=UllmannEpoxides>Template:Ullmann</ref>
A compound containing the epoxide functional group can be called an epoxy, epoxide, oxirane, and ethoxyline. Simple epoxides are often referred to as oxides. Thus, the epoxide of ethylene (C2H4) is ethylene oxide (C2H4O). Many compounds have trivial names; for instance, ethylene oxide is called "oxirane". Some names emphasize the presence of the epoxide functional group, as in the compound 1,2-epoxyheptane, which can also be called 1,2-heptene oxide.
A polymer formed from epoxide precursors is called an epoxy. However, few if any of the epoxy groups in the resin survive the curing process.
Synthesis
The dominant epoxides industrially are ethylene oxide and propylene oxide, which are produced respectively on the scales of approximately 15 and 3 million tonnes/year.<ref>Template:Ullmann</ref>
Aside from ethylene oxide, most epoxides are generated when peroxidized reagents donate a single oxygen atom to an alkene. Safety considerations weigh on these reactions because organic peroxides are prone to spontaneous decomposition or even combustion.
Both t-butyl hydroperoxide and ethylbenzene hydroperoxide can be used as oxygen sources during propylene oxidation (although a catalyst is required as well, and most industrial producers use dehydrochlorination instead).<ref name=PO>Template:Cite book</ref>
Ethylene oxidation
The ethylene oxide industry generates its product from reaction of ethylene and oxygen. Modified heterogeneoussilver catalysts are typically employed.<ref>Template:Cite journal</ref> According to a reaction mechanism suggested in 1974<ref name="kilty">Template:Cite journal</ref> at least one ethylene molecule is totally oxidized for every six that are converted to ethylene oxide: <chem display=block>7 H2C=CH2 + 6 O2 -> 6 C2H4O + 2 CO2 + 2 H2O</chem>
Only ethylene produces an epoxide during incomplete combustion. Other alkenes fail to react usefully, even propylene, though TS-1 supported Au catalysts can selectively epoxidize propylene.<ref>Template:Cite journal</ref>
Organic peroxides and metal catalysts
Metal complexes are useful catalysts for epoxidations involving hydrogen peroxide and alkyl hydroperoxides. Metal-catalyzed epoxidations were first explored using tert-butyl hydroperoxide (TBHP).<ref name=name2>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Simplified mechanism for metal-catalyzed epoxidation of alkenes with peroxide (ROOH) reagents
Electron-deficient olefins, such as enones and acryl derivatives can be epoxidized using nucleophilic oxygen compounds such as peroxides. The reaction is a two-step mechanism. First the oxygen performs a nucleophilic 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.
Transfer from peroxycarboxylic acids
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.
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/or trans epoxide diastereomers may be formed. In addition, if there are other stereocenters present in the starting material, they can influence the stereochemistry of the epoxidation.
Halohydrins react with base to give epoxides.<ref name="Koppenhoefer1993">Template:OrgSynth</ref> 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.
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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".<ref>Template:March6th</ref> Ring-opening regioselectivity in asymmetric epoxides generally follows the SN2 pattern of attack at the least-substituted carbon,<ref>Template:Cite book</ref> but can be affected by carbocation stability under acidic conditions.<ref>Template:Cite web</ref> This class of reactions is the basis of epoxy glues and the production of glycols.<ref name="UllmannEpxyRes"/>
The reaction of epoxides with amines is the basis for the formation of epoxy glues and structural materials. A typical amine-hardener is triethylenetetramine (TETA).
