The dominant epoxides industrially are
ethylene oxide and
propylene oxide, which are produced respectively on the scales of approximately 15 million and 3 million tonnes/year. 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).
Ethylene oxidation The
ethylene oxide industry generates its product from reaction of
ethylene and
oxygen. Modified
heterogeneous silver catalysts are typically employed. According to a reaction mechanism suggested in 1974 at least one ethylene molecule is totally oxidized for every six that are converted to ethylene oxide: 7 H2C=CH2 + 6 O2 -> 6 C2H4O + 2 CO2 + 2 H2O 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.
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). 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. :
Vanadium(II) oxide catalyzes the epoxidation at specifically less-substituted alkenes.
Nucleophilic epoxidation 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. Typical laboratory operations employ the
Prilezhaev reaction. This approach involves the oxidation of the alkene with a
peroxyacid such as
mCPBA. Illustrative is the epoxidation of
styrene with
perbenzoic acid to
styrene oxide: : 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. The reaction proceeds via what is commonly known as the "Butterfly Mechanism". The peroxide is viewed as an
electrophile, and the alkene a
nucleophile. The reaction is considered to be concerted. The butterfly mechanism allows ideal positioning of the
sigma star orbital for π electrons to attack. Because two bonds are broken and formed to the epoxide oxygen, this is formally an example of a
coarctate transition state. :
Asymmetric epoxidations Chiral epoxides are produced by epoxidation of prochiral alkenes. When the catalyst is chiral or the alkene is chiral, then
asymmetric epoxidation becomes possible. Prominent methodologies are the
Sharpless epoxidation, the
Jacobsen epoxidation, and the
Shi epoxidation.
Dehydrohalogenation and other γ eliminations , is prepared by the chlorohydrin method. It is a precursor in the production of
epoxy resins.
Halohydrins react with base to give epoxides. 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. Starting with
propylene chlorohydrin, most of the world's supply of
propylene oxide arises via this route. (but see also the short-lived
epoxyeicosatrienoic acids which act as signalling molecules. and similar
epoxydocosapentaenoic acids, and
epoxyeicosatetraenoic acids.)
Arene oxides are intermediates in the oxidation of arenes by
cytochrome P450. For prochiral arenes (
naphthalene,
toluene,
benzoates,
benzopyrene), the epoxides are often obtained in high enantioselectivity. == Reactions == Ring-opening reactions dominate the reactivity of epoxides.
Hydrolysis and addition of nucleophiles : 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". Ring-opening
regioselectivity in asymmetric epoxides generally follows the SN2 pattern of attack at the least-substituted carbon, but can be affected by carbocation stability under acidic conditions. This class of reactions is the basis of
epoxy glues and the production of glycols.
Polymerization and oligomerization Polymerization of epoxides gives
polyethers. For example
ethylene oxide polymerizes to give
polyethylene glycol, also known as polyethylene oxide. The reaction of an alcohol or a phenol with ethylene oxide,
ethoxylation, is widely used to produce surfactants: :ROH + n C2H4O → R(OC2H4)nOH With anhydrides, epoxides give polyesters.
Metallation and deoxygenation Lithiation cleaves the ring to β-lithioalkoxides. Epoxides can be deoxygenated using
oxophilic reagents, with loss or retention of configuration. The combination of
tungsten hexachloride and
n-butyllithium gives the
alkene. When treated with
thiourea or
triphenylphosphine sulfide, epoxides convert to the
episulfide (thiiranes). But
triphenylphosphine selenide or a selenathiocarbamate in strong acid deoxygenates to the olefin instead.
Other reactions •
Titanocene monochloride opens epoxides to α-radical alkoxides, which can then be trapped by e.g. a hydrogen donor. • Epoxides undergo ring expansion reactions, illustrated by the insertion of carbon dioxide to give
cyclic carbonates. • An epoxide adjacent to an alcohol can undergo the
Payne rearrangement in base. ==Uses==