In organic chemistry, an epoxide is a cyclic ether (R1R2C−O−R3R4) with a three-atom ring, containing two atoms of carbon and one atom of oxygen, where R1, R2, R3 and R4 stand for any organyl group. This ring approximates an equilateral triangle, which makes it strained, and hence 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.[1]
A generic epoxide
NomenclatureEdit
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, but such materials do not contain epoxide groups (or contain only a few residual epoxy groups that remain unreacted in the formation of the resin).
SynthesisEdit
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.[2]
Heterogeneously catalyzed oxidation of alkenesEdit
The epoxidation of ethylene involves its reaction with oxygen. According to a reaction mechanism suggested in 1974[3] at least one ethylene molecule is totally oxidized for every six that are converted to ethylene oxide:
The direct reaction of oxygen with alkenes is useful only for this epoxide. Modified heterogeneoussilver catalysts are typically employed.[4] Other alkenes fail to react usefully, even propylene, though TS-1 supported Au catalysts can perform propylene epoxidation selectively.[5]
Olefin (alkene) oxidation using organic peroxides and metal catalystsEdit
Aside from ethylene oxide, most epoxides are generated by treating alkenes with peroxide-containing reagents, which donate a single oxygen atom. Safety considerations weigh on these reactions because organic peroxides are prone to spontaneous decomposition or even combustion.
Metal complexes are useful catalysts for epoxidations involving hydrogen peroxide and alkyl hydroperoxides. Peroxycarboxylic acids, which are more electrophilic, convert alkenes to epoxides without the intervention of metal catalysts. In specialized applications, other peroxide-containing reagents are employed, such as dimethyldioxirane. 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. Metal-catalyzed epoxidations were first explored using tert-butyl hydroperoxide (TBHP).[6] 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.[7]
Simplified mechanism for metal-catalyzed epoxidation of alkenes with peroxide (ROOH) reagents
Organic peroxides are used for the production of propylene oxide from propylene. Catalysts are required as well. Both t-butyl hydroperoxide and ethylbenzene hydroperoxide can be used as oxygen sources.[8]
Olefin peroxidation using peroxycarboxylic acidsEdit
The reaction proceeds via what is commonly known as the "Butterfly Mechanism".[12] The peroxide is viewed as an electrophile, and the alkene a nucleophile. The reaction is considered to be concerted (the numbers in the mechanism below are for simplification). The butterfly mechanism allows ideal positioning of the O−Osigma star orbital for C−C π electrons to attack.[13] Because two bonds are broken and formed to the epoxide oxygen, this is formally an example of a coarctate transition state.
Chiral epoxides can often be derived enantioselectively from prochiral alkenes. Many metal complexes give active catalysts, but the most important involve titanium, vanadium, and molybdenum.[14][15]
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.
Ring-opening reactions dominate the reactivity of epoxides.
Hydrolysis and addition of nucleophilesEdit
Two pathways for the hydrolysis of an epoxide
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 are examples "ambident substrates."[22] The regioselectivity of ring-opening reactions of non-symmetrical epoxides is sensitive to conditions, This class of reactions is the basis of epoxy glues and the production of glycols.[18]
Polymerization and oligomerizationEdit
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:[23]
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).
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^Sajkowski, D. J.; Boudart, M. (1987). "Structure Sensitivity of the Catalytic Oxidation of Ethene by Silver". Catalysis Reviews. 29 (4): 325–360. doi:10.1080/01614948708078611.
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^Indictor N., Brill W. F. (1965). "Metal Acetylacetonate Catalyzed Epoxidation of Olefins with t-Butyl Hydroperoxide". J. Org. Chem. 30 (6): 2074. doi:10.1021/jo01017a520.
^Thiel W. R. (1997). "Metal catalyzed oxidations. Part 5. Catalytic olefin epoxidation with seven-coordinate oxobisperoxo molybdenum complexes: a mechanistic study". Journal of Molecular Catalysis A: Chemical. 117: 449–454. doi:10.1016/S1381-1169(96)00291-9.
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^Nikolaus Prileschajew (1909). "Oxydation ungesättigter Verbindungen mittels organischer Superoxyde" [Oxidation of unsaturated compounds by means of organic peroxides]. Berichte der Deutschen Chemischen Gesellschaft (in German). 42 (4): 4811–4815. doi:10.1002/cber.190904204100.
^Harold Hibbert and Pauline Burt (1941). "Styrene Oxide". Organic Syntheses.; Collective Volume, vol. 1, p. 494
^Paul D. Bartlett (1950). "Recent work on the mechanisms of peroxide reactions". Record of Chemical Progress. 11: 47–51.
^John O. Edwards (1962). Peroxide Reaction Mechanisms. Interscience, New York. pp. 67–106.
^Berrisford D. J., Bolm C., Sharpless K. B. (2003). "Ligand-Accelerated Catalysis". Angew. Chem. Int. Ed. Engl. 95 (10): 1059–1070. doi:10.1002/anie.199510591.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Sheldon R. A. (1980). "Synthetic and mechanistic aspects of metal-catalysed epoxidations with hydroperoxides". Journal of Molecular Catalysis. 1: 107–206. doi:10.1016/0304-5102(80)85010-3.
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^Koppenhoefer, B.; Schurig, V. (1993). "(R)-Alkyloxiranes of High Enantiomeric Purity from (S)-2-Chloroalkanoic Acids via (S)-2-Chloro-1-Alkanols: (R)-Methyloxirane". Organic Syntheses.; Collective Volume, vol. 8, p. 434
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^Takuya Nakagiri, Masahito Murai, and Kazuhiko Takai (2015). "Stereospecific Deoxygenation of Aliphatic Epoxides to Alkenes under Rhenium Catalysis". Org. Lett. 17 (13): 3346–9. doi:10.1021/acs.orglett.5b01583. PMID26065934.{{cite journal}}: CS1 maint: uses authors parameter (link)
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