Organic compounds with a carbon-carbon-oxygen ring
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.[1]
Nomenclature
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).
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.[2]
Heterogeneously catalyzed oxidation of alkenes
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 catalysts
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]
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]
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 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]
Halohydrins react with base to give epoxides.[19] 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.
[20]
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 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 are examples "ambident substrates."[23] The
regioselectivity of ring-opening in asymmetric epoxides generally follows the SN2 pattern of attack at the least-substituted carbon,[24] but can be affected by carbocation stability under acidic conditions.[25] This class of reactions is the basis of
epoxy glues and the production of glycols.[18]
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:[26]
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).
^Kilty P. A.; Sachtler W. M. H. (1974). "The mechanism of the selective oxidation of ethylene to ethylene oxide". Catalysis Reviews: Science and Engineering. 10: 1–16.
doi:
10.1080/01614947408079624.
^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.
^Nijhuis, T. Alexander; Makkee, Michiel; Moulijn, Jacob A.; Weckhuysen, Bert M. (1 May 2006). "The Production of Propene Oxide: Catalytic Processes and Recent Developments". Industrial & Engineering Chemistry Research. 45 (10): 3447–3459.
doi:
10.1021/ie0513090.
hdl:1874/20149.
S2CID94240406.
^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.
^
abDietmar Kahlich, Uwe Wiechern, Jörg Lindner “Propylene Oxide” in Ullmann's Encyclopedia of Industrial Chemistry, 2002 by Wiley-VCH, Weinheim.
doi:
10.1002/14356007.a22_239
^March, Jerry. 1985. Advanced Organic Chemistry, Reactions, Mechanisms and Structure. 3rd ed. John Wiley & Sons.
ISBN0-471-85472-7.
^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.
^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.
^Kosswig, Kurt (2002). "Surfactants". In Elvers, Barbara; et al. (eds.). Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, GER: Wiley-VCH.
doi:
10.1002/14356007.a25_747.
ISBN978-3527306732.
^Julie M. Longo; Maria J. Sanford;
Geoffrey W. Coates (2016). "Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure–Property Relationships". Chem. Rev. 116 (24): 15167–15197.
doi:
10.1021/acs.chemrev.6b00553.
PMID27936619.
^Takuya Nakagiri; Masahito Murai; 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.
^K. Barry Sharpless; Martha A. Umbreit; Marjorie T. Nieh; Thomas C. Flood (1972). "Lower valent tungsten halides. New class of reagents for deoxygenation of organic molecules". J. Am. Chem. Soc.94 (18): 6538–6540.
doi:
10.1021/ja00773a045.
^Bruce Rickborn and Wallace E. Lamke (1967). "Reduction of epoxides. II. The lithium aluminum hydride and mixed hydride reduction of 3-methylcyclohexene oxide". J. Org. Chem.32 (3): 537–539.
doi:
10.1021/jo01278a005.
^B. Mudryk; T. Cohen (1995). "1,3-Diols From Lithium Β-lithioalkoxides Generated By The Reductive Lithiation Of Epoxides: 2,5-dimethyl-2,4-hexanediol". Org. Synth. 72: 173.
doi:
10.15227/orgsyn.072.0173.
^Sasaki, Hiroshi (February 2007). "Curing properties of cycloaliphatic epoxy derivatives". Progress in Organic Coatings. 58 (2–3): 227–230.
doi:
10.1016/j.porgcoat.2006.09.030.
^Niederer, Christian; Behra, Renata; Harder, Angela; Schwarzenbach, René P.; Escher, Beate I. (2004). "Mechanistic approaches for evaluating the toxicity of reactive organochlorines and epoxides in green algae". Environmental Toxicology and Chemistry. 23 (3): 697–704.
doi:
10.1897/03-83.
PMID15285364.
S2CID847639.
Organic compounds with a carbon-carbon-oxygen ring
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.[1]
Nomenclature
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).
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.[2]
Heterogeneously catalyzed oxidation of alkenes
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 catalysts
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]
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]
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 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]
Halohydrins react with base to give epoxides.[19] 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.
[20]
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 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 are examples "ambident substrates."[23] The
regioselectivity of ring-opening in asymmetric epoxides generally follows the SN2 pattern of attack at the least-substituted carbon,[24] but can be affected by carbocation stability under acidic conditions.[25] This class of reactions is the basis of
epoxy glues and the production of glycols.[18]
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:[26]
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).
^Kilty P. A.; Sachtler W. M. H. (1974). "The mechanism of the selective oxidation of ethylene to ethylene oxide". Catalysis Reviews: Science and Engineering. 10: 1–16.
doi:
10.1080/01614947408079624.
^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.
^Nijhuis, T. Alexander; Makkee, Michiel; Moulijn, Jacob A.; Weckhuysen, Bert M. (1 May 2006). "The Production of Propene Oxide: Catalytic Processes and Recent Developments". Industrial & Engineering Chemistry Research. 45 (10): 3447–3459.
doi:
10.1021/ie0513090.
hdl:1874/20149.
S2CID94240406.
^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.
^
abDietmar Kahlich, Uwe Wiechern, Jörg Lindner “Propylene Oxide” in Ullmann's Encyclopedia of Industrial Chemistry, 2002 by Wiley-VCH, Weinheim.
doi:
10.1002/14356007.a22_239
^March, Jerry. 1985. Advanced Organic Chemistry, Reactions, Mechanisms and Structure. 3rd ed. John Wiley & Sons.
ISBN0-471-85472-7.
^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.
^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.
^Kosswig, Kurt (2002). "Surfactants". In Elvers, Barbara; et al. (eds.). Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, GER: Wiley-VCH.
doi:
10.1002/14356007.a25_747.
ISBN978-3527306732.
^Julie M. Longo; Maria J. Sanford;
Geoffrey W. Coates (2016). "Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure–Property Relationships". Chem. Rev. 116 (24): 15167–15197.
doi:
10.1021/acs.chemrev.6b00553.
PMID27936619.
^Takuya Nakagiri; Masahito Murai; 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.
^K. Barry Sharpless; Martha A. Umbreit; Marjorie T. Nieh; Thomas C. Flood (1972). "Lower valent tungsten halides. New class of reagents for deoxygenation of organic molecules". J. Am. Chem. Soc.94 (18): 6538–6540.
doi:
10.1021/ja00773a045.
^Bruce Rickborn and Wallace E. Lamke (1967). "Reduction of epoxides. II. The lithium aluminum hydride and mixed hydride reduction of 3-methylcyclohexene oxide". J. Org. Chem.32 (3): 537–539.
doi:
10.1021/jo01278a005.
^B. Mudryk; T. Cohen (1995). "1,3-Diols From Lithium Β-lithioalkoxides Generated By The Reductive Lithiation Of Epoxides: 2,5-dimethyl-2,4-hexanediol". Org. Synth. 72: 173.
doi:
10.15227/orgsyn.072.0173.
^Sasaki, Hiroshi (February 2007). "Curing properties of cycloaliphatic epoxy derivatives". Progress in Organic Coatings. 58 (2–3): 227–230.
doi:
10.1016/j.porgcoat.2006.09.030.
^Niederer, Christian; Behra, Renata; Harder, Angela; Schwarzenbach, René P.; Escher, Beate I. (2004). "Mechanistic approaches for evaluating the toxicity of reactive organochlorines and epoxides in green algae". Environmental Toxicology and Chemistry. 23 (3): 697–704.
doi:
10.1897/03-83.
PMID15285364.
S2CID847639.