Core Concepts
In this organic chemistry tutorial, we learn about the epoxide functional group, including its definition, basic properties, naming conventions, and important reactions.
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What is an Epoxide?
An epoxide is a unique functional group found in many organic compounds. The group involves two carbons and oxygen forming a three-membered ring structure.
Due to the nature of three-membered ring structures, epoxides tend to have little stability. However, they have an important use as a reaction intermediate in many important organic synthesis pathways.
Quick Facts on Epoxides
- Structure: a three-membered ring structure of two carbons and one oxygen
- General formula: COC
- Acidity: Very acidic (pKa = -3) when in protonated form
- Solubility: Moderately soluble in water and other polar solvents
- C-O bond length: 1.47Å
- C-O bond enthalpy: 350 kJ mol-1
- IR Spectroscopy: Epoxides tend to show two strong peaks at 750-880 and 810-950cm-1
Epoxide Naming Conventions
As we see in the next section, most epoxides form as derivatives of alkenes or some other species with a pi bond. Thus, most epoxide naming conventions include reference to the parent alkene.
For instance, the IUPAC conventions involve citing the numbers of the two involved carbons with the suffix “epoxy-“. Thus, the simplest epoxide, C2H4O, has the IUPAC name of 1,2-epoxyethane. Other examples include 3,4-epoxyheptane, with an epoxy group between carbons 4 and 5, and 1,2-epoxy-3,5-cyclohexadiene.
Another common epoxide naming convention involves simply naming the parent alkene followed by the term oxide. Thus, 1,2-epoxyethane becomes ethene oxide, 3,4-epoxyheptane becomes 3-heptene oxide, and 1,2-epoxy-3,5-cyclohexadiene becomes benzene oxide.
Epoxide Synthesis Reactions
As we explore in the next section, many synthetically important reactions can happen with epoxides. As a result, organic chemists have developed many techniques for forming epoxides from other, less synthetically useful groups.
Oxidation of Ethene
To produce ethene oxide, one simply must react ethene with diatomic oxygen. Importantly, chemists use a silver–aluminum catalyst to stimulate the reaction. Stoichiometrically, two ethenes react per one oxygen gas molecule. However, for every six molecules of ethene oxide, this reaction fully oxidizes one ethene completely to carbon dioxide.
Unfortunately, larger alkenes cannot oxidize to epoxides through this mechanism, as such alkenes have far less reactivity.
Reaction with Peroxides
Another, more general way of synthesizing epoxides involves reacting alkenes with a peroxycarboxylic acid, also known as a peroxyacid. Chemists often call this reaction “olefin peroxidation”, with “olefin” serving as an alternate term for an alkene. These peroxide groups involve an electrophilic oxygen species that can be attacked by the pi electrons of the alkene. Further, peroxycarboxylic acids also have a carbonyl group which can act as a proton acceptor.
This reaction proceeds through the “Butterfly Mechanism,” which involves the concerted movement of eight electrons, resulting in an epoxide.
Alkenes can also react with generic peroxides (R-OOH) to yield epoxides. However, this reaction requires a metal catalyst, similar to the oxidation of ethene. Chemists often refer to this reaction with peroxides as “olefin oxidation”.
Intramolecular SN2
Additionally, epoxides can form as the result of an oxide group serving as a nucleophile in an intramolecular SN2 reaction. Specifically, this involves forming a halohydrin group, which contains an alcohol adjacent to a halide group.
Next, if the halohydrin exists in basic conditions, the alcohol becomes deprotonated, forming an oxide. Finally, the oxide attacks the carbon with the halide, which becomes a leaving group as the epoxide forms. This reaction can be thought of as an intramolecular Williamson Ether Synthesis.
Reactions with Epoxides
Nucleophilic Addition
Due to the ring strain of epoxide’s three-membered structure, chemists often make easy use of nucleophilic reagents to open the ring. Specifically, the nucleophile attacks one of the carbons which breaks that carbon’s bond with the oxygen serving as a “leaving group” in a quasi-nucleophilic substitution. The end product involves an alcohol or oxide group attached to the non-attacked carbon.
Interestingly, if the epoxide’s carbons have different substituents, this reaction can be regioselective depending on the pH of the reaction mixture. If nucleophilic addition takes place under basic conditions, the nucleophile attacks the less-substituted carbon, due to steric hindrance.
However, if the reaction occurs under acidic conditions, the more-substituted carbon becomes more likely to react with the nucleophile. This is because the epoxide becomes protonated before the reaction. As a result, this places a partial positive charge on the epoxide’s carbons. As we know from studying SN1 reactions, more-substituted carbons form more stable cations. This weakens the bond between the oxygen and more-substituted carbon, making it easier to break.
Polymerization
Once the epoxide ring opens, the resulting alcohol can serve as an effective nucleophile in a subsequent epoxide ring opening. As a result, epoxides easily polymerize into polyether structures.
Often, to catalyze the polymerization, an alcohol compound participates in the reaction. The result is that an alcohol and the compound’s R group cap the polyether chain.
Additionally, when anhydride species exist in the reaction mixture, this polymerization reaction makes polyester chains.
Reduction to Alkene
Aside from ring-opening reactions, chemists can also reduce epoxides back to alkenes. This often involves a heavy metal catalyst, such as tungsten or rhenium, as well as a reductant, such as hydrogen gas (H2), triphenyl phosphate (P(Ph)3), or sodium sulfite (Na2SO3).
