11.8
Structuur en nomenclatuur van epoxiden
Cyclische ethers zijn heterocyclische verbindingen met een zuurstofatoom in de ring samen met koolstofatomen. De benoeming hangt af van het aantal koo…
Cyclic analogs of ethers with an oxygen atom incorporated in the ring are called cyclic ethers. The general formula for cyclic ethers is (CH2)nO, where n represents the number of carbon atoms in the ring. Accordingly, cyclic ethers can be three-membered, four-membered, five-membered, or six-membered ring systems.
The three-membered cyclic ethers are called epoxides. Their basic structure consists of one oxygen atom and two carbon atoms.
The three-membered ring system approximates an equilateral triangle and is highly strained.
The strain arises due to the deviation of the interior bond angles from the ideal tetrahedral angle, that is, 109.5°, leading to the compression of bond angles. This ring strain makes epoxides highly reactive as compared to other ethers.
Epoxides derive their common names from the name of the parent alkene from which they are formed, followed by the word “oxide”. For example, ethylene oxide, which is the simplest epoxide, derives its name from its parent alkene — ethylene.
Other examples include cis-2-butene oxide, and cyclopentene oxide, formally derived from cis-2-butene and cyclopentene, respectively.
In the IUPAC nomenclature, the simplest epoxide, with no substitutions on the carbon atoms, is named oxirane. Substituted epoxides can have up to four R groups and are named as derivatives of oxiranes.
For example, (2R,3S)-2,3-dimethyloxirane and (2R,3S)-2,3-diethyl-2,3-dimethyloxirane have two and four substituents, respectively. The numbering of the oxirane starts from the oxygen atom, and the substituents are listed alphabetically, followed by the word oxirane.
In cases where the epoxide is present as a part of another hydrocarbon system, the prefix “epoxy” is added before the parent’s name.
For example, 1,2-epoxycyclopentane has the epoxide ring as part of another ring system. Similarly, 2,3-dimethyl-2,3-epoxy pentane has the epoxide as part of a straight hydrocarbon chain.
The parent chain is numbered in such a way that epoxy carbon atoms receive the lowest possible numbers.
View the full transcript and gain access to JoVE Core videos
Q1: What is the basic structure of an epoxide?
An epoxide is a three-membered cyclic ether containing one oxygen atom and two carbon atoms. This compact ring structure approximates an equilateral triangle and is highly strained because the interior bond angles deviate significantly from the ideal tetrahedral angle of 109.5 degrees, compressing the bonds and making epoxides highly reactive compared to other ethers.
Q2: Why are epoxides more reactive than other ethers?
Epoxides are more reactive due to ring strain in their three-membered ring system. The compressed bond angles deviate substantially from the ideal tetrahedral angle of 109.5 degrees, creating significant angular strain. This strain makes the ring unstable and prone to ring-opening reactions, which is why epoxides undergo acid-catalyzed ring opening of epoxides and base-catalyzed ring opening of epoxides more readily than larger cyclic ethers.
Q3: How are epoxides named using common nomenclature?
Common names for epoxides are derived from their parent alkene by adding the suffix 'oxide.' For example, ethylene oxide comes from ethylene, cis-2-butene oxide from cis-2-butene, and cyclopentene oxide from cyclopentene. This naming convention directly reflects the alkene precursor from which each epoxide is formed.
Q4: What is the IUPAC name for the simplest epoxide?
The simplest epoxide with no substituents is named oxirane in IUPAC nomenclature. Substituted epoxides are named as derivatives of oxirane, with numbering starting from the oxygen atom and substituents listed alphabetically. For example, (2R,3S)-2,3-dimethyloxirane has two methyl substituents on the oxirane ring.
Q5: How is the epoxy prefix used in IUPAC nomenclature?
When an epoxide is part of a larger hydrocarbon system rather than the main functional group, the prefix 'epoxy' is added before the parent structure name. For example, 1,2-epoxycyclopentane has the epoxide ring incorporated into a cyclopentane system, and 2,3-dimethyl-2,3-epoxypentane has the epoxide within a pentane chain. Parent chain numbering ensures epoxy carbons receive the lowest possible numbers.
Q6: What are the different types of cyclic ethers based on ring size?
Cyclic ethers are classified by the number of carbons in their ring system. Three-membered rings are called oxiranes or epoxides, four-membered rings are oxetanes, five-membered rings are oxolanes, and six-membered rings are oxanes. Ring strain increases as ring size decreases, making three-membered epoxides significantly more strained and reactive than larger cyclic ethers.
Q7: How does ring size affect strain in cyclic ethers?
Ring strain is inversely related to ring size in cyclic ethers. Three-membered rings experience the greatest strain because their bond angles deviate most from the ideal tetrahedral angle of 109.5 degrees. As ring size increases to four, five, or six-membered systems, the strain decreases because the rings can accommodate more normal bond angles, making larger cyclic ethers significantly less reactive than epoxides.