11.8
环氧化物的结构和命名
环醚是环中含有氧原子和碳原子的杂环化合物。 它们根据其环系统中存在的碳原子数来命名。 三元环系的环醚称为"环氧乙烷"，四元环系的环醚称为"氧杂环丁烷"，五元环系的环醚称为"四氢呋喃"，六元环系的环醚称为"氧杂环己烷"。 这些环的环状结构会产生角应变，并且这种应变在碳原子数较少的环中更大。 例如，三元…
环状醚是指环中含有氧原子的醚类环状类似物。环状醚的通式为 (CH2)nO，其中 n 代表环中碳原子的数目。因此，环状醚可以是三元环、四元环、五元环或六元环体系。
三元环醚被称为环氧乙烷。其基本结构由一个氧原子和两个碳原子组成。
三元环结构近似于等边三角形，具有很高的环张力。
由于环内键角偏离理想的四面体角（即109.5°），导致键角受到压缩，从而产生张力。这种环张力使得环氧乙烷类化合物比其他醚类具有更高的反应活性。
环氧化物的普通名称由其母体烯烃的名称后加“氧化物”构成。例如，最简单的环氧化物——环氧乙烷，其名称即来源于它的母体烯烃——乙烯。
其他例子包括 cis-2-丁烯氧化物和环戊烯氧化物，它们分别由 cis-2-丁烯和环戊烯形式上衍生而来。
根据IUPAC命名法，最简单的环氧化物在碳原子上无取代基时称为环氧乙烷（oxirane）。带有取代基的环氧化物最多可含有四个R基团，并以环氧乙烷衍生物的方式命名。
例如，(2R,3S)-2,3-二甲基环氧乙烷和(2R,3S)-2,3-二乙基-2,3-二甲基环氧乙烷分别具有两个和四个取代基。环氧乙烷的编号从氧原子开始，取代基按字母顺序列出，后接“环氧乙烷”一词。
当环氧基作为其他烃类体系的一部分存在时，需在母体化合物名称前加上“epoxy”作为前缀。
例如，1,2-环氧环戊烷的环氧环是另一个环系的一部分。同样，2,3-二甲基-2,3-环氧戊烷中的环氧结构则是直链烃链的一部分。
主链的编号方式应使环氧碳原子获得尽可能小的编号。
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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.