Overall ring strain seems to be a big issue when it comes to organic chemistry. That is why cyclopentane may be in an "envelope form" or why cyclobutane may be in a kinked, kite form. Both of these example molecules are not planar. So, why is epoxide (oxacyclopropane) a stable molecule, if at all?
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Well, you have to ask: how stable? and compared to what? Epoxides, including ethylene oxide, are generally considered relatively unstable molecules, with a high chemical activity and involvement in numerous reactions, including polymerization and thermal decomposition. However, ethylene oxide does exist as a molecule, as does cyclopropane, which also contains a three-membered ring. So, let's compare for example the ring strain in ethylene oxide and cyclopropane. I found some computation data on their relative stability from these lecture notes, which indicate that “ethylene oxide has less ring strain than cyclopropane”. I would attribute this to the fact that the C–O–C angle has less strain because its “relaxed” value would be 104.5°, compared to the “relaxed” C–C–C angle of 109.4°. |
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$\newcommand{c}[1]{\mathsf{\small #1}}$ You can't quite compare rings consisting only of carbon atoms with rings with oxygens in them. In the latter, the $\c{C}-\c{O}$ bonds are polar, the electrons are closer to the oxygen atom which means the acuteness at the $\c{C}$ atoms is not as much of a problem as it would be in a hydrocarbon ring. |
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