What makes an epoxide stable?

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?

• Chemists talk about conformation, not "form". What do you mean by "stable"? Epoxides seem pretty reactive to me :) – CHM Apr 30 '12 at 0:42
• ""a stable molecule, if at all?"" Do You doubt the existence of epoxides? Some molecule being stable without giving conditions means "You can fill that in a bottle and it will suvive for some days at least" in lab slang. – Georg May 1 '12 at 12:28
• You could say it is kinetically stable, but thermodynamically unstable. – user95 Jul 12 '12 at 5:18

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°.

Epoxides are stable because, first and foremost, they are ethers. Ethers are an exceptionally unreactive functional group. Why? Because alkoxides are poor leaving groups that do not participate in many common nucleophile-electrophile reactions (e.g., SN2). Of course, there are ways to force ethers to react, such as through an SN1 pathway utilizing strong acid catalysis.

Among the ethers, epoxides are (among) the most reactive due to the ring strain, which enables chemists to perform reactions with epoxides that do not work with other ethers. For example, the reaction of an epoxide with a Grignard reagent is one of the first carbon-carbon bond forming reactions introduced in introductory organic chemistry.

I believe it's for this reason, their early importance in organic chemistry education, that epoxides have a reputation as exceptionally reactive. However, most epoxides are very readily prepared, isolated through aqueous workup, and purified on silica gel with no deleterious effects. In fact, epoxides can be survive being subjected to strong nucleophiles if a more reactive group is present in the molecule. For example, an ester can be reduced with DIBAL in the presence of an epoxide, and recently I was able to carry out a synthesis that used butyllithium in the presence of an unhindered epoxide with no trouble.

You can't quite compare rings consisting only of carbon atoms with rings with oxygens in them. In the latter, the $\ce{C-O}$ bonds are polar, the electrons are closer to the oxygen atom which means the acuteness at the $\ce{C}$ atoms is not as much of a problem as it would be in a hydrocarbon ring.