Epoxides Cyclic three-membered ring ethers reactions

The one general exception to the rule that ethers don t typically undergo Sn2 reactions occurs with epoxides, the three-membered cyclic ethers that we saw in Section 7.8. Epoxides, because of the angle strain in the three-membered ring, are much more reactive than other ethers. They react with aqueous acid to give 1,2-diols, as we saw in Section 7.8, and they react readily with many other nucleophiles as well. Propene oxide, for instance, reacts with HC1 to give l-chloro-2-propanol by Snj2 backside attack on the less hindered primary carbon atom. We ll look at the process in more detail in Section 18.6. 

Alkenes undergo reaction with peroxycarboxylic acids (RCO3H) to give three-membered-ring cyclic ethers called epoxides. For example, 4-octene reacts with a peroxyacid to yield 4,5-epoxyoctane ... 

Under the same conditions, the reactivity of three-membered cyclic ethers in anionic copolymerization with cychc anhydrides is higher than that of four-membered ethers Higher membered cyclic ethers can polymerize or copolymerize with anhydrides only by a cationic mechanism whereby not only alternating copolymer but also a great number of polyether sequences are formed. This difference in reactivity is evidently associated with the basicity of cychc ethers, three-membered ethers having the lowest basicity The lower basicity causes a lower reactivity of the epoxide (cychc ether) in competitive reactions or in copolymerization with other cychc monomers compared with the expected reactivity which follows from the strain in the ring. The strain energy, taken as the difference between the experimental and calculated heats of formation was found to be 54.4kJ/mol for ethylene oxide... 

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