epoxide
/ ee-POX-ide /
An epoxide is a three-membered ring made of two carbons and one oxygen — the smallest possible cyclic ether. Picture a tiny triangle: two carbons joined to each other and each also joined to the same oxygen at the apex. The simplest one, ethylene oxide (oxirane), is the building block of antifreeze and a major industrial chemical.
What makes an epoxide special is that it is a strained, reactive ether. Ordinary ethers are famously inert, but squeezing the C-O-C angle down to about 60 degrees in a three-membered ring forces the bonds far from their preferred angle, loading the molecule with ring strain. That strain is stored energy aching to be released, and it transforms the usually-sleepy ether oxygen into an excellent leaving group: a nucleophile attacking one of the ring carbons relieves the strain by popping the ring open. So an epoxide reacts where a normal ether would not.
Epoxides are immensely useful precisely because they react cleanly with so many nucleophiles, opening to give a product with two functional groups on adjacent carbons (one from the nucleophile, one a new -OH from the former ring oxygen). You usually make an epoxide by treating an alkene with a peroxyacid (such as mCPBA), which delivers an oxygen across the double bond in one stereospecific step. In nature and the lab they are powerful electrophiles — which is also why some epoxides are toxic or carcinogenic, since they can alkylate DNA.
Ethylene oxide is a triangle of -CH2-CH2- closed by an oxygen bridge. Treating an alkene like cyclohexene with mCPBA (a peroxyacid) delivers an oxygen across the double bond to give the epoxide in one step.
A strained three-membered ring ether — that strain is what makes it react.
An epoxide is technically an ether, but do not expect ether-like inertness: ring strain makes it dramatically more reactive, so it opens readily under both acid and base where an ordinary ether would not budge.