epoxide ring opening

Ring opening is what epoxides are for. A nucleophile attacks one of the strained ring carbons, the C-O bond on that side breaks, and the three-membered ring springs open into a chain — relieving the ring strain and leaving a new -OH where the oxygen used to be. The result is a molecule with the incoming nucleophile on one carbon and an -OH on the adjacent carbon: a 1,2- or vicinal arrangement of two functional groups.

Here is the elegant part: an epoxide opens under either basic or acidic conditions, and the two routes attack at different carbons. Under basic (or neutral) conditions, a strong nucleophile attacks directly by an SN2-like step at the less hindered carbon (the one with fewer alkyl groups), because that carbon is easier to reach — sterics decide. Under acidic conditions, the oxygen is protonated first; this stretches and weakens the C-O bonds and builds up partial positive charge on the more substituted carbon (the one better able to bear positive charge), so the nucleophile is drawn to the more hindered carbon instead. Same molecule, opposite regiochemistry, depending on the conditions.

Either way the attack is backside (SN2-like) at the carbon, so the stereochemistry is clean: the nucleophile and the -OH end up anti (on opposite faces, trans), and an attack on a stereocenter inverts it. This combination of predictable regiochemistry plus reliable anti stereochemistry makes epoxide opening a favourite tool for building two adjacent functional groups with control — installing two new bonds in one well-behaved step.