Reactions of Epoxides

Epoxides, cyclic ethers, undergo ring-opening reactions under either acidic or nucleophilic conditions to give an anti-product. A ring opening is a reaction where a ${\rm {C{-}C}}$ bond is broken in a carbon ring, a cyclic compound containing carbons.

In ring-opening reactions with nucleophilic (basic) reagents and unsymmetrical epoxides, the reaction will always yield the anti-product, with the hydroxide going to the more-substituted carbon and the nucleophile going to the less-substituted carbon. The anti-product is when two groups are on opposite sides of the molecule. This mimics the standard SN2 pathway.

In ring-opening reactions with acidic reagents and unsymmetrical epoxides, the reaction will always yield the anti-product, but the regiochemistry is a little more complex. When one side is primary and the other is secondary, the regiochemistry is the same as the nucleophilic ring opening. But when one side is primary and the other is tertiary, the product forms with the hydroxide going to the less-substituted side.
In nucleophilic ring-opening reactions, the nucleophile attacks the epoxide on the least-substituted carbon atom to yield the anti-product. For acid-catalyzed ring-opening reactions, the reaction starts with a protonation of the ether followed by nucleophilic attack on the protonated epoxide to form the anti-product. For example, when H₃O⁺ is added to an epoxide, an anti-diol is formed.