Ethers, Sulfides, Epoxides, and Sulfur Functional Groups

Reactions of Epoxides

Epoxides undergo a ring-opening reaction with either an acid or a nucleophile to give an anti-product. When nucleophiles are used in ring-opening reactions, the product will always give the anti-product, with the OH going to the more-substituted carbon.

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 CC{\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.

Epoxide Reaction

When an epoxide undergoes ring opening, the stereochemistry of the product will be determined by the conditions (acidic or basic) of the reaction and the substitution pattern of the epoxide.
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.

Nucleophilic Ring Opening

The nucleophilic ring opening is regioselective and attacks the less-substituted carbon of the epoxide ring.

Acid-Catalyzed Ring Opening

The acid-catalyzed ring opening of oxirane (C2H4O) produces ethylene glycol (C2H6O2). Protonation gives the oxirane a positive charge. The nucleophile attacks the more positive side of the epoxide ring, breaking the bond with the oxygen and attaching to the more-substituted carbon.