Acid and Base Catalyzed Epoxide Ring Opening

Ring Opening of Epoxides or Oxiranes

Epoxides, also called oxiranes, are three-membered cyclic ethers, i.e. the oxygen atom is in the ring. This ring strain, with bond angles of approximately 60° (far from the ideal tetrahedral angle of 109.5°), makes them inherently reactive and susceptible to nucleophilic attack. The relief of this strain is the driving force behind ring-opening reactions.

Acid-Catalysed Ring Opening

The ring opening of alkyl-substituted oxiranes occurs readily with acid catalysis by the SN₁ mechanism and bond cleavage at the more substituted carbon. However, the intermediate is stabilized by a weak Lewis acid–base interaction between the cationic centre and the lone pair of the internal oxygen atom.

If a protonated epoxide is not capable of yielding a viable carbenium ion, ring-opening will be by an SN₂ reaction.

Base-Catalysed Ring Opening

In contrast to normal ethers, epoxides are susceptible to ring-opening attack by powerful nucleophiles; these reactions occur readily without catalysis owing to the release of ring strain. Grignard reagents, for example, open epoxides in a reaction which is useful for the synthesis of alcohols with an additional two-carbon unit. In substituted epoxides, reaction proceeds via SN₂ reaction, attack occurs at the least substituted carbon of the epoxide.

Weak nucleophiles, however, require base catalysis. Consequently, solvolytic reactions with water or an alcohol containing a base proceed via HO⁻ or RO⁻ formed from the solvent.

With or without base catalysis, nucleophile-induced ring opening of an epoxide may be regarded as an SN₂ substitution in which the nucleofuge is tethered to the β C of the electrophilic alkyl group. For steric reasons, therefore, nucleophilic attack and consequent C–O bond cleavage occur at the less substituted carbon, as illustrated above.

Show the structure of the main product when 2,2-dimethyloxirane reacts in methanol (a) in the presence of an acid, and (b) with added sodium methoxide, and explain the results.