Question

Complete the mechanism for the base-catalyzed opening of the epoxide by adding any missing atoms, bonds, charges, nonbonding electron pairs, and curved arrows. 

Answer 1

Opening of unsymmetrical epoxide is a regioselective reaction. The opening of unsymmetrical epoxide is depended upon the reaction medium i.e. either in acidic medium or basic medium.

In an acidic medium, the opening of epoxide takes place at the sterically more crowded side because the first step is the protonation of epoxide oxygen. Then it opens at which side depends upon the stability of carbocation formed. Always the sterically more crowded side opens.

In a basic medium, the opening of epoxide is always at a sterically less crowded side takes place. Because the protonation at epoxide oxygen does not take place.

 

Step 1: In this step, the nucleophile opens the epoxide from the sterically less crowded side.

 

Step 2: In this step the protonation of oxygen takes place. The alcohols are good proton donors. So, the oxygen abstracts hydrogen from the cyclopentanone.

 

Step 3: In this step, the epoxide is converted into alcohol.