Question

How are epoxides formed?

Answer 1

A common way to do it is using a peroxyacid on an alkene.

This is a concerted epoxidation mechanism---it happens in one step. These things occur:

• The alkene donates its `pi` electrons into the bottom oxygen's antibonding orbitals, and receives the bonding electrons from the left `"H"-"O"` bond. This resembles the `\mathbf("Br"_2)` addition onto an alkene.
• The carbonyl oxygen intramolecularly donates its `pi` electrons to grab the peroxy proton within the same peroxyacid.
• The bottom oxygen's `sigma` bond breaks and a `pi` bond forms between the carbonyl carbon and the carboxyl oxygen. This was facilitated through the proton acquisition by the carbonyl oxygen.

Since this is a syn addition of a `-"O"-`, it doesn't matter whether the alkene is more substituted on one of its carbons or not. It'll add the same way, generating only one product.