Opening of epoxides with acid and water to give trans diols
Description: When aqueous acid is added to epoxides, trans-diols are formed.
Notes: The stereochemistry of this reaction is trans, since the epoxide must undergo a backside attack to open. Aqueous acid is often written as “H3O+”, although sulfuric acid (H2SO4) and water can be written as well.
Mechanism: Protonation of the epoxide (Step 1, arrows A and B) makes the carbons adjacent to the oxygen much more susceptible to nucleophilic attack. Attack preferentially comes on the carbon best able to support positive charge – that is, the most substituted carbon. Attack on this carbon from the backside (Step 2, arrows C and D) leads to a trans product, which is then deprotonated (Step 3, arrows E and F).
Notes: There’s nothing special about HSO4(-) here, it’s just the conjugate base formed when H2SO4 protonates water to give H3O(+). It’s a “spectator ion” in this reaction – just put it in to show that the charges are balanced.