chicagomel said:
What if the epoxide is asymmetrical?
Epoxides are a very rare MCAT topic, but I think they're cool, so here goes.
When dealing with asymmetrical epoxides:
Acid catalyzed ring opening: The epoxide O is protonated by acid, creating a good leaving group b/c it leaves as an uncharged hydroxyl group. Although the ring doesn't "open" per se until the water/base comes in to attack one of the carbons, it is helpful to "think" of the ring opening to understand the mechanism. When the ring opens, you're going to generate a carbocation (analogous to an Sn1 reaction) on one of the epoxide carbons, thus, you want to form the most stable carbocation (ie. most substituted carbon gets the charge, the least substituted carbon gets the hydroxyl). Thus the water/base will attack at the most substituted (the "carbocation" carbon).
Base catalyzed ring opening of epoxides: This time a base catalyst is coming in and attacking a carbon, backside to the epoxide, analogous to an Sn2 reaction. Thus, you need a strong base (strong nucleophile, thus negatively charged), and that base wants to attack at a carbon with the least steric hindrence ie. the least substituted carbon, pushing the epoxide O to the most substituted carbon. The negatively charged O is then protonated by removing a proton from your base, thus regenerating your base catalyst.
So in summary, both react to form the most stable intermediate in different ways:
Acid catalysis ("Sn1" carbocation) : hydroxyl (epoxide O) on least substitued carbon, base on most substituted carbon
Base catalysis ("Sn2" steric hindrence): hydroxyl group (epoxide O) on
most substitued carbon, base attacks
least substituted carbon.
Good luck!
