Epoxide Reactions
Triangle Reactions
The next few reactions we will look at are what I call triangle reactions. It is prudent to review them now. No, seriously, if you don't remember these, click on that link and go review the analogy I use to describe these triangle reactions. If you don't know what I am talking about when I refer to an attacker, a pickpocket, and a victim, this won't make much sense. But, if you do, you will have a MUCH better understanding.
Epoxide Reactions
with HX
An epoxide reacting with a strong acid like HBr or HCl is a triangle reaction. In the presence of the acid, the oxygen atom of the epoxide is protonated. Once it is protonated and has a positive charge, the epoxide triangle is holding a sign overhead announcing it has positive money. The halide can backside attack the more substituted carbon atom forming a halohydrin with anti-stereochemistry.
in acid with water to make glycols
If instead of using MCPBA in a chlorinated solvent, a peroxyacid in water is used, the epoxide formed can further react. This is a triangle reaction. In this reaction an acid (the peroxyacid) is present. Once the epoxide is formed, the peroxyacid can protonate the epoxide. The epoxide is holding a sign overhead announcing it has positive money. Water, a weak nucleophile, can backside attack the more substituted carbon atom resulting in a glycol with anti-stereochemistry.
in acid with alcohols
A very similar reaction occurs when an epoxide reacts with acid when an alcohol is the solvent. The same protonation occurs. But, in this instance, alcohol attacks the protonated epoxide at the more substituted, more positive carbon atom. This makes an ether that is anti to a hydroxyl group.
with base or Grignards
The previous reactions were triangle reactions in the presence of acid. They all made a positively charged epoxide that was attacked by weak nucleophiles. But, if the nucleophile is a strong, negatively charged nucleophile, epoxides react differently. In the following reaction, an epoxide is attacked by an aggressive alkoxide. In this triangle reaction, the epoxide triangle victim is holding up a sign with an “O” on it. The aggressive thief attacks the less protected side of the epoxide victim. This leads to an anti ether/hydroxyl system, but the ether is on the less substituted carbon atom. This is opposite to what happened in the acid catalyzed reaction.
Grignard reagents attack epoxides in the same way. In the case of a Grignard reagent, the nucleophile is an aggressive negative carbon atom.
5. Draw the products of the following reactions.
Answers
5.
