Epoxidation of Alkenes
Somewhere in one of your exams, you will see at least one question on epoxidation of alkenes.
The reaction: What is epoxidation? An epoxide is a 3-membered ring containing two carbon atoms and one oxygen atom. It is interesting because it is easily opened due to small ring strain and due to the electronegativity of the oxygen atom.
The reagents and starting materials:
1) What is an alkene? An alkene is an unsaturated hydrocarbon containing at least one double bond.
2) What is epoxidation of an alkene? This is the reaction where an alkene is subjected to a peroxyacid to convert it into an epoxide. Another way to say it is epoxidation is the electrophilic addition of oxygen to the double bond of the alkene.
3) What reagents can you use to create the epoxide? Generally, peroxy acids are used in this electrophilic addition to the alkene. There are several types of commonly used peroxyacid such as proxy trifluoroacetic acid, peroxyacetic acid, hydrogen peroxide, and mCPBA.
The mechanism: The mechanism for the reaction is relatively complex. While it is considered a single step reaction, it involves several changes. The double bond is our nucleophile and attacks the more electrophilic oxygen. This breaks the weak oxygen-oxygen bond and creates a new carbonyl. Once this carbonyl is formed, rearrangement occurs and the more electrophilic oxygen is released to become the oxygen of the epoxide.
The stereochemistry: The stereochemistry associated with this reaction is interesting and important. As the reaction can occur on a cis or trans alkene, we see the two different products come from these two different starting materials. The oxygen can only attack from one face of the alkene. This means that the stereochemistry of the alkene is retained. Translation: if you start with a cis alkene you will get a cis epoxide. If you start with a trans alkene, you will get a trans epoxide.
Further, remember that if you start with a di or tri-substituted alkene, you very well may create new stereocenters. However, if you remember the golden rule of chirality, you will know that you need to start with chirality in order to finish with it. These alkenes are not chiral to start with, therefore we will end with a racemic mixture. If there is some chirality in the molecule, somewhere near the double bond then that chirality can influence which face the peroxyacid is attacked from, but will not exclusively give a chirally pure product.
The reaction the reaction is versatile, and works on many different alkenes. Please remember that the reaction will not work on the double bonds of an aromatic compound.
We rate the importance of this reaction, the epoxidation of alkenes, as four beakers out of five.