Chapter 6: Reactions of Alkenes: Addition Reactions

Epoxidation of Alkenes

Summary

• Overall transformation :  C=C to epoxide
• Reagent : a peracid or peroxyacid, RCO₃H
• Regioselectivity : not relevant since both new bonds are the same, C-O
• Stereoselectivity : syn since the two new C-O σ bonds form at the same time from the peracid.
• The reaction is an example of a concerted process (i.e. all bonding changes occur in one step)
• Relative reactivity (relative rates) : H₂C=CH₂ (1), CH₃CH=CH₂ (24), CH₃CH=CHCH₃ (500), (CH₃)₂C=CHCH₃ (6500)
• The relative rates reflect the increased nucleophilicity of the alkene (due to the electron donating alkyl groups)
• Since the reaction is concerted, the stereochemistry of the alkene is preserved in the product.
• For example if the alkyl groups of the alkene are cis- then they are also cis- in the  epoxide and trans-alkenes give trans-epoxides:

QUESTIONS

• What other functional group description could be given to an epoxide ? ANSWER
• What other addition reactions convert an alkene into a three membered ring ? ANSWER

MECHANISM FOR REACTION OF ALKENES WITH PERACID
A single step reaction involving several changes. Start at the C=C as the nucleophile, make a bond to the slightly electrophilic O, break the weak O-O, make a new C=O ,break the original C=O to make a new O-H bond, break the original O-H to form the new C-O bond ! (phew !) bond to give the epoxide.

© Dr. Ian Hunt, Department of Chemistry