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Formation of epoxides from alkenes using m-CPBA

Description: meta-chloroperoxybenzoic acid (m-CPBA) or other peroxyacids such as peracetic acids will convert alkenes into epoxides.

Notes: The stereochemistry of this reaction is always “syn” (both bonds to oxygen are formed on the same side of the alkene).

Examples:

Notes: Note that example 2 just shows m-CPBA written out – it’s in no way different from the others. Examples 3 and 4 show that the reaction is stereospecific – that is, trans alkenes give trans products, and cis alkenes give cis products. Also note there is no reaction with alkynes.

Mechanism:

This is a reaction which the arrow pushing formalism isn’t very good at describing. Instead, a transition state is shown here, which shows breaking of the C–C π bond at the same time the C–O bonds are forming. At the same time the O–O bond is breaking and the hydrogen from the OH is being picked up by the carbonyl oxygen.

Notes: Note that the byproduct here is m-chlorobenzoic acid (in grey).