The Addition of a Peroxyacid to an Alkene

An alkene can be converted to an epoxide by a peroxyacid. An epoxide is an ether in which the oxygen is incorporated into a three-membered ring, and a peroxyacid is a carboxylic acid with an extra oxygen. The overall reaction amounts to the transfer of an oxygen from the peroxyacid to the alkene. It is an oxidation reaction because it increases the number of C—O bonds. 

The peroxyacid commonly used for epoxidation is MCPBA (wi to-chloroperoxy benzoic acid). 

the oxygen atom is both the electrophile and the nucleophile. This is another example of a concerted reaction—that is, all bond-making and bond-breaking occur in the same step. 

The mechanism for the addition of oxygen to a double bond to form an epoxide is analogous to the mechanism we have just seen for the addition of bromine to a double bond to form a cyclic intermediate (Section 6.9). 

The unstable cyclic bromonium ion intermediate is an electrophile that subsequently reacts with a nucleophile. The epoxide, however, is stable enough to isolate because none of the ring atoms has a positive charge. 

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The Addition of a Peroxyacid to an Alkene 261 Remember that an O—O bond is weak and is therefore easily broken (Section 1.4). 

The addition of a peroxyacid to an alkene to form an epoxide is a concerted reaction the oxygen atom adds to the two sp carbons at the same time (Section 6.10). Therefore, it must be a syn addition. 

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