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Peroxy-acids are commonly used epoxidizing agents. They have an extra oxygen atom between the carbonyl group and their acidic hydrogen, and are electrophilic at oxygen. Attack at this position by a nucleophile displaces carboxylate, which is a good leaving group. An example of one such reaction is shown below involving ethylene and peroxyformic acid. The mechanism is essentially an electrophilic attack, with a proton being transferred from the epoxide oxygen to the carboxylic acid by-product.
Firstly the nucleophilic π bond donates its electrons to the oxygen, breaking the O-O bond to form the new carbonyl bond. The electrons from the old O-H bond make up the second new C-O bond, and the original carbonyl group uses its electrons to pick up the proton. The transition state for the reaction makes the bond-forming and bond-breaking processes much clearer.