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NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition phase

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A. Baeyer and V. Villiger, found that treating a ketone with a peroxy-acid (RCO3H) can produce an ester. An oxygen atom is 'inserted' next to the carbonyl group. The oxygen-oxygen single bond is very weak and monovalent oxygen cannot bear to carry a positive charge. Once the peracid has added, loss of carboxylate is concerted with a rearrangement driven by formation of a carbonyl group.

G.-J. ten Brink, I. W. C. E. Arends and R. A. Sheldon, Chem. Rev., 2004, 104, 4105–4124.