The peroxycarboxylic acid has an unusual property of having an electrophilic oxygen atom on the COOH group. The reaction is initiated by the electrophilic oxygen atom reacting with the nucleophilic carbon-carbon double bond. The mechanism involves a concerted reaction with a four-part, circular transition state. The result is that the originally electrophilic oxygen atom ends up in the epoxide ring and the C(O)OOH group becomes C(O)OH.

Mechanism

Peroxycarboxylic acids are generally unstable. One stable example is meta-chloroperoxybenzoic acid, shown in the mechanism below. Often abbreviated MCPBA, it is a stable crystalline solid that is popular for laboratory use. However, MCPBA can be explosive under some conditions.

Peroxycarboxylic acids are sometimes replaced in industrial applications by monoperphthalic acid, or the monoperoxyphthalate ion bound to magnesium, which gives magnesium monoperoxyphthalate (MMPP). In either case, a nonaqueous solvent such as dichloromethane, ether, acetone, or dioxane is used. This is because in an aqueous medium with any strong acid or base catalyst present, the epoxide ring is hydrolyzed to form a vicinal diol, a molecule with two OH groups on neighboring carbons. (For more explanation of how this reaction leads to vicinal diols, see below.)  However, in a nonaqueous solvent, the hydrolysis is prevented and the epoxide ring can be isolated as the product. Reaction yields from this reaction are usually good. The reaction rate is affected by the nature of the alkene, with more nucleophilic double bonds resulting in faster reactions.

Example