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A concerted reaction is defined as the simultaneous breaking of bonds a-b and c-d to form two new bonds a-c and b-d after going through a common transition state. Though two bonds have to be broken, only one transition state exists, e.i. only one energy barrier has to be overcome to reach the final product. Since the reaction proceeds through a cyclic transition state, they are summarized as pericylcic reactions according to Woodward and Hoffman. Examples are cycloadditions and sigmatropic reactions.