Sandmeyer reactions. Part 3. Estimation of absolute rate constants for the transfer of chloride ligands from CuII to 2-benzoylphenyl radical (Pschorr radical clock) and further investigations of the relative rates of transfer of chloride and water ligands to other substituted phenyl radicals

Abstract

Absolute rate constants have been estimated for the transfer of chloride ligands from CuCl(OH₂)₅⁺, CuCl₂(OH₂)₄ and CuCl₃(OH₂)₃^– to 2-benzoylphenyl radical in aqueous solution at ionic strength 5 mol dm^–3 by comparison with its rate of cyclisation. The radical reacts with the monochlorocopper(II) complex with k=(8.8 ± 1.3)× 10⁶ dm³ mol^–1 s^–1, but it does not discriminate between the di- and tri-chloro complexes, k=(1.0 ± 0.1₅)× 10⁸ dm³ mol^–1 s^–1. Relative rate constants for the transfer of chloride and water ligands to a series of 2-, 3- and 4-substituted phenyl radicals have also been obtained; three copper complexes are implicated in chloride transfer for the conditions examined. Radicals with 3-and 4-substituents discriminate the monochlorocopper(II) complex from the two more highly chlorinated complexes, the discrimination increasing the more nucleophilic and reactive the radical. Radicals with strongly electron-withdrawing 2-substituents (CN, F) do not discriminate between any of the chlorocopper(II) complexes. A mechanistic rationale of the observations is proposed.