Reaction kinetics of electrogenerated 9,10-bis(4-substituted-phenyl)anthracene cation radicals with methanol: A change in mechanism brought about by remote substituents

Abstract

The reactions of 9,10-bis(4-nitrophenyl)- and 9,10-bis(4-methoxyphenyl)anthracene cation radicals (DNA˙⁺ and DAA˙⁺) with methanol (MeOH) in acetonitrile were analyzed using a pulse electrolysis stopped-flow method. The reaction of DNA˙⁺ was found to proceed via the same rate law as the 9,10-diphenylanthracene cation radical (DPA˙⁺), i.e. −d[DNA˙⁺]/dt = k[DNA˙⁺][MeOH]², with a reaction rate 6 times faster. In contrast, the rate law governing the reaction of DAA˙⁺ with MeOH was different from that of DPA˙⁺, and was found to be −d[DAA˙⁺]/dt = k[DAA˙⁺]²[MeOH]. Reflecting this difference in the rate laws, an acceleration of the reaction was observed in the decay of DAA˙⁺ relative to DPA˙⁺, though methoxy groups usually stabilize aromatic cation radicals. This unusual remote substituent effect, which involves a change in the reaction mechanism, is discussed in terms of stabilization of intermediates in the proposed reaction mechanisms.