Pulse radiolysis study of salt effects on reactions of aromatic radical cations with Cl–. Rate constants in the absence and presence of quaternary ammonium salts

Abstract

The effect of quaternary ammonium salts on the decays of the radical cations of biphenyl, trans-stilbene, anthracene and pyrene generated by pulse radiolysis in chlorohydrocarbons has been investigated. The decays, which are due to neutralization reactions with Cl^–, are retarded by the addition of salts having non-nucleophilic PF^–₆, BF^–₄ and ClO^–₄, whereas the radical cations are rapidly quenched by salts having I^– and BPh^–₄. The retarding effect of the salts is attributed to the formation of ion pairs between the reacting ions and the counter-ions from the salts. The rate constants for the neutralization reactions in 1,2-dichloroethane have been determined for the free-ion and ion-paired states; the latter state is attained by the addition of Bu₄NPF₆. The rate constants for the reactions of the radical cations except for Py^·+(Py = pyrene) are in the ranges (1.3–1.9)× 10¹¹ dm³ mol^–1 s^–1 for the free ions and (2.8–7.0)× 10¹⁰ dm³ mol^–1 s^–1 for the ion pairs. The rate constant determined in the absence of the salt for Py^·+ is one order of magnitude smaller than those for the others. The salt effect is also smallest on the reaction of Py^·+. The charge delocalization of the large aromatic radical cation may be responsible for the exceptional results for Py^·+. The largest salt effect was observed on the reaction of Ph₂CH⁺, a charge-localized carbenium ion investigated for comparison. The solvent effect on the neutralization reactions is also discussed.