Substituent effects on the free-radical addition reactions of arylthiyl radicals with arylacetylenes

Abstract

Absolute rate constants for addition reactions of arylthiyl radicals (YC₆H₄S˙) to arylacetylenes (XC₆H₄CCH) have been determined by a flash-photolysis method. The rate constants (in dm³ mol^–1 s^–1) vary from 1.0 × 10⁵(X =m-NO₂ and Y =p-MeO) to 1.0 × 10⁷(X =p-MeO and Y =p-Br). For all arylthiyl radicals, a Hammett plot of the σ⁺-constants for the X-substituents yields negative ρ⁺(Y) values. The ρ⁺(Y) values depend also on the Y-substituents on the arylthiyl radicals [ρ⁺(Y)=–0.22 for Y =p-MeO and ρ⁺(Y)=–0.53 for Y =p-Br]. These ρ⁺(Y) values are slightly smaller than the ρ⁺(Y) values reported for the corresponding arenesulphonyl radicals, reflecting the low electrophilicity of the arylthiyl radicals. On the other hand, for each arylacetylene a Hammett plot of the rate constants for YC₆H₄S˙vs. σ⁺(Y) yields positive ρ⁺(X) values which increase with the increasing electron density on the triple bond.