Evaluation of Addition Rates of Thiyl Radicals to Vinyl Monomers by Flash Photolysis. 3. Polar Effects in Addition Reactions of Substituted Benzenethiyl Radicals

Article abstract of DOI:10.1021/ja00409a041

Rate constants for addition (k₁) of para-substituted benzenethiyl radicals (p-XC6H4S*) to vinyl monomers (CH2=CHY) such as isobutyl vinyl ether and vinyl acetate (nonconjugated monomers) and acrylonitrile (conjugated one) have been determined by means of flash photolysis.The rate constants for the reverse reaction (k_-1) and the equilibrium constants (K = k₁/k_-1) have been estimated in the forms of k_-1/k₂ and Kk₂, respectively, where k₂ is the rate constant for the reaction between p-XC6H4SCH2C*HY and oxygen; oxygen was used as a selective radical trap to the carbon-centered radicals.From the linear correlations obtained in the Hammett plots of log k₁, log (k_-1/k₂), and log Kk₂ vs. the substituent constants (?⁺), the reaction constants (ρ⁺) were estimated for each vinyl monomer.The ρ⁺ (Kk₂) values were invariant with a change in vinyl monomers (ρ⁺(Kk₂) = 1.37).The ρ⁺(k₁) values increase with an increase in the electron densities of the double bonds in vinyl monomers.This suggests that a part of the polar effects in the reactivities is determined by the polar effect in the thermodynamic stabilities of the p-XC6H4S* and that another part of the polar effects is caused by the polar resonance structures in the transition state of which contribution varies with the electron-donating or -withdrawing ability of vinyl monomers.