Quantitative rate constants for the reaction of dyes and alkenes with α-hydroxyalkyl radicals, measured by laser flash photolysis

Abstract

Rate constants are measured for the addition reactions of 1-hydroxy-1-cyclohexyl (1HC) and 2-hydroxy-2-propyl (2HP) radicals to 7 alkenes and for the 1-electron reduction of 16 organic dyes by 1HC, and a subset of 5 of these dyes by 2HP. This was done to determine to what extent the many reported rate constants for reactions of 2-hydroxy-2-propyl radicals (2HP) may be used to predict the rates of reactions of other tertiary α-hydroxy-alkyl radicals, and to give a better understanding of the factors that control dye reduction. The dyes were chosen to represent a wide range of dye types (azo, anthraquinone, phthalocyanine, triaryl-methane, indocyanine and azine dyes). Radicals were produced by laser flash photolysis of the corresponding tertiary α-hydroxyketone giving carbonyl and tertiary α-hydroxy-alkyl radicals. Control experiments with a bis-acylphosphine oxide were carried out which clearly demonstrated that the carbonyl radicals did not interfere with the kinetics. On average the addition and reduction rate constants for 1HC are only 20% lower than for 2HP. Larger decreases are observed for sterically congested alkenes due to the increased steric bulk of 1HC. The rate constants for 1-electron reduction of the dyes are in the range 4 × 10⁷ to 6 × 10⁹ mol⁻¹ l s⁻¹ and may be predicted, reasonably well using the Marcus equation with a reorganisation energy, λ = 182 kJ mol⁻¹.