Acid Base Reactions Between Acridine Orange and Substituted Phenols in Benzonitrile Part II. Kinetics: An Extended Eigen Mechanism of Proton Transfer

Abstract

The kinetics of proton transfer between five substituted phenols (AH) and the acridine orange base (B) are studied in benzonitrile solution using the temperature jump method. In the overall reactions (1)–(3) the formation and dissociation of an intermediate ion pair A‐.. HB+ is rate determining. The rate constants of the bimolecular recombination of the ions to the ion pair are kb1, kb2, and kb3. Their values are in the range (0.18‐ 1.7) · 109 M‐1 s‐1 (25 °C, I = 0.1 M). kb2 and kb3 do not depend on the pK of the phenol involved, although these rates are not diffusion controlled. The rate of proton transfer from the protonated acridine orange (BH+) to the free anions (A‐) does depend to some degree on the acidity of the corresponding phenol. In the case of the 3‐methyl‐ 4‐nitrophenolate ion this rate is diffusion controlled, as shown by the dependence of kb1 on the temperature and the ionic strength. Lower limits for the formation constants of the ion pairs are estimated from the rate data. The equilibria and the kinetics are discussed in terms of an extended Eigen scheme of proton transfer, and the separate steps of anion solvation by hydrogen bonding to phenol molecules shown to depend in different ways on the basicity of the anion.