Interaction Kinetics of I2(aq) with Substituted Phenols and Humic Substances

Abstract

We evaluate the hypothesis that the reactivity trend for iodination of natural humic substances (HS) resembles that for the iodination of some substituted phenols. The hypothesis was tested by comparing the rates of reaction of I2(aq) with HS and a series of eight substituted phenolic compounds. Rates of iodination for all of the phenolic compounds, except salicylate, are described with an empirical rate law R = kobs[phenolic compound]1[I3-]1[[H+]-1[I-]-2] with the values of kobs related to the structure of the substituted phenol. The values of kobs, corresponding to iodination of the simple substituted phenols, range from 5.6 × 10-8 to 4.7 × 10-5 M s-1 at 25 °C. These rate coefficients can be predicted over at least three orders-of-magnitude from a modified Hammett relation. The rates of iodination of HS fall within the range measured for substituted phenols, suggesting that iodination of the natural HS proceed by similar pathways. The humic substances differ markedly in their reactivity toward I2(aq) in...