Structure–reactivity relationships and substituent effect additivity in the aqueous oxidation of chlorophenols by cerium( iv )

The oxidation of all 19 chlorophenols and unsubstituted phenol by cerium(IV) was studied in an acidic aqueous solution in order to carry out a systematic test of chlorine substituent effects on the reactivity. All reactions were found to show 2 ∶ 1 cerium(IV) ∶ phenol stoichiometry and a simple second-order rate equation. Rate constants did not correlate well with characteristic parameters such as pK values, carbon-13 NMR chemical shifts or Hammett substituent constants. Nevertheless, a strict additivity of chlorine substituent effects was found in both characteristic and reactivity parameters. The data suggest that a proton-coupled electron transfer mechanism could be operative. 2,4,6-Trichlorophenol was found to show exceptionally high reactivity towards cerium(IV).