Substrate Selectivity and Orientation in Aromatic Substitution by Molecular Fluorine

Article abstract of DOI:10.1021/ja00530a034

Direct elemental fluorination of representative aromatic substrates, including PhH, PhCH3, PhF, PhCl, PhBr, PhNO2, PhCN, and PhOCH3, has been investigated in inert solvents, e.g., CCl3F and other fluorocarbons, over the temperature range -154 to 40 deg C.In order to achieve the necessary control of the extremely reactivve electrophile, and to minimize unwanted modifications of the reaction environment, the fluorination has been carried out at extremely low rate and correspondingly low conversions, generally below 0.01percent, using as a reagent gaseous mixtures of F2 highly diluted (<1 mol percent) in N2 or Ar.The partial rate factors, calculated from a consistent set of relative reactivity and orientation data measured at -78 deg C in CCl3F, correlate smoothly with the ?⁺ constants for all the substituents investigated, giving a ρ⁺ value of -2.45 for aromatic substitution by elemental fluorine with a correlation coefficient of 0.993.These results characterize F2 as a highly reactive, and correspondently unselective, reagent, and support a polar electrophilic substitution mechanism that is discussed and compared with other plausible fluorination pathways.