Gerard Forat

Fluorination of aromatic compounds by halogen exchange with fluoride anions (“halex” reaction)

Publisher Summary Halogen exchange with a fluoride anion is one of the two main techniques to introduce a fluorine atom on an aromatic nucleus, which is a useful complement of diazotization of anilines in hydrogen fluoride or thermal decomposition of arenediazonium fluoroborates (Schiemann reaction). It is used on an industrial scale to produce, for example, 2, 6-difluorobenzonitrile (the precursor of the insecticide Diflubenzuron) or 2, 4-difluoroaniline (the precursor of Diflufenican or other herbicides). Because of the high lattice energy of inorganic fluorides, substitutive fluorination with alkaline fluorides in aprotic solvents is always a two-phase reaction which is very dependent on the origin and the preparation of the solid reagent. In order to get new improvements, it is necessary to have a better knowledge of the reaction at the molecular scale and, especially, to determine if the process is limited by physics (that means by the solubilization of the fluoride) or by chemistry. In the latter case, the problem is to know if the reaction occurs in the liquid phase or on the surface of the solid. Some insight could be given by kinetic measurements and comparison of the influences of the reaction parameters on rate values.