Highly selective photochemical synthesis of perfluoroalkyl bromides and iodides

Article abstract of DOI:10.1016/S0022-1139(98)00112-2

Highly fluorinated alkyl iodides are conveniently synthesized by telomerization of a fluoroalkyl-iodide, RI, with, e.g., C₂F₄. Normally, the reaction, often carried out in the liquid phase with a radical initiator, gives products with a broad distribution of molecular weights. In this work, we report a method that obtains selectively products of a desired molecular weight: this method is a photochemically induced reaction in the gas phase; the gas is circulated through a trap or a rectification still which continuously removes the heavier products, whereas the more volatile molecules return to the photoreactor. An analysis by rate equations shows which control parameters are important, and by a suitable choice of these parameters, we obtained a better selectivity for, e.g., C₈F₁₇I than previously. This method also works with BrC₂F₄Br instead of an iodide. In this case, we demonstrated in a small laboratory setup with simple low-pressure Hg lamps (5 × 30 W), a productivity of more than 0.5 kg/day. In the telomerization of CF₃Br or HC₂F₄Br with C₂F₄ we found, however, a few percent of dibromide side products which are sometimes difficult to separate because of similar boiling points. For this case, it is better to synthesize the iodides instead, and then exchange the I for Br, if desired.