Pentafluorobenzoic acid explained

Pentafluorobenzoic acid (PFBA) is an organofluorine compound with the formula C₆F₅CO₂H. It is a white crystalline powder that has a high solubility in water. Its pK_a of 1.48 indicates that it is a strong acid.^[1]

Preparation

Pentafluorobenzoic acid is prepared by treating pentafluorophenyllithium (or pentafluorophenyl Grignard reagent) with carbon dioxide. These reagents are usually prepared in situ from pentafluorobenzene and bromopentafluorobenzene.^{[1] [2]}

It arises via the reaction of perfluorotoluene with trifluoroacetic acid and antimony pentafluoride.^[3]

Substitution reactions

Substitution of fluoride occurs typically at the para position. This reaction has been used to anchor the −C₆F₄CO₂H group to surfaces. Magnesium methoxide results in ortho methoxylation. Cleavage of this ether gives tetrafluorosalicylic acid. Via similar ortho-directed reactivity, nickel complexes catalyse the defluoridation of 2 and 5 positions. Without nickel, defluoridation occurs with para-selectivity.^[1]

Notes and References

1. Prakash, G. K. S.; Hu, J. "Pentafluorobenzoic Acid" in e-EROS Encyclopedia of Reagents for Organic Synthesis, 2005.
2. Harper . R. J. Jr. . Soloski . E. J. . Tamborski . C. . 1964 . Reactions of Organometallics with Fluoroaromatic Compounds . J. Org. Chem. . 29 . 8. 2385 . 10.1021/jo01031a067 .
3. 10.1016/j.jfluchem.2007.05.020. Transformation of perfluorinated benzocycloalkenes and alkylbenzenes to their carbonyl derivatives under the action of CF₃COOH/SbF₅. Journal of Fluorine Chemistry. 128. 9. 1058–1064. 2007. Zonov. Yaroslav V.. Karpov. Victor M.. Platonov. Vyacheslav E..