Pentafluorobenzoic acid

Pentafluorobenzoic acid 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.

Preparation

Pentafluorobenzoic acid is prepared by treating pentafluorophenyllithium with carbon dioxide. These reagents are usually prepared in situ from pentafluorobenzene and bromopentafluorobenzene.
It arises via the reaction of perfluorotoluene with trifluoroacetic acid and antimony pentafluoride.

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.