title: Pentafluorobenzoic acid
text: # Pentafluorobenzoic acid
  
  Pentafluorobenzoic acid (PFBA) is an organofluorine compound with the formula $C_6F_5CO_2H.$ 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 (or pentafluorophenyl Grignard reagent) 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 $−C_6F_4CO_2H$ 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.
  
  