Benzoic Acids

4-Fluoro-3-hydroxybenzoic acid is a colorless solid that can be produced through the sulfonation of 4-fluorophenol with sulfuric acid. This process usually takes place in two steps: first, the phenol is refluxed with sulfur trioxide and then aqueous potassium hydroxide is added to the mixture. The reaction produces a sulfite salt, which is then treated with sodium sulfite to produce the desired acid. 4-Fluoro-3-hydroxybenzoic acid can also be synthesized by adding sulfur dioxide to an aqueous solution of 3% hydrogen peroxide and adding potassium hydroxide dropwise until effervescence ceases. This method produces an insoluble precipitate, which is filtered out and washed with water. The product can then be purified by recrystallization from hot water or by washing with ether.