Diphenic acid is prepared from anthranilic acid by
diazotization, followed by reduction with copper(I). : It can also be synthesized from the oxidation of
phenanthrene by
peracetic acid, which is first prepared from
acetic acid and 90%
hydrogen peroxide: :CH3COOH + H2O2 ⇌ CH3COOOH + H2O :4 CH3COOOH + C14H10 → 4 CH3COOH + C14H10O4 Phenanthrene can also be treated with other
oxidizing agents (such as hydrogen peroxide,
chromium trioxide,
potassium dichromate, or
potassium permanganate), which first yields
phenanthrenequinone and gives diphenic acid on further oxidation. Similarly, phenanthrenequinone can be boiled in alcoholic potash (potassium hydroxide in alcohol) to give the potassium salt of diphenic acid, and can alternatively be photo-oxidized to diphenic acid. ==References==