Anthranilic acid

Anthranilic acid is an aromatic acid with the formula C₆H₄(NH₂)(CO₂H) and has a sweetish taste. The molecule consists of a benzene ring, ortho-substituted with a carboxylic acid and an amine. As a result of containing both acidic and basic functional groups, the compound is amphoteric. Anthranilic acid is a white solid when pure, although commercial samples may appear yellow. The anion [C₆H₄(NH₂)(CO₂)]⁻, obtained by the deprotonation of anthranilic acid, is called anthranilate. Anthranilic acid was once thought to be a vitamin and was referred to as vitamin L₁ in that context, but it is now known to be non-essential in human nutrition.

Although not usually referred to as such, it is an amino acid. Solid anthranilic acid typically consists of both the amino-carboxylic acid and the zwitterionic ammonium carboxylate forms, and has a monoclinic crystal structure with space group P2₁. It is triboluminescent. Above 81 °C (178 °F; 354 K), it converts to an orthorhombic form with space group Pbca, which is not triboluminescent; a non-triboluminescent monoclinic phase with similar structure is also known.

In 1840-1841, Carl Julius Fritzsche was able to extract and crystallize two acids from the products of reaction of indigo dye with caustic potash, which he called chrysanilic and anthranilic acids after their colors before purification (golden yellow and black respectively) and the plant anil (Indigofera suffruticosa). The former was identified as ortho-carboxy anil of indoxyl-2-aldehyde only in 1910 while the latter was identified as salicylamide already in 1843 by Cahours.

Many routes to anthranilic acid have been described. Industrially it is produced from phthalic anhydride, beginning with amination:

The resulting sodium salt of phthalamic acid is decarbonylated via a Hofmann rearrangement of the amide group, induced by hypochlorite:

A related method involves treating phthalimide with sodium hypobromite in aqueous sodium hydroxide, followed by neutralization. In the era when indigo dye was obtained from plants, it was degraded to give anthranilic acid.

Anthranilic acid was first obtained by base-induced degradation of indigo.

Anthranilic acid is biosynthesized from chorismic acid by the action of anthranilate synthase. In organisms capable of tryptophan synthesis, anthranilate is a precursor to the amino acid tryptophan via the attachment of phosphoribosyl pyrophosphate to the amine group. After then, cyclization occurs to produce indole.