O-Phenylenediamine
O-phenylenediamine (OPD) is an organic compound with the formula C6H4(NH2)2. This aromatic diamine is an important precursor to many heterocyclic compounds. It is isomeric with m-phenylenediamine and p-phenylenediamine. Preparation
Commonly, 2-nitrochlorobenzene is treated with ammonia and the resulting 2-nitroaniline, whose nitro group is then reduced:
ClC6H4NO2 + 2 NH3 → H2NC6H4NO2 + NH4Cl

H2NC6H4NO2 + 3 H2 → H2NC6H4NH2 + 2 H2O

In the laboratory, the reduction of the nitroaniline is effected with zinc powder in ethanol, followed by purification of the diamine as the hydrochloride salt. This compound darkens upon exposure to air. Sodium dithionite and activated carbon are utilized to purify a hot aqueous solution. The solution is then cooled and crystallized to remove impurities. Reactions and uses
o-Phenylenediamine condenses with ketones and aldehydes to give rise to various valuable products. Its reactions with carboxylic acids and their derivatives produce benzimidazoles. The herbicides benomyl and fuberidazole are made in this manner. Thiophanate-methyl is another herbicide produced from o-phenylenediamine. Quinoxalinedione may be prepared by condensation of o-phenylenediamine with dimethyl oxalate. Mercaptoimidazole are commonly used as antioxidants in rubber production, obtained by condensing xanthate esters. Treatment with nitrous acid gives benzotriazole, a corrosion inhibitor. Condensation of substituted o-phenylenediamine with diketones yields various pharmaceuticals. Phenylenediamine is a significant precursor of ligands in coordination chemistry. Schiff base derivatives, such as those derived from salicylaldehyde, are excellent chelating ligands. Oxidation of its metal-phenylenediamine complexes affords the diimine derivatives, which are intensely colored and often exist in multiple stable oxidation states. Safety
With an LD50 of 44 mg/L (in water), o-phenylenediamine is about 1000 times less toxic than the para-isomer. Anilines are typically handled as if they are carcinogenic. For many applications, OPD has been replaced by safer alternatives such as 3,3',5,5'-tetramethylbenzidine. References

Diamines
Anilines
Chelating agents