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. 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. Commonly, 2-nitrochlorobenzene is treated with ammonia and the resulting 2-aminonitrobenzene is then hydrogenated: 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 in air; impurities may be removed by treating a hot aqueous solution with sodium dithionite (reducing agent) and activated carbon, and allowing the product to cool crystallize. o-Phenylenediamine condenses with ketones and aldehydes to give rise to a variety of useful products. Reactions with carboxylic acids and their derivatives afford benzimidazoles. The herbicide benomyl is prepared in this manner. Also, quinoxalinedione may be prepared by condensation of o-phenylenediamine with dimethyl oxalate. Condensation with xanthate esters affords mercaptoimidazoles, which are used as antioxidants in rubber products. Treatment with nitrous acid gives benzotriazole, a corrosion inhibitor. Condensation of substituted o-phenylenediamine with various diketones is used in the preparation of a variety of pharmaceuticals. In coordination chemistry, phenylenediamine is an important ligand precursor. 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. With an LD50 of 44 mg/L (aquatic), 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 For many applications, OPD has been replaced by safer alternatives such as 3,3',5,5'-tetramethylbenzidine.