(also diaminobenzenes), C6H4(NH2)2, aromatic diamines. Ortho, meta, and para isomers are known, occurring as colorless crystals that darken upon exposure to light and air. Their melting points are 102°, 63°–64°, and 147°C, respectively. Phenylenediamines are soluble in hot water, alcohol, and diethyl ether; they form salts with mineral acids and are readily oxidized and acylated. The meta and para isomers undergo diazotization, a property that forms the basis of their use in the synthesis of dyes (azine, oxazine, thiazine). Polyamides, for example, Phenylone, which are obtained through the condensation of meta-phenylenediamine with isophthalic acid, are important in industry.
Phenylenediamines are used in analytical chemistry. Ortho-phenylenediamine is a reagent for diketones, carboxylic acids, and aldehydes, and meta-phenylenediamine is a reagent for the nitrite ion. The sulfate of N, N-diethyl-para-phenylenediamine is used as a developer in color photography. Phenylenediamines are produced synthetically. Ortho-phenylenediamine is obtained through the reduction of ortho-nitroaniline and through the action of an aqueous ammonia solution on ortho-chloroaniline and ortho-dichlorobenzene; meta-phenylenediamine is obtained through the reduction of meta-dinitrobenzene; and para-phenylenediamine is produced by the reduction of para-nitro-aniline and by the action of aqueous ammonia on para-dichloro-benzene in the presence of CuSO4.