Phenylenediamine is the common name for diaminobenzene, which exists as three isomers: 1,2‐, 1,3‐, and 1,4‐diaminobenzene. The 1,2‐isomer is commonly called o‐phenylenediamine; the 1,3‐isomer is m‐phenylenediamine; and the 1,4‐isomer is p‐phenylenediamine.
The toluenediamines, with six possible isomers, are commonly named by using numbers to designate the position of the amino groups with respect to the toluene methyl group, i.e., 2,3‐diaminotoluene or 2,3‐toluenediamine (3‐methyl‐1,2‐phenylenediamine) [2687‐25‐4]; 2,4‐diaminotoluene or 2,4‐toluenediamine (4‐methyl‐1,3‐phenylenediamine) [95‐80‐7]; 2,5‐diaminotoluene or 2,5‐toluenediamine(2‐methyl‐1,4‐phenylenediamine) [95‐70‐5]; 2,6‐diaminotoluene or 2,6‐toluenediamine (2‐methyl‐1,3‐phenylenediamine) [823‐40‐5]; 3,4‐toluenediamine (4‐methyl‐1,2‐phenylenediamine) [496‐72‐0]; and 3,5‐toluenediamine (5‐methyl‐1,3‐phenylenediamine) [108‐71‐4]. An 80 : 20 mixture of the 2,4‐ and 2,6‐isomers is used and sold as m‐toluenediamine. Commercially available o‐toluenediamine is a 60 : 40 mixture of the 3,4‐ and 2,3‐isomers.
Like most simple aromatic amines, the phenylene‐ and toluenediamines can be prepared by electrophilic aromatic nitration, followed by reduction of the introduced nitro group (usually catalytic hydrogenation) to an amino group. This chemistry was developed in the early 1800s. Until the mid‐1900s, phenylene‐ and toluenediamines were used principally for the preparation of dyes, but they are currently more important in the production of high‐performance textile fibers, agricultural chemicals, and diisocyanates for the preparation of a variety of polyurethane products.