Nitrophenols

products of the substitution of nitro groups, —NO₂, for the hydrogen atoms in the phenol ring, C₆H₅OH. The nitrophenols shown in Table 1 are of commercial significance.

Nitrophenols are crystalline substances. In industry, ortho- and para-nitrophenols are produced by heating the corresponding nitrochlorobenzenes with an aqueous solution of sodium hydroxide; 2,4-dinitrophenol (2,4-D) is obtained in an analogous manner from 2,4-dinitrochlorobenzene. These nitrophenols are blood poisons, but they also act on the nervous system and irritate the mucous membranes and skin, leading to dermatitis and chronic eczema. For example, the maximum permissible concentration of 2,4-D in air in work areas is 0.05 mg/m³. The main use of ortho- and para-nitrophenols is in the production of ortho- and para-aminophenols. Picric acid, sulfur black dye, and 2,4-diaminophenol (amidol developer) are produced from 2,4-D; it is also used as a herbicide.

Picric acid, or 2,4,6-trinitrophenol (2,4,6-T), was used as a yellow dye for silk (the first synthetic organic dye) and as an explosive (melinite and lyddite); it is poisonous, and its maximum permissible concentration in air in work areas is 1 mg/m³. With metals, 2,4,6-T forms dangerously explosive, easily flammable salts called picrates.