2-Amino-4-nitrophenol

Identified by CAS 99-57-0 and EC 202-767-3, 2-amino-4-nitrophenol is an aromatic nitroamino compound with molecular formula C₆H₆N₂O₃ and molar mass 154.12 g/mol. The substance is supplied as an orange-brown to dark brown crystalline powder, and the melting range for technical material is generally specified between 142 °C and 146 °C under USP <741> capillary conditions. The substitution pattern places the amino group at position 2 and the nitro group at position 4 relative to the phenolic hydroxyl. This ortho-amino/para-nitro arrangement produces an intramolecular hydrogen-bonded ring system in which the hydroxyl and amino groups act as electron donors and the nitro group acts as an electron acceptor. The product is differentiated by end-use grade rather than by a model code. Technical grade is directed to azo dye intermediate synthesis; purified grade is supplied for personal care intermediate use with reduced iron, copper, and lead limits. Milled powder and wet presscake forms are available to accommodate different dosing, filtration, and drying operations on the downstream production line. The presscake form may contain residual water and must be corrected for dry solids content before batch charging.

What Distinguishes 2-Amino-4-nitrophenol from the 4-Amino-2-nitro Isomer?

In 2-amino-4-nitrophenol, the amino group is ortho to the phenolic hydroxyl and the nitro group is para to the hydroxyl. In the 4-amino-2-nitro isomer, CAS 119-34-6, the positions are reversed, placing the nitro group ortho and the amino group para. The ortho-amino/para-nitro motif strengthens intramolecular hydrogen bonding between the amino and hydroxyl groups while coupling the amino/hydroxyl donor region to the electron-withdrawing nitro acceptor across the ring. This shifts the electron density at the carbon atoms available for electrophilic attack and changes the visible absorption profile of the derived azo colorants. The 2-amino-5-nitro isomer positions the nitro group meta to the hydroxyl; this removes the direct para conjugation between the nitro group and the amino/hydroxyl pair and yields a different solubility and color-transition profile. Because all three isomers share C₆H₆N₂O₃ and the identical molar mass of 154.12 g/mol, incoming inspection cannot rely on molecular weight alone. FT-IR fingerprinting, melting range, and HPLC retention time are required to distinguish the positional isomers before batch release. The para-nitro substitution in 2-amino-4-nitrophenol decreases the electron density at the aromatic positions involved in diazo coupling, but the ortho hydroxyl-amino pair restores electron density at specific ring positions. This composite electronic profile is not reproduced by simple nitroaniline or aminophenol derivatives, and it is the basis for the compound’s use as a selective intermediate rather than a drop-in replacement for its isomers.

Certificates of analysis for 2-amino-4-nitrophenol generally state assay, water content, residue on ignition, and heavy metal content. A representative specification profile is shown below.

For personal care intermediate applications, the heavy metal specification is typically tightened to 10 mg/kg for iron and 5 mg/kg for copper. The powder is dried at 50 °C under vacuum when received moisture exceeds 0.5 %. Analytical confirmation should include HPLC at 254 nm, USP <741> melting range, and ISO 760 water determination. If particle size is specified, 98 % through a 75 µm sieve is a common fine-dispersion target for dye paste preparation. The product is typically packed in fiber drums with polyethylene liners at a net weight of 25 kg.

When Nitro-Substituted Aminophenols Are Selected for Leather and Textile Azo Dye Intermediates

In azo dye intermediate synthesis, 2-amino-4-nitrophenol is diazotized and coupled to aromatic coupling components such as β-naphthol, pyrazolones, or sulfonated naphthalenes. The diazonium salt is generated by controlled addition of sodium nitrite to an acidic slurry of the aromatic amine. Because the para-nitro group increases the electrophilicity of the diazonium species, the diazonium salt is more active than that derived from unsubstituted o-aminophenol, but it is also less thermally stable. Process control maintains the reaction mass at 0 °C to 5 °C; above 10 °C, decomposition of the aryl diazonium intermediate can become exothermic and generate tars. Coupling to β-naphthol is conducted at pH 8.0 to 9.5 with continuous pH monitoring because the free phenol concentration and the diazonium electrophilicity must be balanced. The resulting monoazo dyes are applied to leather and textile substrates after conversion to sulfonated or metal-complex forms. The ortho-amino/para-nitro arrangement tends to deepen the color of the final azo molecule relative to the para-amino/ortho-nitro isomer, and it can improve the stability of chromium complex dyes under ISO 105-C06 washing conditions. Production equipment for reproducible diazotization includes glass-lined reactors with jacket cooling, calibrated nitrite dosing pumps, and redox potential probes; excess nitrous acid is avoided because it leads to diazo tar formation and elevated total organic carbon in effluent. On production filtration lines, presscake from azo coupling can blind filter media when the diazonium feed rate is too high, causing differential pressure excursions across plate-and-frame units. Incomplete diazotization leaves free amine that may fail the residual aromatic amine specification in the finished dye, while overcoupling at high pH can produce undesired polyazo impurities. These process limits are more severe than those for unsubstituted o-aminophenol because the nitro group reduces the solubility of the diazonium salt and increases the decomposition hazard.

In personal care oxidative hair color formulations, 2-amino-4-nitrophenol is generally incorporated as a preformed aromatic nitro dye or as a shade modifier rather than as a primary oxidative coupler. The nitro group suppresses the ring from acting as a strong coupler; instead, the compound provides a yellow-to-brown component in semipermanent and oxidative color systems. Formulation concentrations are typically in the range 0.1 % to 2.0 % w/w of the colorant base. Cosmetic use requires verification of the current entry in Annex III of EC 1223/2009, and nitrosamine formation must be controlled through raw material quality and processing conditions. Formulators must also confirm that the residual heavy metal profile does not exceed the limits set for oxidative colorants, because iron and copper at the part-per-million level can catalyze decomposition of the peroxide developer and shift the final shade during the 30–45 min processing window.

Stability Boundaries in Storage and Aqueous Processing

The solid is sensitive to light, heat, and moisture. It should be stored in sealed packaging at ≤ 30 °C and below 60 % relative humidity; exposure to higher humidity can raise water content above 0.5 % and cause caking during drum unloading. The compound is incompatible with strong oxidizers, nitrosating agents in acidic media, and concentrated mineral acids unless the diazotization reaction is intended and temperature-controlled. Sodium nitrite and hydrochloric acid must never be mixed with the dry powder outside a reactor with cooling capacity. Aqueous slurries of the material are mildly acidic; the pH varies with particle size and dissolved ionic impurities, so direct discharge to process drains without neutralization is not permitted. The purified grade should also be protected from transition metal contamination because iron and copper catalyze oxidative degradation and shift the shade of downstream colorants. When a storage container is opened at relative humidity above 60 %, the material should be dried under vacuum at 50 °C before gravimetric charge to avoid batch-to-batch variation in water content.