Aminophenols

Abstract

Abstract Aminophenols and their derivatives are of commercial importance, both in their own right and as intermediates in the photographic, pharmaceutical, and chemical dye industries. They are amphoteric and can behave either as weak acids or weak bases, but the basic character usually predominates. 3‐Aminophenol is fairly stable in air unlike 2‐aminophenol and 4‐aminophenol, which easily undergo oxidation to colored products. The former are generally converted to their acid salts, whereas 4‐aminophenol is usually formulated with low concentrations of antioxidants which act as inhibitors against undesired oxidation. The simple aminophenols exist in three isomeric forms depending on the relative positions of the amino and hydroxyl groups around the benzene ring. At room temperature they are solid crystalline compounds. 2‐Aminophenol forms white orthorhombic, bipyramidal needles when crystallized from water or benzene, which readily become yellow‐brown on exposure to air and light. 3‐Aminophenol forms white prisms when crystallized from water or toluene. 4‐Aminophenol forms white plates when crystallized from water. The base is difficult to maintain in the free state and deteriorates rapidly under the influence of air to pink‐purple oxidation products. The acidity of the hydroxyl function in the aminophenols is depressed by the presence of an amino group on the benzene ring. The amino group behaves as a weak base, giving salts with both mineral and organic acids. The aminophenols are chemically reactive, undergoing reactions involving both the aromatic amino group and the phenolic hydroxyl moiety. Aminophenol undergoes a variety of cyclization reactions. Important reactions include alkylation, acylation, diazonium salt formation, cyclization reactions, condensation reactions, and reactions of the benzene ring. Aminophenols are made either by reduction of nitrophenols or by substitution. Reduction is accomplished with iron or hydrogen in the presence of a catalyst. Catalytic reduction is the method of choice for the production of 2‐ and 3‐aminophenol. Most production of the technical grade materials (95% purity) occurs on‐site, as they are chiefly used as intermediate reactants in continuous chemical syntheses. World production of the fine chemicals (99% purity) is probably no more than a few hundred metric tons yearly. Under atmospheric conditions, 3‐aminophenol is the most stable of the three isomers. Both 2‐ and 4‐aminophenol are unstable. The use of activated iron oxide in a separate cellophane bag inside the storage container inhibits the discoloration of aminophenols. The salts are more resistant to oxidation and should be used where possible. In general, aminophenols are irritants. Their toxic hazard rating is slight to moderate. 4‐Aminophenol is a selective nephrotoxic agent and interrupts proximal tubular function. Care should be taken in handling these compounds, with the wearing of chemical‐resistant gloves and safety goggles; prolonged exposure should be avoided. The aminophenols are versatile intermediates and their principal use is as synthesis precursors; their products are represented among virtually every class of stain and dye. The derivatives of the aminophenols have important uses in both the photographic and pharmaceutical industries. They are also extensively employed as precursors and intermediates in the synthesis of more complicated molecules, especially those used in the staining and dye industry.