2-Aminophenol und Anwendungen

2-Aminophenol is an organic compound with the chemical formula C6H7NO.Along with its isomer 4-aminophenol,it is an amphiphilic molecule and reducing agent.It is a useful reagent for the synthesis of dyes and heterocyclic compounds.Reflecting its slightly hydrophilic nature, the white powder is moderately soluble in alcohols and recrystallizes from hot water.2-hydroxyaniline, 2-amino-1-hydroxybenzene, C.I. oxidized base 17, C.I. 76520, C6H7NO, Mr 109.13, forms white orthogonal bipyramidal needles when crystallized from water or benzene Structure, melting point 174 C.The basic unit cell of the crystal has eight molecules with a density of 1.328 g/cm3.Under reduced pressure (1.47 kPa), 2-aminophenol sublimes rapidly at 153 C without decomposition.The acid-base dissociation constants are pK1 4.72 (water at 21 C), 4.66 (1 vol% ethanol aqueous solution at 25 C); pK2 9.66 (water at 15 C), 9.71 (. Salts: hydrochloride needle salt, mp 207 C; formate, mp 120 C; oxalate, melting point 167.5 C (decomposition); acetate, mp 150 C.

Synthesis and structure 

2-Aminophenol (and its isomer 4-aminophenol) are synthesized industrially by reducing the corresponding nitrophenols with hydrogen in the presence of various catalysts.Nitrophenols can also be reduced with iron.This compound exhibits intramolecular and intermolecular hydrogen bonding involving adjacent amine and hydroxyl groups.Thus, 2-aminophenol has a relatively high melting point (174 °C) compared to other compounds of similar molecular weight; for example, 2-methylphenol melts at 31 °C.

Applications

2-Aminophenol has many uses. As a reducing agent, it is marketed under the names Atomal and Ortol for developing black and white photographs. 2-Aminophenol is an intermediate in the synthesis of dyes.It is especially useful in the production of metal complex dyes when diazotized and coupled with phenol, naphthol, or other aromatic or resonant dye species.Metal complex dyes using copper or chromium are often used to produce dull colors.Tridentate ligand dyes are useful because they are more stable than their bidentate or monodentate counterparts.

Physical Properties

Simple aminophenols exist in three isomeric forms depending on the relative positions of the hydroxyl and amino groups around the benzene ring.At room temperature, they are solid crystalline compounds.Commercial-grade compounds are often impure due to contamination by oxidation products and may have a yellowish-brown or pinkish-purple tinge, which is more common with 2- and 4-aminophenols than with the 3-isomer.

Chemical Properties

The chemical properties and reactions of aminophenols can be found in detail in many standard chemistry textbooks, only a summary is given here.The presence of amino groups on the benzene ring reduces the acidity of phenols; this phenomenon is most pronounced in 4-aminophenols.They also behave as weak bases, forming salts with inorganic and organic acids.Aminophenols are true ampholytes and do not have a zwitterionic structure; therefore, they exist as neutral molecules or as ammonium cations or phenate anions depending on the pH of the solution.However, deviations from the theoretical curves observed during acid-base titrations lead to the assumption of the presence of the hemi-salt complex cation B+2, formed by the association of the ammonium cation B+ with the neutral molecule B.This association was most pronounced for 4-aminophenol.Aminophenols are chemically reactive and undergo reactions involving the aromatic amino and phenolic hydroxyl moieties,as well as substitutions on the benzene ring.Oxidation leads to the formation of highly pigmented polymeric quinoid structures.2-Aminophenol undergoes various cyclization reactions

Alkylation

All possible mono-di and trimethylated aminophenols are known.Monoalkylation occurs when aminophenols are heated with appropriate alkyl halides or with alcohol and Raney nickel; equivalent or even better results can be obtained using aldehydes or ketones instead of alcohol. Specific alkylation of hydroxyl groups to form methoxyanilines (anisidine) or ethoxyanilines (phenethylamines) is difficult due to the reactivity of the amino groups; often mixed alkylated products are obtained. 3-Methoxyaniline can be prepared by methylating 3-aminophenol under basic conditions, but it is more common to protect the amino group, methylate 3-acetamidophenol, and then hydrolyze.Other anisidines and phenetidines are prepared indirectly by reduction of nitro analogs:

Acylation

Acylation of aminophenols (using acetic anhydride in base or pyridine, acetyl chloride, and pyridine in toluene, or ketene in ethanol) usually leads to N-acylated products.However, if an excess of reagents is used, especially 2-aminophenol, O,N-diacylated products will form.Aminophenyl carboxylates (O-acylated aminophenols) are usually prepared by reducing the corresponding nitrophenyl carboxylates, which is especially important for 4-aminophenol derivatives. Both 2- and 4-aminophenol acylation products are known to undergo acyl migration from the O-position to the N-position.2-Aminophenyl ethyl carbonate slowly rearranges to 2-hydroxyphenyl ethyl carbamate in dilute acid; the corresponding 4 derivative does not undergo this particular reaction.