Aminophenols separation

Oxazin-2-one ring from o-aminophenols Identification of o-aminophenols Separation of aminophenols... 

A) Benzoyl Derivative. Since acetylation and benzoylation do not always proceed smoothly with nitrophenols, it is best to reduce them to the aminophenol as in (3) above. Add an excess of 20% aqueous sodium hydroxide to the reaction mixture after reduction, cool and then add a small excess of benzoyl chloride, and shake in the usual way. The dibenzoyl derivative wiU separate. Filter, wash with water and recrystalUse. (M.ps., p. 551.)... 

The use of an amperometric detector is emphasized in this experiment. Hydrodynamic voltammetry (see Chapter 11) is first performed to identify a potential for the oxidation of 4-aminophenol without an appreciable background current due to the oxidation of the mobile phase. The separation is then carried out using a Cjg column and a mobile phase of 50% v/v pH 5, 20 mM acetate buffer with 0.02 M MgCl2, and 50% v/v methanol. The analysis is easily extended to a mixture of 4-aminophenol, ascorbic acid, and catechol, and to the use of a UV detector. 

Iron Reduction. The reduction of nitrophenols with iron filings or turnings takes place in weakly acidic solution or suspension (30). The aminophenol formed is converted to the water soluble sodium aminopheno1 ate by adding sodium hydroxide before the iron-iron oxide sludge is separated from the reaction mixture (31). Adjustment of the solution pH leads to the precipitation of aminophenols, a procedure performed in the absence of air because the salts are very susceptible to oxidation in aqueous solution. 

In another process variant, only 88% of the nitrobenzene is reduced, and the reaction mixture then consists of two phases the precious metal catalyst (palladium on activated carbon) remains in the unreacted nitrobenzene phase. Therefore, phase separation is sufficient as work-up, and the nitrobenzene phase can be recycled direcdy to the next batch. The aqueous sulfuric acid phase contains 4-aminophenol and by-product aniline. After neutralization, the aniline is stripped, and the aminophenol is obtained by crystallization after the aqueous phase is purified with activated carbon (53). 

Several gas-Hquid chromatographic procedures, using electron-capture detectors after suitable derivatization of the aminophenol isomers, have been cited for the deterrnination of impurities within products and their detection within environmental and wastewater samples (110,111). Modem high pressure Hquid chromatographic separation techniques employing fluorescence (112) and electrochemical (113) detectors in the 0.01 pg range have been described and should meet the needs of most analytical problems (114,115). 

Under atmospheric conditions, 3-aminophenol is the most stable of the three isomers. Both 2- and 4-aminophenol are unstable they darken on exposure to air and light and should be stored in brown glass containers, preferably in an atmosphere of nitrogen. The use of activated iron oxide in a separate cellophane bag inside the storage container (116), or the addition of staimous chloride (117), or sodium bisulfite (118) inhibits the discoloration of aminophenols. The salts, especially the hydrochlorides, are more resistant to oxidation and should be used where possible. 

Aromatic amines that have been used include o-toluidine, p-aminosali-cylic acid, p-aminobenzoic acid, diphenylamine and p-aminophenol. Their ability to react preferentially with a particular carbohydrate or class of carbohydrate is often useful, e.g. p-aminophenol, which shows some specificity for ketoses compared with aldoses and is useful for measuring fructose. These reagents have proved particularly useful for the visualization and identification of carbohydrates after separation of mixtures by paper or thin-layer chromatography, when colour variations and the presence or absence of a reaction aid the interpretation of the chromatogram. 

C.-E. Lin, Y.-T. Chen and T.-Z. Wang, Separation of benzenediamines, benzenediols and aminophenols in oxidative hair dyes by micellar electrokinetic chromatography using cationic surfactants. J. Chromatogr.A, 837 (1999) 241-252. 

This system resolved the aniline peak (retention time (rt) = 2.67 min) from the benzidine peak (rt = 2.27 min) as can be seen in Figure 2. Other potential interferences were selected for study by looking at the expected fragments from the reduction of various dyes. Reduced dye samples were spiked with aniline (rt = 2.67 min), -aminophenol (rt = 1.97 min), -phenylenediamine (rt = 1.93 min) and -nitroaniline (rt = 3 16 min). None of these materials interfered with the detection of the benzidine peak. To determine if other types of dyes might interfere with the analysis, two sets of filters were spiked at low and high levels separately with C.I. Direct Red 28 (13 7 yg and 137 yg), C.I. Direct Blue 53 formulation (o-tolidine-based) (21.2 yg and 212 yg) and C.I. Direct Blue 8 formulation (o-dianisidine-based)(23.3 yg and 233 yg). 

Separation and detection of p-aminophenol and ascorbic acid has also been evaluated in Topas microchips using different end-channel amperometric detectors. Thus, platinum- and gold-wire, screen-printed carbon electrode and gold film have been used as working electrodes [77]. 

Pumera et al. [34] used gold nanoparticle enhanced open channel NCEC for the separation of p-aminophenol, o-aminophenol, and m-aminophenol in a bare glass channel and polydiallyl dimethyl ammonium chloride (PDADMAC) gold coated channel. The mobile phase used was acetate buffer (20 mM, pH 5.0) with electrokinetic sample injection at 1.5kV/cm for 3 seconds and 2.0kV/cm as separation voltage (Fig. 7.15). Broyles... 

Figure 7.15 Chromatogram of the separation of (1) /j-aminophcnol, (2) o-aminophenol, and (3) m-aminophenol, (A) without treatment and (B) after PDADMAC gold coating chips [34].

Aminophenols which were formerly obtained only by the reduction of nitro-compounds in concentrated sulphuric aoid can now be prepared by reducing dilute acid suspensions of nitro-compounds, provided the mixture be well stirred and the cathode surfaces made up. of two or more metals. This improved process, which it is claimed gives good yields of amino-hydroxy bodies, is due to the Society of Chemical Industry, Basle.1 When an indifferent cathode is employed, the addition of certain metals in the form of salts or finely powdered metal to the electrolyte increases the yield of amine at the expense of amino-hydroxy compound such are copper, iron, or lead if added separately. If, however, two at least of these and other metals be added, reduction to aminophenol is favoured. 

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