Aminophenols, oxidation

SYNS 4-AMINO-2-NITROPHENOL C.I. 76555 FOURRINE 57 FOURRINE BROWTS PR FOURRINE BROWN PROPYL 4-HYDROXY-3-NITROANILINE NCI-C03963 o-NITRO-p-AMINOPHENOL OXIDATION BASE 25... 

Incorporation of electroactive ferrocene groups into PAMAM dendrimers for enhanced electrochemical signal has also been reported [62]. In this work ferrocene functionalized dendrimers were immobilized onto a SAM-modified Au electrode, as shown in Fig. 14.13, and served as an immobilization matrix for the capture probe DNA, and as an electrocatalyst for p-aminophenol oxidation, p-aminophenol was produced by alkaline phosphatase labeled detection probe DNA used in a sandwich-t3 e enzyme-linked DNA assay. The authors show that use of the ferrocene functionalized dendrimers lead to a significant enhancement in electrochemical signal resulting in a 100 pM detection limit. 

It is thus possible to come up with some general remarks on the reactivity of possible oxidants for NADH. Two-electron-proton acceptors are better oxidants than one-electron-nonproton acceptors, aromatic diimines are better than quinones and oxidized aminophenols, oxidants with the catalytic functionality... 

Metol (A -methyl-p-aminophenol) oxidation products were extracted from water and separated on a C g column (A = 271 nm) using a 15-min 95/5 100/0 water... 

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. 

Oxidation H ir Colorant. Color-forming reactions are accompHshed by primary intermediates, secondary intermediates, and oxidants. Primary intermediates include the so-called para dyes, -phenylenediamine, -toluenediamine, -aminodiphenylamine, and p- am in oph en o1, which form a quinone monoimine or diimine upon oxidation. The secondary intermediates, also known as couplers or modifiers, couple with the quinone imines to produce dyes. Secondary intermediates include y -diamines, y -aminophenols, polyhydroxyphenols, and naphthols. Some of the more important oxidation dye colors are given in Figure 1. An extensive listing is available (24,28). 

Also present but not essential in permanent hair colorants are nitro dyes which dye hair without oxidation. These dyes, nitro derivatives of aminophenols and benzenediamines, impart yellow, orange, or red tones. Although they have good tinctorial value, they are not as colorfast as the oxidative dyes. They also are used in semipermanent hair colorants. 

Resorcinol Derivatives. Aminophenols (qv) are important intermediates for the syntheses of dyes or active molecules for agrochemistry and pharmacy. Syntheses have been described involving resorcinol reacting with amines (91). For these reactions, a number of catalysts have been used / -toluene sulfonic acid (92), zinc chloride (93), zeoHtes and clays (94), and oxides supported on siUca (95). In particular, catalysts performing the condensation of ammonia with resorcinol have been described gadolinium oxide on siUca (96), nickel, or zinc phosphates (97), and iron phosphate (98). 

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 (2) is fairly stable in air unlike 2-aminophenol (1) and 4-aminophenol (3) which easily undergo oxidation to colored products. The former are generally converted to their acid salts, whereas 4-amiaophenol is usually formulated with low concentrations of antioxidants which act as inhibitors against undesired oxidation. 

Aminophenol. This compound 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 crystals exist in two forms. The a-form (from alcohol, water, or ethyl acetate) is the more stable and has an orthorhombic pyramidal stmcture containing four molecules per unit cell. It has a density of 1.290 g/cm (1.305 also quoted). The less stable P-form (from acetone) exists as acicular crystals that turn into the a-form on standing they are orthorhombic bipyramidal or pyramidal and have a hexamolecular unit (15,16,24) (see Tables 3—5). 

The aminophenols are chemically reactive, undergoing reactions involving both the aromatic amino group and the phenoHc hydroxyl moiety, as weU as substitution on the benzene ring. Oxidation leads to the formation of highly colored polymeric quinoid stmctures. 2-Aminophenol undergoes a variety of cyclization reactions. 

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. 

Another method employed is the treatment of aqueous solutions of aminophenols with activated carbon (81,82). During this procedure, sodium sulfite, sodium dithionite, or disodium ethylenediaminotetraacetate (82) is added to increase the quaUty and stabiUty of the products and to chelate heavy-metal ions that would catalyze oxidation. Addition of sodium dithionite, hydrazine (82), or sodium hydrosulfite (83) also is recommended during precipitation or crystallization of aminophenols. 

More specifically, 2-aminophenol can be detected in solution using an iron(II) sulfate—hydrogen peroxide reagent (94). 3- Aminopheno1 has been analyzed colorimetrically by oxidation in base and subsequent extraction of a violet quinoneimide dye (95). A colorimetric method using... 

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. 

Aminophenol is a selective nephrotoxic agent and intermpts proximal tubular function (121,122). Disagreement exists concerning the nephrotoxity of the other isomers although they are not as potent as 4-aminophenol (123,124). Respiration, oxidative phosphorylation, and ATPase activity are inhibited in rat kidney mitochondria (125). The aminophenols and their derivatives are inhibitors of 5-Hpoxygenase (126) and prostaglandin synthetase... 

Because the aminophenols are oxidized easily, they tend to remove oxygen from solutions. Hence, if they are released from industrial waste waters into streams and rivers, they will deplete the capacity of these environments to sustain aquatic life. Concern has also been raised that chlorination of drinking water may enhance the toxicity of aminophenols present as pollutants (138) chlorinated aminophenols are known to be more toxic (139). 

Aminophenol [95-55-6] M 109.1, m 175-176 , pKj 4.65, pKj 9.75. Purified by soln in hot water, decolorised with activated charcoal, filtered and cooled to induce crystn. Maintain an atmosphere of N2 over the hot phenol soln to prevent its oxidation [Charles and Preiser J Am Chem Soc 74 1385 7952]. Can also be crystd from EtOH. 

Pyrogels used by the US armed forces include (1) PT1, which is a complex mixt based on a paste of Mg and an oxidizer, bound with petroleum distillate and asphalt. Isobutyl methacrylate is used as a thickener. (2) PT2, which contains 5% isobutyl methacrylate as a thickener, together with Ba nitrate and a small quantity of asphalt. (3) PTV, which is described as an improved oil and metal incendiary mixt composed of 5% poly butadiene, 6% Na nitrate, 28% Mg, and a trace of p-aminophenol in 60% gasoline (Ref 5)... 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3, so-called neutral diazotization) or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. Kozlov and Volodarskii (1969) measured the rates of diazotization of l-amino-2-naphthol-4-sulfonic acid in the presence of one equivalent of 13 different sulfates, chlorides, and nitrates of di- and trivalent metal ions (Cu2+, Sn2+, Zn2+, Mg2+, Fe2 +, Fe3+, Al3+, etc.). The rates are first-order with respect to the added salts. The highest rate is that in the presence of Cu2+. The anions also have a catalytic effect (CuCl2 > Cu(N03)2 > CuS04). The mechanistic basis of this metal ion catalysis is not yet clear. 

Polypyrrole shows catalytic activity for the oxidation of ascorbic acid,221,222 catechols,221 and the quinone-hydroquinone couple 223 Polyaniline is active for the quinone-hydroquinone and Fe3+/Fe2+ couples,224,225 oxidation of hydrazine226 and formic acid,227 and reduction of nitric acid228 Poly(p-phenylene) is active for the oxidation of reduced nicotinamide adenine dinucleotide (NADH), catechol, ascorbic acid, acetaminophen, and p-aminophenol.229 Poly(3-methylthiophene) catalyzes the electrochemistry of a large number of neurotransmitters.230... 

Protecting diisopropyl ether against oxidation requires 16 ppm of N-benzyl-4-aminophenol or 50 ppm of diethylenetriamine. 

Polyhydroxybenzenes or aminophenol derivatives can be considered as leuco quinones or leuco quinoneimines. They are easily oxidized and couple... 

A tridentate N02 donor set complex was formed from 2-pyridinecarboxaldehyde N-oxide and 2-aminophenol and gave mononuclear and trinuclear compounds. The trinuclear zinc complex was structurally characterized and revealed bridging acetate groups, [ZnLZn(OAc)4ZnL].865... 

Oxidation of the tetrahydroindolizine 338 with sodium chlorite gives the lactam 339, which rapidly lactonizes to give the pyranoindolizine 340 (Equation 122) <2001T8647>, and the one-pot , three-component reaction between the keto ester 235, acrolein, and o-aminophenol (formally analogous to that of Equation 63) gives the benzoxazolo-naphthyridine 341 (Equation 123) <20010L2145>. 

Hemoglobin is another heme-containing protein, which has been shown to be active towards PAH, oxidation in presence of peroxide [420], This protein was also modified via PEG and methyl esterification to obtain a more hydrophobic protein with altered activity and substrate specificity. The modified protein had four times the catalytic efficiency than that of the unmodified protein for pyrene oxidation. Several PAHs were also oxidized including acenaphthene, anthracene, azulene, benzo(a)pyrene, fluoranthene, fluorene, and phenanthrene however, no reaction was observed with chrysene and biphenyl. Modification of hemoglobin with p-nitrophenol and p-aminophenol has also been reported [425], The modification was reported to enhance the substrate affinity up to 30 times. Additionally, the solvent concentration at which the enzyme showed maximum activity was also higher. Both the effects were attributed to the increase in hydrophobicity of the active site. 

Alternative metabolic pathways involve ring-oxidation and peroxidation of arylamines. Although ring-oxidation is generally considered a detoxification reaction, an electrophilic iminoquinone (X) can be formed by a secondary oxidation of the aminophenol metabolite (18,19). Lastly, reactive imines (XI) can be formed from the primary arylamines by peroxidase-catalyzed reactions that involve free radical intermediates (reviewed in 20). 

In AChE-based biosensors acetylthiocholine is commonly used as a substrate. The thiocholine produced during the catalytic reaction can be monitored using spectromet-ric, amperometric [44] (Fig. 2.2) or potentiometric methods. The enzyme activity is indirectly proportional to the pesticide concentration. La Rosa et al. [45] used 4-ami-nophenyl acetate as the enzyme substrate for a cholinesterase sensor for pesticide determination. This system allowed the determination of esterase activities via oxidation of the enzymatic product 4-aminophenol rather than the typical thiocholine. Sulfonylureas are reversible inhibitors of acetolactate synthase (ALS). By taking advantage of this inhibition mechanism ALS has been entrapped in photo cured polymer of polyvinyl alcohol bearing styrylpyridinium groups (PVA-SbQ) to prepare an amperometric biosensor for... 

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