Weakly acidic medium

Kinetic mles of oxidation of MDASA and TPASA by periodate ions in the weak-acidic medium at the presence of mthenium (VI), iridium (IV), rhodium (III) and their mixtures are investigated by spectrophotometric method. The influence of high temperature treatment with mineral acids of catalysts, concentration of reactants, interfering ions, temperature and ionic strength of solutions on the rate of reactions was investigated. Optimal conditions of indicator reactions, rate constants and energy of activation for arylamine oxidation reactions at the presence of individual catalysts are determined. 

Note It is necessary to remove acid mobile phases completely, since the color reaction only occurs in neutral to weakly acid medium. This is often difficult when cellulose layers are employed so that interference can occur. 

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. 

Although Hammett convincingly explained the nitrosation of aliphatic amines and the diazotization of aniline under the conditions employed by Schmid and others, one unsatisfactory point remained namely the second-order kinetic equation obtained by Hantzsch and the workers who followed him for diazotization in a more weakly acidic medium. Comparison of experimental details shows that at concentrations of free mineral acid below 0.05 m the reaction is apparently second-order, but it becomes third-order at higher concentrations of acid. 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

A series of metal cations (Ni, Fe, Co, Cu, Pt) form colored complexes with dimethylglyoxime in ammonia solution or weakly acidic medium. 

Florence [98] carried out anodic stripping voltammetry of bismuth in a weakly acidic medium, with a polished vitreous carbon electrode mercury-... 

Lastly, iodate (I03 ) in a weak acidic medium quantitatively oxidizes KI to an equivalent amount of iodine, as expressed below ... 

Hg2f, Cd21, Zn21, As3+, hsch2.chsh.ch2oh Precipitation in weakly acidic medium... 

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. 

When these CCH conditions were applied to o-iodonitrobenzene (1), cr iodoaniline (2) together with iodoaniline (4) and nitrobenzene (1) were obtained in the yields indicated in Scheme 1 (88% mass balance), which corresponds to a 62% selectivity for the formation of o iodoaniline (3). Under the same conditions, the ECH of o-iodonitrobenzene (2) gave aniline (90% yield) as the sole product. There was complete hydrogenolysis of the C-I bond (no selecti vity). Thus, in basic medium (pH 12.5), CCH method is much more selective than ECH. However, in weakly acidic medium (pH 3, pyridine.HCl buffer), it has been reported that ECH at a RCu cathode in methanol-water 95 5 (v/v) gave o-iodoaniline in a 97% yield (1, 3). 

Problem 18.36 Explain the following conditions used in coupling reactions (a) excess of mineral acid during diazotization of arylamines, (b) weakly acidic medium for coupling with ArNH, (c) weakly basic solution for coupling with ArOH. ... 

In the case of phenols, either C- or (9-protonated ion 12 or 13 are formed, depending upon the acidity of the medium. In relatively weak acid medium such as H2SO461 only O-protonation is observed. However, in HSO3F only the C-protonated species is formed.62 In a much stronger acid system such as HSC F-SbFs (Magic Acid), both C-or O-protonated dication 14 can be observed. Both the C- and O-protonated ions were shown to exist in the gas phase by IR photodissociation spectroscopy.63... 

Prakash, Olah, and co-workers256 have prepared Mosher s acid analogs by the hydroxyalkylation of substituted benzenes with ethyl trifluoropyruvate [Eq. (5.95)]. Deactivated aromatics (fluorobenzene, chlorobenzene) required the use of excess triflic acid indicative of superelectrophilic activation.3 5 In contrast to these observations, Shudo and co-workers257 reported the formation gem-diphenyl-substituted ketones in the alkylation of benzene with 1,2-dicarbonyl compounds [Eq. (5.96)]. In weak acidic medium (6% trifluoroacetic acid-94% triflic acid), practically no reaction takes place. With increasing acidity the reaction accelerates and complete conversion is achieved in pure triflic acid, indicating the involvement of diprotonated intermediates. 

Among the reactive groups in wool the most important are amino, imino, and hydroxyl groups. Reactions occur in a weakly acidic medium (pH 3-5) and include nucleophilic substitution of leaving groups (usually Cl, F, and, rarely, sulfonate or ammonium groups) or addition reactions to polar aliphatic double bonds. 

Since the 1 2 dye complexes are formed in a weakly acidic medium, they can be applied to wool at a higher pH. This enables a gentle mode of dyeing. 

Little is known quantitatively about substituent effects in the nitration of derivatives of azanaphthalenes. In preparative experiments 4-hydroxy-quinoline, -cinnoline, and -quinazoline give the 6- and 8-nitro compounds, but with nitric acid alone 4-hydroxyquinoline and 2,4-di-hydroxyquinoline react at C(3).31 With nitric acid, 4-hydroxycinnoline still gives mainly 4-hydroxy-6-nitrocinnoline, but some of the 3-nitro compound can also be isolated.81 51 The change of orientation with reagent could be due to a change to free-base nitration in the more weakly acidic medium, or to the occurrence in nitric acid of nitration via nitrosation.31... 

Oxidative cleavage of alkenes to carboxylic acids.1 Alkenes are oxidized to carboxylic acids by H202 (35%) catalyzed by H2W04 in a weakly acidic medium (pH 4-5) maintained by addition of KOH. The oxidation probably involves initial oxidation to a 1,2-diol followed by dehydrogenation to an a-ketol, which is then cleaved to a mono- or dicarboxylic acid. 

The Irradiated Nl foils are dissolved in HHOg end the Cu activity is separated by spontaneous deposition on Zb dust In weakly acidic medium or by pptn. of Cu(I) thiocyanate after reduction with sulfurous acid. Then the filtrate Is counted for the zinc. 

Hydrofluoric acid Neutral or weak acid medium - [28]... 

Dihydroxy tartaric acid Neutral or weakly acid medium 10-100 [33]... 

The alumino and gallophosphates were formed [7] in the systems amine-Al(Ga)203—P205—H20 using the same way of the synthesis and the same range of temperatures and pressure. However, the introduction of phosphoric acid in the synthesis lowered the pH of the solutions to a neutral or weakly acidic medium which favours higher coordinations for aluminium and gallium ions. The combined effect of pH and of the shape of the amines lead to ca two hundred structure types. 

The imine 81 has been transformed into several decarboxybetalains. With (5)-cyclodopa (7) the reaction takes place in 1 N HCl and affords the blue-violet 17-decarboxybetanidin (19) in 34% yield. For the reaction with L-proline, neutral conditions have to be used because of the instability of the resulting yellow 13-decarboxyindicaxanthin (83). Indoline reacts in a weakly acidic medium and affords the red 14-decarboxyindobetalain (82) in 78% yield. Compounds 19 and 83 were identical with the compounds obtained from thermolysis of the natural products (33,34). It is noteworthy that the highest wavelength absorption maxima are shifted toward shorter wavelengths in comparison to the corresponding be-... 

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