Cations aqueous, acidity

The largest protonated cluster of water molecules yet definitively characterized is the discrete unit lHi306l formed serendipitously when the cage compound [(CyHin)3(NH)2Cll Cl was crystallized from a 10% aqueous hydrochloric acid solution. The structure of the cage cation is shown in Fig. 14.14 and the unit cell contains 4 [C9H,8)3(NH)2aiCUHnOfiiai- The hydrated proton features a short. symmetrical O-H-0 bond at the centre of symmetry und 4 longer unsymmetrical O-H - 0 bonds to 4... 

Quinoxalin-2-one is a very weak base (pK — 1.37) and so the different orientation of substitution in acetic and sulfuric acids may mean that in acetic acid the principal species undergoing nitration is the neutral molecule, and in sulfuric acid, the mono-cation. Treatment of quinoxaline-2,3-dione, or its iViV -dimethyl derivative in sulfuric acid, with 1 equivalent of potassium nitrate, results in nitration at position 6 with 2 equivalents of potassium nitrate, 6,7-dinitro compounds are formed. When quinoxaline is boiled with aqueous nitric acid, 6-... 

The significantly lower dediazoniation rate of the l/f-3,5-dimethylpyrazole-4-di-azonium ion (8.22) compared with that of the benzenediazonium ion was the central subject of an MNDO study by Brint et al. (1985). The diazonium ion 8.22 has been recovered unchanged after heating for 3 h at 100 °C in aqueous hydrochloric acid. It is not completely decomposed after a similar treatment for 48 h (Reilly and Madden, 1925). Brint et al. calculated the heats of formation of this diazonium ion and of the corresponding heteroaryl cation 8.23 (Scheme 8-16). They found that the values of A//f for the diazonium ion 8.22 and for the benzenediazonium ion are almost identical, whereas that for the cation 8.23 is much greater. The energy required to dissociate the pyrazolediazonium ion is therefore nearly twice that required for the benzenediazonium ion (A//f = 329 and 194 kJ mol-1, respectively). 

The proximity of the reaction centre to the second phenyl ring makes the aryl cation, formed by heterolytic dediazoniation, a serious competitor to the aryl radical. This is evident in Table 10-6 from various examples where the yield obtained in aqueous mineral acid (varying from 0.1 m to 50% H2S04) is higher than in the presence of an electron-transfer reagent. This competition was studied in three types of product analyses by Cohen s group (Lewin and Cohen, 1967 Cohen et al., 1977), by Huisgen and Zahler (1963 a, 1963 b), and by Bolton et al. (1986). 

The stable triphenylcyclopropenium cation (81) undergoes an electron-transfer reaction when photolyzed in acidic medium (van Tamelen et al., 1968, 1971). Irradiation of 81 for 4 hours in 10% aqueous sulfuric acid resulted in a 49% yield of hexaphenylbenzene (82). The reaction is presumed to proceed by initial charge transfer to produce the cyclopropenyl radical 83, which then couples to give 84. This compound in... 

Rate-determining protonation to give a vinyl cation rather than 1,4 addition of water has been proposed as the most consistent mechanism (25) for the acid-catalyzed hydration of arylpropiolic acids in aqueous sulfuric acid. Hydration of arylpropiolic acid closely resembles the acid-catalyzed isomeriza-... 

A number of miscellaneous reactions involving diazonium ions and possible vinyl cations have been reported. Treatment of amine 138 with sodium nitrite in 20% aqueous acetic acid is reported to give methyl cyclopropyl ketone as one of four products (116). The reaction has been postulated to involve a vinyl cation, presumably by the following sequence of reactions (116) ... 

Because of their basic properties, aikaioids were among the first naturai substances that eariy chemists extracted and purified. Morphine was isoiated from poppies in 1805 and was the first aikaioid to be characterized. When treated with aqueous strong acid, aikaioids accept protons to produce water-soiubie cations. The protonated aikaioids dissoive, ieaving the rest of the piant materiais behind. Adding strong base to the aqueous extract reverses the proton-transfer reaction, converts the aikaioid back to its neutrai base form, and causes pure aikaioid to precipitate from the soiution ... 

Nitration can be catalyzed by lanthanide salts. For example, the nitration of benzene, toluene, and naphthalene by aqueous nitric acid proceeds in good yield in the presence of Yb(03SCF3)3.5 The catalysis presumably results from an oxyphilic interaction of nitrate ion with the cation, which generates or transfers the N02+ ion.6 This catalytic procedure uses a stoichiometric amount of nitric acid and avoids the excess strong acidity associated with conventional nitration conditions. 

For a few membrane electrodes the ion selectivity is limited, e.g., the Metrohm EA301-Me2+ electrode shows the same sensitivity for various divalent cations in fact, even non-selectivity may be accepted, e.g., where only one type of cation is present as in aqueous hydrofluoric acid where the pH can be determined with a cation-exchange resin electrode40. 

Nickel and Weber [30] reported aqueous titrations of carboxylic acids, phenols, acidic drugs containing NH groups, cationic acids (ammonium salts) in dimethyl-formamide solution against 0.1 M potassium hydroxide aqueous solution as the... 

The catalytic constants for acids in aqueous solution correlate well with pK, and there is no apparent difference between uncharged and cation acids (carboxylic acids and pyridinium cations), though some steric inhibition is evident for 2,6-substituted pyridinium cations. The catalytic constants for bases behave somewhat more erratically, and it has been suggested by Gruen and MoTigue (1963b) that errors are intro-... 

Anodic oxidation of methoxybenzenes in aqueous sulphuric acid also leads to loss of the methoxy substituent, this time through jju o-substitution on the radical-cation by water. Anisole and 4-niethoxyphenol are both converted to quinone [81]. The elimination of methanol is catalysed by protons by the mechanism illustrated in Scheme 6.8. Diphenyl derivatives have also been isolated from oxidation of some methoxybenzenes. They arise through the competitive reaction involving a... 

Thianthrene, 68 X = Y = S, radical-cation is obtained by oxidation in trifluoro-acetic acid containing perchloric acid and the evaporation of the solvent [232], It shows electrophilic behaviour on the sulphur atom. When the electrochemical oxidation of thianthrene is carried out in aqueous acetic acid, the monoxide is obtained... 

Amines are stable to electrochemical oxidation in acid solution because the nitrogen lone pair is protonated and inaccessible for reaction. This is not the case for N-acetylamines, which are oxidisable at a lead dioxide anode in aqueous sulphuric acid [99]. The primary electron tiansfer step involves the amide function and leads to a radical-cation, which loses a proton from the carbon atom adjacent to nitrogen. Subsequent steps lead to an acylimmonium ion, which is trapped by water. N-acetylated primaiy amines are converted to the corresponding carboxylic acid. 

Highly significant was J. C. Warf s observations (1949) that cerium(IV) can be separated readily and completely from the trivalent cations by extraction from an aqueous nitric acid solution using tri-n-butyl phosphate. 

Only limited information about the electrochemical properties of astatine is available. The formation of a singly positively charged cation, At+, in aqueous and other solvent environments has been observed. A study in a nitric acid environment investigated the adsorption into ion exchange materials of this cation [206]. In a similar study in aqueous perchloric acid, the mobility of At+ was found to change with pH and the presence of a strongly water bound species, At(H20)2", with a pA of ca. 1.5 was proposed [207]. 

The more exact forms of the aqueous cations, with their primary hydration shells, are normally omitted from the diagrams. As is the case with Crvl, which in acid solution exists as the dichromate ion, Cr2072-, the forms of any oxo anions are indicated by their formulae in the diagrams. The diagram for chromium summarizes the following important properties ... 

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