Thallium ions, reactions

Thallium(I) acetate, 24 630-632 Thallium compounds, in organic reactions, 24 635 Thallium formate, 24 630 Thallium halides, 24 632 Thallium ion, 24 629 Thallium nitrate, uses for, 24 636 Thallium salts, 24 630, 632 Thallium sulfate, uses for, 24 636 Thallium (III) compounds, in organic reactions, 24 635-636 Thallium(III) fluoride, 24 632 Thallium(III) ion, 24 630 Thallium(III) salts, 24 632 Thallium(III) trifluoroacetate, 24 635 Thallium (Tl), 24 627-641... 

Thallium(i) salts have long been used in reactions with organic and organometallic halide complexes as a means of activating the halide by removal as insoluble T1X. However, the thallium ions proved not to be innocent bystanders, and numerous examples were reported in COMC (1995) where the metal-bound thallium complexes were formed. Deliberate reactions of thallium(i) and thallium(m) salts with metal carbonyl anions have yielded a variety of complexes of the form T1 MLJ3. In the past decade, new examples of metal carbonyl derivatives of thallium have been prepared (see Table 2). In addition, the propensity for Tl+ to form adducts with 16-electron noble metal complexes has been exploited. 

The related reaction shown in Equation (104)117 leads to a butterfly arrangement with two thallium ions bridging between two gold atoms, 127. Here, the Tl-Tl distance is 360.27 pm and is thought to contribute significantly to the physical properties of the complex. The compound shows solvent-dependent luminescent behavior in solution as well as in the solid state. 

One of the few catalysts to give reasonably selective metathesis to form ethene and hex-3-ene, reaction (6), is M0O3/AI2O3 doped with 2% alkali metal ions (Bradshaw 1967 Alkema 1968) or thallium ions (Kobylinski 1972). The suppression of the isomerization is more effective the larger the cation Cs+ > T > Rb" > K > Na" " > Li it also correlates with a diminished ability to adsorb ammonia. The polarizability of the cation thus appears to be an important factor in reducing the surface acidity which is the cause of isomerization. 

Solvatomercuration (40) is a polar addition which follows the Markovnikov rule. Because acetoxymercuric ion is a soft acid, the reaction proceeds facilely. Thallium ions are also soft, hence they attack multiple bonds efficiently. As Tl(III) is also an oxidant, the initial solvatothallation products often undergo rearrangements (41,42) unobserved during solvatomercuration. 

A similar situation exists in the hydroxymercuration and thallium(m) oxidation of cycloalkenes and cycloalkanes, the rate laws showing strict second-order behaviour for both metal ions. In the thallium(ni) reactions two products have been observed, a 1,2-diol and a ketone or aldehyde. Two-electron coupling with trifluoroacetate has been shown to promote oxidative phenol coupling. The mechanism of oxidation of olefins by palladium(n) has been investigated. In the presence of PdClg in aqueous media, the data conform to the rate expression... 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

After reaction, the activity of a 25.0-mL water sample is 745 counts per minute (cpm), caused by the presence of Tl+-204 ions. The activity of Tl-204 is 5.53 X 105 cpm per gram of thallium metal. Assuming that 02 is the limiting reactant in the above equation, calculate its concentration in moles per liter. 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

Selectivity to primary metathesis products is usually less than 100%, as a consequence of side reactions, such as double-bond migration, dimerization, oligomerization, and polymerization. The selectivity can be improved by adding small amounts of alkali or alkaline earth metal ions, or, as has recently been shown, thallium 40), copper, or silver ions (41)-... 

The equation for a net chemical reaction represents the overall transformation of reactants into products. Thus, thallium Ill) ions oxidize iron(II) ions according to Eq. (1-1), and a secondary amine reacts with an aryl chloride as in Eq. (1-2). 

Consider the oxidation of mercurous ion by thallium(3+) ion in aqueous solution.13 The reaction and rate law are... 

The mechanisms of these reactions are presumably analogous to those of the Pr6vost and Woodward-Prevost reactions. In the first step of the reaction of iodine and thallium(I) acetate with cyclohexene in both parts A and B of this procedure, trans-2-iodocyclohexyl acetate is formed. The second equivalent of thal-lium(I) acetate scavenges iodide ion during formation of the 1,3-... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

The utility of thallium(III) salts as oxidants for nonaromatic unsaturated systems is a consequence of the thermal and solvolytic instability of mono-alkylthallium(III) compounds, which in turn is apparently dependent on two major factors, namely, the nature of the associated anion and the structure of the alkyl group. Compounds in which the anion is a good bidentate ligand are moderately stable, for example, alkylthallium dicar-boxylates 74, 75) or bis dithiocarbamates (76). Alkylthallium dihalides, on the other hand, are extremely unstable and generally decompose instantly. Methylthallium diacetate, for example, can readily be prepared by the exchange reaction shown in Eq. (11) it is reasonably stable in the solid state, but decomposes slowly in solution and rapidly on being heated [Eq. (23)]. Treatment with chloride ion results in the immediate formation of methyl chloride and thallium(I) chloride [Eq. (24)] (55). These facts can be accommodated on the basis that the dicarboxylates are dimeric while the... 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

During oxidation of tin(II) ions by hydrogen peroxide, iodine, bromine, mercury(ir) and thallium(III) the induced reduction of cobalt(in) complexes cannot be observed. Therefore, it was concluded that these reactions proceed by 2-equivalent changes in the oxidation states of the reactants. 

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

For practical applications it is important to minimize the production of the intermediate peroxide, and to ensure that the reaction goes all the way to water. Sometimes this can be ensured by the addition of a suitable catalyst. A case in point is oxygen reduction on gold from alkaline solutions. At low and intermediate overpotentials the reaction produces only peroxide in a two-electron process at high overpotentials the peroxide is reduced further to water. The addition of a small amount of Tl+ ions to the solution catalyzes the reaction at low overpotentials, and makes it proceed to water. Thallium forms a upd layer at these potentials it seems that a surface only partially covered with T1 is a good catalyst, but the details are not understood [3]. 

The only new report of a group 7 complex with thallium is the reaction of [Re7C(CO)2i]3 ion with T1PF6 (Equation (92)).94 The thallium adds in a triply bridging fashion opposite to the capping Re(CO)3 unit 99.94 The complex is stable in dichloromethane but dissociates in coordinating solvents. In acetone, infrared data indicated that the complex would be 99% dissociated at concentrations of the cluster of about 10-4M. Addition of halide ions to dichloromethane solutions causes a precipitation of the thallium(i) halide. 

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