Thallium ions, oxidation

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). 

We will use the example of thallium ion. The potential of the working electrode will be stepped from a potential at which only Tl" " is the stable form to a potential at which only is the stable form. Figure 6.2(a) shows a plot of potential against time - note that the rise in potential here is essentially vertical. It would be completely vertical but for the requirement to charge the doublelayer around the electrode. The potential before the step is, e.g. 0 V, i.e. it is well cathodic (negative) of, .,+(= 1.252 V), so Tl is the only stable redox form, and no Tl " " will form. Thie potential after the step is, e.g. 1.6 V, i.e. it is well anodic of 3+. p,+, so Tl is the only stable redox form here, thus causing Tl" to oxidize to Tl +. 

The value of[Tl ] in the solution bulk remains essentially constant since only a tiny proportion of the overall amount of Tl is oxidized, but at the surface of the electrode we can say, to a good approximation, that [Tt ] = 0. Very soon after the potential is stepped, Tl from the bulk diffuses toward the electrode, thereby attempting to even out the concentration gradient, i.e. to replenish the Tt" that was consumed at the commencement of the step. W need to recognize, however, that these thallium ions will not remain as Tl for long as they will be oxidized immediately to form Tl, i.e. as soon as they impinge on the electrode. The end result is that a concentration gradient will soon form after the potential has been stepped. 

Among some metal oxygen compounds which add, palladium and thallium ion both oxidize olefins and apparently the initial step is the addition of a metal hydroxide across the olefin double bond. The intermediates have not been isolated because they go on to other products but kinetic and other evidence indicates that the addition of the hydroxide is the initial step. In the well known mercury acetate addition to olefins in alcohol solution one can isolate the /S-hydroxv or alkoxy ethylmercury derivatives. 

Scheme 4 shows the ionic substitution of a phenol by a metal phenolate compound, by a concerted mechanism. Although similar to Scheme 3, a phenoxonium ion is avoided, with probable energetic advantages thallium(III) oxidations of phenols may follow such a path. 

Of these three metal compounds, thallium compounds such as T1(N03)3 and T1(0C0CF3)3 have been widely utilized in organic synthesis. Both TP+ and Pb" + ions are isoelectronic and the former is a less powerful oxidant than the Pb(IV) ion. Oxidizing reactivities of Tl+ salts vary with the anion associated with the metal, the solvent and other factors. On treatment with T1(N03)3 [TTN], phenols generally undergo two-electron oxidation forming phenoxonium ions which will be attacked by a variety of nucleophiles. 

A mechanism involving an intermediate enediolate ion with the rate of reaction being equal to the rate of enolizatiori was proposed. The kinetics and mechanism of thallium(III) oxidation of cellobiose in acid medium have been determined the acid-catalyzed reaction was first order in each reactant, and the active oxidant was thallium(III) diacetate. The sugar products were identified as D-gluconic acid and glucose. ... 

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. 

The oxidative bromination of Cs-Cg olefins can be carried out using a system composed of catalytic thallium(iii) oxide (0.2 M of Tl(iii)) and a vanadium-based heteropolyacid cocatalyst (HPA-n = H3+ PMOi2 V 04o n = 2-8) in the presence of hydrogen bromide solution (1.0 M of Br ) and oxygen as co-oxidant. The HPA-n cocatalyst can promote oxidation of thal-lium(i) to thallium(iii) in the presence of bromide ions. For instance, the reaction of 1-hexene with the thallium(iii)/HPA-6/bromide system at 100 °C and 8 bar of oxygen led to a mixture of 1,2-dibromohexane and hex-enebromohydrin along with a small amount of 2-hexanone in different ratios. In the absence of thallium(m) species, the oxidation process proceeds slowly, presumably due to the HPA-6 anion-mediated oxidation of bromide to bromine. 

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... 

In small-scale syntheses, a wide variety of oxidants have been employed in the preparation of quinones from phenols. Of these reagents, chromic acid, ferric ion, and silver oxide show outstanding usefulness in the oxidation of hydroquinones. Thallium (ITT) triduoroacetate converts 4-halo- or 4-/ f2 -butylphenols to l,4-ben2oquinones in high yield (110). For example, 2-bromo-3-methyl-5-/-butyl-l,4-ben2oquinone [25441-20-3] (107) has been made by this route. 

Unlike boron, aluminum, gallium, and indium, thallium exists in both stable univalent (thaHous) and trivalent (thaUic) forms. There are numerous thaHous compounds, which are usually more stable than the corresponding thaUic compounds. The thaUium(I) ion resembles the alkaU metal ions and the silver ion in properties. In this respect, it forms a soluble, strongly basic hydroxide and a soluble carbonate, oxide, and cyanide like the alkaU metal ions. However, like the silver ion, it forms a very soluble fluoride, but the other haUdes are insoluble. Thallium (ITT) ion resembles aluminum, gallium, and indium ions in properties. 

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

The oxidation of mercurous ions by thallium (3+) ion shows an inverse first-order dependence on [Hg2+], which means that one Hg2+ ion must be subtracted out in figuring the composition.3 Thus, we have... 

The exhibition of variable valency is indeed a characteristic of transition metals. Main group metal ions such as those of groups 1 or 2 exhibit a single valence state. Other main group metals may show a number of valencies (usually two) which are related by a change in oxidation state of two units. This is typified by the occurrence of lead(iv) and lead(ii) or thallium(iii) and thallium(i). However, all the transition metals exhibit a range of valencies that is generally not limited in this manner. 

Mercury(II), thallium(III), and lead(IV) are isoelectronic and, as can be seen from the data in Eqs. (19)-(22) (77) the redox potential for thallium is intermediate between those of mercury and lead. Consequently, the relative oxidizing ability of the three metal ions should be in the order Hg(II) <... 

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... 

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. 

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

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