Iodine disproportionation reactions

Reaction (11.4) is really a disproportionation reaction of the halate(I) anion 3XO 2X -E XO. ) Reaction (11.3) is favoured by the use of dilute alkali and low temperature, since the halate(I) anions, XO are thermally unstable and readily disproportionate (i.e. reaction (11.4)). The stability of the halate(I) anion, XO , decreases from chlorine to iodine and the iodate(I) ion disproportionates very rapidly even at room temperature. 

It has also been shown that the disproportionation reaction, with the generation of the ionic componnd from thioamide-iodine complexes, exhibits pressure dependence [2]. A pressnre increase, leads to the ionic iodonium salt (iii) from (ii) (Scheme 13.2). The favouring of [MBZIM) ] [I3] (24a) formation, is also proven by compntational stndies, based on energetic gronnds [6]. 

The usual reversibility of disproportionation reactions with increase in acidity provides an effective general route to synthesis of cations in low or fractional oxidation states, as was shown for preparation of Au"+ in Sec. 11.3.4,3 and for iodine cations in Sec. 11.3.4.4. 

The disproportionation of HOI at pH 13 has been reported (49). Disproportionation reactions are likely at pH values near the pKa, where both HOI (an electron acceptor) and OI (an electron donor) species are present. The energies of the LUMO of HOI are close to the HOMO of IO. Thus, a two-electron transfer from IO to HOI is likely (Figure 8). Because the HOMO of IO and the LUMO of HOI are predominantly iodine in character (50), equation 9a is predicted to be more appropriate than equation 9b. 

Above pH 11, iodine disproportionates to hypoiodous acid (HOI), iodate (IO3), and iodide. Define disproportionation, find the oxidation states of I in each compound, and write balanced equations for the reactions. 

The effectiveness of a spray system in removing fission product iodine from the containment atmosphere depends on several parameters, as is known from early investigations, among others those conducted by Row et al. (1969), by Patterson and Humphries (1969) and by Row (1971). Other experiments have demonstrated that spray solutions buffered at pH 9.5 are more effective in iodine removal by a factor of 3 than pure water, probably because acidification of the outer droplet layer is prevented which, in pure water, may be caused by the I2 hydrolysis and disproportionation reactions (Hyder, 1991). Other parameters controlling iodine removal by sprays are specific for the individual system under consideration (internal geometry, spray rate etc.). Spraying experiments performed in the context of the CSE tests (see Section 7.3.3.3.8.) showed a reduction of the airborne iodine concentration of more than one order of magnitude within a spray time of about 10 minutes subsequent spray periods were much less effective (Hilliard and Postma, 1981). In the case of airborne particulate iodide, the chemical composition of the spray solution (water, boric acid, alkaline borate solution) does not affect... 

First, some short remarks on the basic chemistry of iodine in a high-temperature steam environment shall be made. In such a steam phase as is present in the reactor pressure vessel above the core melt zone, as well as in wide regions of the primary system, ionic iodine species such as 1 and lOs" (which are the dominant products in aqueous solution at higher pH) are not stable. As a result, the disproportionation reaction of I2 caused by H2O is not expected to proceed inside the primary system to any significant extent (perhaps with the exception of liquid water pools which may be formed in regions of lower temperature), quite in contrast to the sump water of the containment, where it is one of the essential mechanisms controlling... 

The kinetics of HOI disproportionation (reaction 2) has been analyzed by different investigators (e. g. Thomas et al., 1980 Palmer and Lyons, 1988), with somewhat conflicting results. Measurements of Wren et al. (1986) have shown that the rate of disproportionation in alkaline solutions can be expressed as a function of the concentrations [I2 + L + IO + LOH ], with the iodine +1 oxidation state species IO being the primary component, and the reaction proceeding through the +3 species 102 . In addition, the rate of disproportionation is catalyzed by both phosphate and borate buffers. 

The above formation reactions are disproportionation reactions of iodine resulting from the superimposition of the following half-reduction equilibria ... 

All of the cation radicals of 1-11 have been isolated as solid perchlorate salts. Compounds 1 ( ) and 2 ( ) are best oxidized by perchloric acid in anhydrous solvents. The cation radicals of 3 (O, 5 (2 ), 7, 8 ( ), 10 ( ), and 11 (10) can be made by oxidation with iodine-silver perchlorate. The crystalline cation radical perchlorates can be separated out or used in mixture with the solid silver iodide. The cation radical of 4 (7) y as well as that of 3( ) is obtained by an interesting disproportionation reaction of the parent compound and its 5-oxide in perchloric acid (eq. 2). The reaction probably involves the dication and the... 

Scheme 20 Main equilibria involved in the hydrolytic disproportionation of iodine and reaction with hydrogen peroxide.

The net reaction is the disproportionation of H2O2 to H2O + 5O2 and the starch indicator oscillates between deep blue and colourless as the iodine concentration pulsates. 

In reaction (19) the iodine shown on the left has an oxidation number of zero. After the reaction, some of the iodine atoms have oxidation number +5 and some —1. In other words, the iodine oxidation number has gone both up and down in the reaction. This is an example of selfoxidation-reduction, sometimes called disproportionation. It is a reaction quite typical of, but not at all restricted to, the halogens. 

A reasonable mechanism for the iodine oxidation of 5-Trt cysteine peptides is given in Scheme 6. 45 Reaction of iodine with the divalent sulfur atom leads to the iodosulfonium ion 5 which is then transformed to the sulfenyl iodide 6 and the trityl cation. Sulfenyl iodides are also postulated as intermediates in the iodine oxidation of thiols to disulfides. The disulfide bond is then formed by disproportionation of two sulfenyl iodides or by reaction between the electrophilic sulfur atom of R -S-I and the nucleophilic S-atom of a second R -S-Trt molecule. The proposed mechanism suggests that any sulfur substitution (i.e., thiol protecting group) capable of forming a stabilized species on cleavage, such as the trityl cation, can be oxidatively cleaved by iodine. 

We shall conclude the discussion in this section with the reaction eaq + I2 - I2 which proceeds at a diffusion-controlled rate (122). In this reaction, I2 was shown to be formed at a rate equal to the rate of disappearance of e aq. I2- may act as an iodine atom carrier on interaction with organic radicals (22) and will be reduced easily by e aq or H atoms. Ultimately, it disproportionates to give I + I. ... 

The first step in these oxidation reactions is a fast pre-equilibrium, which can be formally considered as ligand exchange (hydroxy - alkyloxy) on the iodine atom. The product 9 then disproportionates to the carbonyl derivative 4 and the iodosoarene 10 (IBA) [8]. 

In 1947 Szwarc prepared a white polymeric material u by rapid flow pyrolysis of p-xylene under reduced pressure. On the basis of p-xylylene diiodide 2) detected in the reaction mixture of the pyrolysis products with iodine gas he proposed a formation 1,3) of p-xylylene(p-quinodimethane) (QM) in this pyrolysis. He claimed the polymeric material to be poly-p-xylylene(poly-QM)and proposed a mechanism 2) for the formation of poly-QM, involving thermal cleavage of carbon-hydrogen bonds of p-xylene to yield p-xylyl radicals which collide with each other to give p-xylene and QM through disproportionation. QM condenses and polymerizes to produce poly-QM. 

The reaction very likely proceeds through the disproportionation of 4-ethoxyphenyl tellurium monochloride to tellurium and the diaryl tellurium dichloride. Bis[trifluoromethyl] tellurium transferred its trifluoromethyl groups to iodine, sulfur, selenium, phosphorus, arsenic, and antimony on heating with the elements, sulfur dichloride, selenium tetrabromide, or the triiodides of phosphorus, arsenic, and antimony at 170 to 220° in sealed tubes3. 

The rate of disproportionation of IO is very fast at all temperatures, so that it is unknown in solution. Reaction of iodine with base gives IO3 quantitatively according to an equation analogous to that for Br2. 

Chlorine monofluoride may be prepared by direct interaction at 220 to 250°C and it is readily freed from C1F3 by distillation, but it is best prepared by interaction of Cl2 and C1F3 at 250 to 350°C. Bromine monofluoride also results on direct reaction of Br2 with F2, but it has never been obtained in high purity because of its ready disproportionation. Iodine monochloride is obtained as brownish-red tablets (J3 form) by treating liquid chlorine with solid iodine in stoichiometric amount, and cooling to solidify the liquid product. It readily transforms to the a form, ruby-red needles. Bromine monochloride is prone to dissociation ... 

Bromine trifluoride and IFS are formed by the reaction of X2 with AgF in HF to give AgX plus XF, followed by disproportionation of XF, but the products are not easily purified. Iodine trifluoride is a yellow powder obtained by fluorination of I2 in Freon at -78°C it decomposes to I2 and IF5 above -35°C. 

The [AuBr4] ion is similar to [AuC14]. Both are formed by the disproportionation of [AuX2] in aqueous solution in the absence of excess halide. The tetraiodoaur-ate ion, [Aul4], is obtained by oxidation of [Aul2] with iodine. It is unstable, and in acetonitrile the temperature-dependent reaction is reversible ... 

Cationic polymerization is similar to anionic polymerization in that binary termination by recombination or disproportionation cannot occur. The most common initiators are Brensted or Lewis acids and iodine. A plethora of possible side reactions make it difficult to attain high molecular weights or prepare living polymers. Also, theoretical predictions of rates, molecular weights, and molecular-weight distributions are in general not reliable. 

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