Reactions of Iodine Species

Figure 9. An oceanic cycle for iodine indicating the major pathways for transport and reaction of iodine species.

Both molecular iodine and organic iodides are susceptible to thermal and radiolytic decomposition in the containment atmosphere. Of particular interest is the reaction of iodine species with ozone formed by radiolytic processes in the atmosphere ... 

Reactions of Iodine Species Isotopic Exchange Reactions. 

The rates of some redox reactions of iodine species at room temperature are given below. A discussion of the mechanisms of some reactions of Inorganic Iodine compounds may be found elsewhere. (31)... 

In most direct titrations with iodine (iodimetry) a solution of iodine in potassium iodide is employed, and the reactive species is therefore the tri-iodide ion 13. Strictly speaking, all equations involving reactions of iodine should be written with 13 rather than with I2, e.g. 

The oxidative addition of iodine to diorganotellurides has also been examined by stopped-flow spectroscopy. The initial fast reaction of iodine with diphenyltellur-ide (23), di-4-methoxyphenyltelluride (24), A,V-dimethyl-2-(aminomethyl)phenyl-telluride (17), and 2,6-di-tert-butyltelluropyran-4-one (25) displays inverted Arrhenius behavior (negative values of E ), which is consistent with a preequilibrium involving higher-order iodine species as shown in equation (6). The I4 species is the actual oxidant for diorganotellurides as shown in equation (7). Thus, the initial reaction is formation of the r i-association complex of I4 with the... 

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. 

Develop a reaction mechanism for iodine (I2-O2-H2 system) from the information in the NIST Chemical Kinetics Database [256], Start with the H2-O2 reaction subset hydrogen.mec. Using the database, identify the relevant reactions with I2. Add these reactions to the starting mechanism, including product channels and rate constants. List the additional I-containing species formed in reactions of I2. Extend the reaction mechanism with reactions of these species. Continue this procedure until reactions of all relevant iodine species in the I2-O2-H2 system is included in the mechanism. 

When a small amount of iodine is added to a mixture of chlorine and methane, it prevents chlorination from occurring. Therefore, iodine is a free-radical inhibitor for this reaction. Calculate A H° values for the possible reactions of iodine with species present in the chlorination of methane, and use these values to explain why iodine inhibits the reaction. (The I—Cl bond-dissociation enthalpy is 211 kJ/mol or 50 kcal/mol.)... 

This cycle is consistent with the known reaction chemistry of iodine, and he profiles of iodine species in the ocean and in sedimentary pore waters. The cycle allows for cycling of iodine in the marine environment through biological fixation. Thus, iodine appears to be an excellent carbon tracer in oceanic waters because of the difficulty in oxidizing I" to I03. This chemistry is in contrast to that of the sulfide system (25). 

MeOH, Pr OH, or MeO(CHa)aOMe.451e Similarly, the corresponding Br-HaO and Cl-HaO complexes have been observed on laser photolysis of Bra and Cla in water.451" The primary photoprocess following nitrogen-laser irradiation of I37 is (93).462 However, for solutions in various alcohols it has been shown that I47 is produced on laser flash photolysis of Ia/I mixtures.458 This species is formed by reaction of iodine atoms, formed on photolysis of Ia, with I3. I47 has also been observed following y-irradiation of I2 in methyltetrahydrofuran.454... 

Figure 15.5 A schematic of the photochemical reaction process of iodine species in the production and removal of ozone. The figure shows the reaction process of iodine species in the removal of O3 O3 -1- I = 10 -H O2 HO2 + HO2 -H 10 = HOI + O2 HOI -I- hy = OH -H 10 -H NO = l-H NO2.

Stanisavljev, D., Consideration of the thermodynamic stability of iodine species in the Bray-Liebhafsky reaction, Ber. Bunsenges. Phys. Chem., 101, 1036-1039, 1997. 

In addition to the reactions in solution mentioned above, molecular iodine dissolved in water can react with submerged surfaces. Irreversible chemical adsorption of iodine species on submerged surfaces could reduce the overall iodine inventory in the aqueous phase, and thereby reduce the release of molecular iodine to the containment atmosphere. 

In general, it can be assumed that the reaction between silver and iodine species in the gas phase, as well as the reaction of iodine vapor with silver aerosol or with silver deposited on the primary circuit surfaces, is only of minor significance for iodine behavior in the course of a severe accident. The main reasons are the rather short residence time of the silver aerosols in the gas phase, the fact that iodine and silver volatilization from the reactor core may differ considerably over time and, finally, the small proportion of elemental I2 and of HI (compared with the Csl fraction) assumed to be present in the gas phase during transport through the primary circuit. In contrast, Agl formation is expected to proceed to a significant extent later on in the containment sump water (see Section 7.3.3.3.3.). 

In Canada, the Liric model was developed (Wren et al., 1991) and is still undergoing improvement. Liric is a comprehensive model of an essentially mechanistic nature, showing similarities in the reaction sets used for the Inspect code described above. Liric was developed to predict the time-dependent behavior of iodine in the containment under a variety of reactor accident conditions. Out of the large number of reactions involving physical and chemical processes, aqueous thermal reactions of iodine are considered, as well as water radiolysis processes and the interaction of the radiolysis products with aqueous iodine species. In addition, radiolytic decomposition of organic substances in the aqueous phase, formation and decomposition of organoiodine compounds, iodine reactions with aqueous impurities such as buffers and metal ions, mass transfer between the aqueous and gas phases and, finally, adsorption of gaseous I2 onto surfaces and its desorption from them are included in the model. The Liric model has been developed in close cooperation with the experimental work carried out in the Radioiodine Test Facility. 

A more recent investigation by Suginome and collaborators has demonstrated that the selective fragmentation of alkoxyl radicals, one of the principal reactions of these species, can be utilized as the key step in the synthesis of a variety of molecules, including several natural products. Subsequently, Suarez and colleagues have also demonstrated the utility of the (diacetoxyiodojbenzene/iodine procedure in the hypoiodite photolysis in a series of articles. 

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