Iodine redox properties

The exact nature of the reaction (oxidative vs. reductive) will depend on the redox properties of I ) and Q. The electron transfer process is a special case of exciplex formation favored in the strongly polar solvents, such as water. The involvement of an exciplex in a photochemical reaction is generally established by studying the effects of known exciplex quenchers such as amines on the exciplex fluorescence and the product formation. The heavy atom effect, due to the presence of substituents such as bromine or iodine intra- or intermolecularly, causes an exciplex to move to the triplet state preferentially, with a quenching of fluorescence. 

It must be kept in mind that the interference with redox properties alluded to above may be thermodynamic and not kinetic that is, the decreased reducibility when residues 67 or 59 are derivatized may arise not from modifications in the direct electron pathway but from a decrease in the reduction potential of the protein. Margalit and Schejter (230) have observed that the alkaline state IV of cytochrome c, with methionine-80 replaced by another ligand, has a reduction potential which is lowered from the normal - -260 mV to less than 90 mV at pH 11.2 or above. If similar factors are at work in the derivatives iodinated or nitrated at tyrosine-67, where the III to IV transition is lowered from pH... 

Most kinetic determinations of anions involve the iodide ion, which exhibits a strong catalytic effect on the reaction between cerium(IV) and arsenic(III) and a few others as a result of the redox properties of the I2/ I couple. Other anions that can be determined using their intrinsic catalytic effect include sulfur-containing species such as sulfite, sulfide, and thiosulfate, which are quantified by means of the iodine/sodium azide system, and phosphates, which are measured through their effect on the formation of molybdenum blue. Table 5 gives illustrative examples of determinations for these anions and a few others. 

Poly(vinylbenzyl mercaptan) has been described by several authors. The synthesis always follows the same path chloromethylation of polystyrene followed by reaction with thiourea and hydrolysis of the isothiuronium salt 1,53,55,63-65). The redox properties of a polymer obtained from chloromethylated styrene-divinylbenzene copolymers have been evaluated (3). Redox capacities givrai for mercaptyl resins were determined to be 2. W>-5.27 milliequivalents of iodine reduced per gram of (dry) resin in aqueous potassium iodide. The oxidized form of the resin could be easily reduced with 10% aqueous bisulfite fmr a complete redox cycle. Recently polyvinylbenzyl mercaptan resins were prepared directly by treating chloromethylat polystyrene with KSH in dimethyl formamide (66). 

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

Perhaps the most important application of redox chemicals in the modern laboratory is in oxidation or reduction reactions that are required as part of a preparation scheme. Such preoxidation or prereduction is also frequently required for certain instrumental procedures for which a specific oxidation state is essential in order to measure whatever property is measured by the instrument. An example in this textbook can be found in Experiment 19 (the hydroxylamine hydrochloride keeps the iron in the +2 state). Also in wastewater treatment plants, it is important to measure dissolved oxygen (DO). In this procedure, Mn(OH)2 reacts with the oxygen in basic solution to form Mn(OH)3. When acidified and in the presence of KI, iodine is liberated and titrated. This method is called the Winkler method. 

Astatine has properties very similar to those of iodine, but has a more metallic character. The oxidation of astatide to astatine occurs at a potential ca. 0.4 V less positive compared to that for the oxidation of iodine. Fundamental information about the redox chemistry of astatine is available in an earlier review [158] and standard data collections [14]. A summary of the approximate standard potentials and known redox states is given in Scheme 8. 

Applications of iodine as a redox reagent are extensive because (7) its E° is intermediate and therefore it can act as either an oxidizing agent (as Ij) or a reducing agent (as I3 ), and (2) its is nearly independent of pH (at values less than about 8), a useful property when conditions for favorable conditional equilibrium constants are being selected. We include in this chapter a few examples to illustrate the range of application. 

Bromine is very reactive chemically it has properties that are intermediate between those of chlorine and iodine. The most stable valence states of bromine in its salts are — I and + V, although + I, + III, and + VII are also known. As indicated by the differences in standard redox potentials, bromine [Eo(Br2, Br ) +1.07 V] is less active oxidizing agent than chlorine [Eo(Cl2, Cl ) + 1.36 V]. Because of its reactivity, bromine does not occur naturally in the free elemental state, but it is widespread and abundant in the form of its ions, bromides (Greenwood and Earnshaw 1997). 

Thiols exhibit reducing properties. They are oxidized into disulfides by iodine. The couple disulfide/thiol obeys the half-redox equilibrium... 

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