Oxidation of Iodide to iodine

One of the most required methods of determination of iodide-ions in praetiee of ehemieal analysis is photometrie determination of produets of iodination of organie eompounds. The oxidation of iodide to iodine ean be earned out suffieiently seleetively. But in ease of presenee of great abundanee of bromide-ions the seleetive oxidation of iodide-ions is problematie. The variants of determination of iodide-ions with different organie reagents are known, but the absenee of bromide-ions in a system is supposed in most of them. In natural objeets these halides are present simultaneously. 

They form a monolayer that is rich in defects, but no second monolayer is observed. The interpretation of these results is not straightforward from a chemical point of view both the electrodeposition of low-valent Ge Iy species and the formation of Au-Ge or even Au Ge h compounds are possible. A similar result is obtained if the electrodeposition is performed from GeGl4. There, 250 20 pm high islands are also observed on the electrode surface. They can be oxidized reversibly and disappear completely from the surface. With Gel4 the oxidation is more complicated, because the electrode potential for the gold step oxidation is too close to that of the island electrodissolution, so that the two processes can hardly be distinguished. The gold step oxidation already occurs at -i-lO mV vs. the former open circuit potential, at h-485 mV the oxidation of iodide to iodine starts. 

A small group of cyclic thioureas have been used in the treatment of excessive thyroid function. They include 6-n-propylthiouracil (176 R1 = H, R2 = Prn), 5-iodo-2-thiouracil (176 R1 = I, R2 = H), l-methyl-2-mercaptoimidazole (methimazole) and its ethoxycarbonyl derivative carbimazole (177), which lacks the bitter taste of the unacylated compound. These compounds block the synthesis of thyroxin by inhibiting the oxidation of iodide to iodine and the oxidative coupling of iodotyrosine residues. 

Oxidation of iodide to iodine (Eo = -0.615 V) promoted by sonication was one of the earliest tests demonstrating the sonochemical effects on solutions. The following effects were detected in water-sonicated systems with and without CCI4 and (or) iodide ... 

In the presence of Kl, a pink colour appeared due to the oxidation of iodide to iodine and extraction of I2 into the organic phase. 

Iodide ions (as F or I3 ) can be extracted as ion-pairs with basic dyes such as Crystal Violet [8,9], Brilliant Green [10,11], Rhodamine B (in toluene) [12], or Butylrhodamine (in benzene) [13]. After oxidation of iodide to iodine, the iodine is extracted into CCI4 and then stripped into the aqueous phase (as F) by shaking with thiosulphate. Finally, the F ions associated with Methylene Blue, are extracted into 1,2-dichloroethane [14]. 

Selenium has been determined with 5,5-dimethyl-1,3-cyclohexanedione [51], 6-amino-1-hydroxynaphthalenesulphonic acid [52,53], and l-aminonaphthalene-7-sulphonic acid [54], Determinations of Se involved also the following dyes Rhodamine B [55], Methylene Blue [56], Xylenol Orange [57], and Rhodamine 6G (by the amplification method, in iodide medium, after oxidation of iodide to iodine, and reaction of the I03 with the dye) [58]. 

At one time, they added sodium thiosulfate, but it sounded too chemical, and people became worried. So, the manufacturers switched to dextrose, which is as effective and sounds innocuous. Sometimes, they add sodium bicarbonate, because the oxidation of iodide occurs readily in an acid solution and not in a base bicarbonate produces basic conditions. And occasionally they might use disodium phosphate or sodium pyrophosphate to provide the alkaline conditions. These are also sequestering agents, which bind trace metals that catalyze the oxidation of iodide to iodine. 

Reaction with ei,fi-Unsaturated Sulfoxides. The reaction of TMSI with a, -unsaturated sulfoxides in chloroform at ambient temperature is a mild, efficient, and general method for the preparation of carbonyl compounds (eq 63). The proposed reaction mechanism is shown in eq 63. Formation of a strong oxygen-silicon bond is followed by reduction of the sulfur function and oxidation of iodide to iodine, the latter precipitating in chloroform. The trimethylsiloxy anion attacks the unsaturated carbon linked to the sulfur function, which leaves the substrate, allowing the formation of the sUyl enol ether species. Finally, hydrolysis converts the silyl enol ether into the carbonyl compound. ... 

In acidic solution, hydrogen peroxide is a powerM oxidizing agent (see Table 1.5, Section 1.5). The oxidation of iodide to iodine by acidified hydrogen peroxide is a good example for our purpose ... 

Oxidation of iodide to iodine is undoubtely involved in these reactions, but the details of the mechanism have not been explored. lodination of moderately reactive aromatics can be effected by mixtures of iodine and silver or mercuric salts. Hypoiodites are presumably the active iodinating species. Some examples of iodina-tion procedures are included in Scheme 11.2. 

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