Iodine interaction with solvent

Abstract This presentation is a brief review on the resnlts of our work on iodine interaction with thioamides, selenoamides and amides. The thioamides, benzothia-zole-2-thione (BZT) (1), 6-n-propyl-2-thiouracil (PTU) (2), 5-chloro-2-mercap-tobenzothiazole (CMBZT) (3), N-methyl-benzothiazole-2-thione (NMBZT) (4), benzimidazole-2-thione (BZIM) (5), thiazolidine-2-thione (TZD) (6), 2-mercapto-pyridine (PYSH) (7), 2-mercapto-nicotinic acid (MNA) (8), 2-mercapto-benzoic acid (MBA) (9) and 2-mercapto-pyrimidine (PMT) (10) react with producing three type of complexes of formulae [(HL)IJ(l2) (HL= thioamide and n= 0, 1), [(HL) [I3 ] and [(HL-L)]+[l3 ]. The interaction of seleno-amides, derived from, 6-n-propyl-2-thiouracil (RSelJ) (R= Me- (11), Et- (12), n-Pr- (13) and i-Pr- (14)) with I, have also been studied and produced the complexes [(RSeU)IJ of spoke structure. These complexes are stable in non-polar solvents, but they decompose in polar solvents, producing dimeric diselenide compounds or undertake deselenation. 

EtZnI (117) behaves differently because no clear solutions of this compound could be obtained in apolar solvents. Crystals of 117 suitable for an X-ray crystal determination could be obtained by crystallization of crude 117 from EtI as a solvent . The solid state structure of 117 involves the packing of four EtZnI units in an orthorhombic unit cell in such a way that each of the zinc atoms has additional bonding interactions with two iodine atoms of neighbouring EtZnI units, thus forming a coordination polymer (Figure 57) . ... 

Since iodine is a solid and sulphur dioxide is a gas, a polar solvent must be used for the reaction and as a dilution agent. Methanol is generally used as the solvent however, methylmonoether glycol or diethylene glycol can also be used. In these conditions, sulphur dioxide is not simply dissolved in the solvent but actually interacts with it. For example, with methanol, S02 is transformed into methylhydrogen sulphite that reacts with I2 in the presence of water. This leads to the following reaction ... 

Iodoacetylenes as well as iodine cyanide are soft Lewis acids (Laurence etal. 1981), which interact with basic solvents yielding characteristic wavenumber shifts Av (C-I) (e g., for ICN relative to the wavenumber in CCI4 solutions). These shifts differ for soft solvents, with sulfur or selenium donor atoms or n systems, and hard solvents, with oxygen or nitrogen donor atoms. However, these authors have not converted this observation and their data to a solvent softness scale. In fact, if prorated values of A v (O-H), for phenol, relative to CCI4 solutions, see B0 H above, representing the hard basicity of the solvents, are subtracted, the remainder measures the solvent softness. Quantitatively,... 

The mechanism of glycal addition was further examined by Horton and coworkers,86 who have used this method to form (7S,9S)-4-demethoxy-7-0-(2,6-dideoxy-2-iodo-a-L-mannopyranosyl)-adriamycinone87 and -daunomycinone,88 which are iodinated analogs of natural antitumor compounds. The nature of the solvent was found to be critical. In particular, they were able to generalize that in non-coordinating polar solvents, where no possibility for interaction with iodine exists—that is, no lone-pair interactions—the formation of the iodonium intermediate was irreversible, and the resultant stereochemistry reflects the electronic... 

Solutions of iodine in organic solvents illustrate an important aspect of solution chemistry. Iodine vapor has a deep purple color as a result of absorption of light in the visible region, which shows a maximum at 538 nm. Solutions of iodine have absorption maxima that vary with the nature of the solvent, and when I2 is dissolved in a solvent such as CC14 or heptane the solution has a blue-purple color. However, if the solvent is benzene or an alcohol the solution is brown. This difference in color is the result of the change in the relative energy of the n and n orbitals of the I2 molecule as a result of their interaction with the solvent. The interaction is very weak for solvents such as CCI4 and heptane so in... 

Since iodine is a solid and sulfur dioxide is a gas, an auxiliary polar solvent is used which serves as both a diluent and as a reaction medium. Generally, methanol is used, though in rare cases the methylmonoether glycol, or diethylene glycol, are also used. Under these conditions sulfur dioxide is not simply dissolved in the solvent but interacts with it, leading with methanol to methylhydrogenosulfite. This latter becomes the species that reacts with the iodine in the presence of water. 

The model of iodine is very simple 1° interacts with water by the non-electrostatic part of the I"-water interactions [115]. Figure 33 shows the solvent contribution to the potential of mean force (PMF) for the ion and the neutral atom. As for the ion, the solvent PMF is purely repulsive for the atom. In the region from z = -8 A to z = — 5 A both curves are almost identical. This is in keeping with the interpretation of this part of the free energy increase being due to steric interactions rather than hydration forces (which are absent for the neutral I , as the hydration energy becomes almost zero after removal of the electrostatic contributions to the interaction potential). 

Zeolites can also be considered as solid electrolytic solvents with a donor-acceptor interaction between the framework and the counter cation. The color of iodine is due to a n —>0 transition in the visible range and is known to be largely sensitive to the solvent [3]. Because of a donor-acceptor interaction between iodine and the solvents the a orbital is pertmbed and shifted to higher... 

A noteworthy conclusion in the publication cited is that the explanation of the deviation from a linear correlation may be the interaction between the iodine and the individual solvents, i.e., the formation of charge-transfer complexes. This would alter the diffusion coefficients, and additionally, positronium may react at different rates with free iodine and with iodine in the complex form. 

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