Iodine equilibria

Grovenstein and Kilby218 showed that the kinetic isotope effect kH/kD is 3.97 for the iodination 2,4,6-trideuterophenol by iodine in aqueous buffer at 25 °C, and this is in accord with the base catalysis described above. However, this large isotope effect means that the intermediate is in fairly rapid equilibrium with the reactants, so that it is difficult to determine from kinetic studies which iodinating species is involved. Thus it might be positive iodine, equilibria (112), (113), (115)... 

CT complexes between carbonyl groups and iodine (equilibrium 16) have been well known for a long time179, as well as complexes with bromine180. 

The graphs of extent of adsorption of iodine wifli total iodine equilibrium concentration. Figure 8.2, for the charcoal and the 850 °C PVDC carbon exhibit areas within which any point is a position of equilibrium in the system. This area is restricted in the upper region by the limiting solubility of iodine in the iodide solution and in the lower region by the... 

This equilibrium is established when hydrogen iodide is heated, hydrogen-iodine bonds being broken. 

The amount of halic(I) acid formed when the halogen reacts reversibly with water decreases from chlorine to iodine and the concentration of iodic(I) acid in a saturated solution of iodine is negligible. However the equilibrium... 

Comparable results are not obtained with the less reactive iodine, because the hydrogen iodide formed tends to reduce the iodo compound and a condition of equilibrium is produced ... 

Radical substitution reactions by iodine are not practical because the abstraction of hydrogen from hydrocarbons by iodine is endothermic, even for stable radicals. The enthalpy of the overall reaction is also slightly endothermic. Thus, because of both the kinetic problem excluding a chain reaction and an unfavorable equilibrium constant for substitution, iodination cannot proceed by a radical-chain mechanism. 

Chlorination is cariied out in a manner similar- to bromination and provides a ready route to chlorobenzene and related ar-yl chlorides. Fluorination and iodination of benzene and other arenes are rarely perfor-med. Fluorine is so reactive that its reaction with benzene is difficult to control. Iodination is very slow and has an unfavorable equilibrium constant. Syntheses of aryl fluorides and aryl iodides are nor-mally cariied out by way of functional group transformations of arylffluines these reactions will be described in Chapter 22. 

Mukherjee studied the gas phase equilibria and the kinetics of the possible chemical reactions in the pack-chromising of iron by the iodide process. One conclusion was that iodine-etching of the iron preceded chromis-ing also, not unexpectedly, the initial rate of chromising was controlled by transport of chromium iodide. Neiri and Vandenbulcke calculated, for the Al-Ni-Cr-Fe system, the partial pressures of chlorides and mixed chlorides in equilibrium with various alloys and phases, and so developed for pack aluminising a model of gaseous transport, solid-state transport, and equilibria at interfaces. 

It is, however, more likely that the discrepancies observed in the periodate oxidation of malonaldehyde concern mainly the hydroxylation step. In the mechanism proposed (5) for this reaction, it is the enol form of malonaldehyde which is hydroxylated. However, titrations of a solution of malonaldehyde, prepared by hydrolysis of an aqueous solution (33) of carefully distilled 1, 3, 3-tri-ethoxypropene (46, 47), both with strong base and with iodine, indicate that only about 80% of the enol form is present in the equilibrium solution. On the other hand, the thio-barbituric acid test (58, 59) gave consistently higher values for the malonaldehyde content of the solution. The fact that only about 80% of the enol form is present in the equilibrium solution is all the more important as it can be shown (56) by titration with strong base that the enolization is slow, and moreover does not seem to go to completion. 

A similar argument can be applied to the equilibrium involved in the sublimation of iodine ... 

At a given temperature, the pressure of iodine vapor is constant, independent of the amount of solid iodine or any other factor. The equilibrium constant expression is... 

At this temperature, K = 0.0169. What are the partial pressures at equilibrium of the hydrogen and iodine if initially a sealed flask at 800 K contains only HI at a pressure of0.200 atm ... 

To determine the equilibrium constant of foe system, identical one-liter glass bulbs are filled with 3.20 g of HI and maintained at a certain temperature. Each bulb is periodically opened and analyzed for iodine formation by titration with sodium thiosulfate, Na O ... 

It is determined that when equilibrium is reached, 37.0 mL of 0200 M Na2S203 is required to titrate the iodine. What is K at the temperature of the experiment ... 

To determine the ftq, value of Hg a solid sample is used, in which some of the iodine is present as radioactive 1-131. The count rate of the sample is 5.0 X 1011 counts per minute per mole of L An excess amount of Hg2I2(s) is placed in some water, and the solid is allowed to come to equilibrium with its respective ions. A 150.0-mL sample of the saturated solution is withdrawn and the radioactivity measured at 33 counts per minute. From this information, calculate the ft, value for Hg2l2. 

The bulbs contain tleft to right) gaseous chlorine and the vapors in equilibrium with liquid bromine and solid iodine. 

Fig. 9-1. Iodine dissolving in an alcohol-water mixture. Equilibrium is recognized by constant color of the solution.

Figure 9-1 shows the addition of solid iodine to a mixture of water and alcohol. At first the liquid is colorless but very quickly a reddish color appears near the solid. Stirring the liquid causes swirls of the reddish color to move out— solid iodine is dissolving to become part of the liquid. Changes are evident the liquid takes on an increasing color and the pieces of solid iodine diminish in size as time passes. Finally, however, the color stops changing (see Figure 9-1). Solid is still present but the pieces of iodine no longer diminish in size. Since we can detect no more evidence of change, we say that the system is at equilibrium. Equilibrium is characterized by constancy of macroscopic properties ...

Figure 9-1C shows a system at equilibrium. Solid iodine has dissolved in an alcohol-water mixture until the solution is saturated. Then no more solid dissolves and the color of the solution remains constant. 

Consideration of the dissolving of iodine in an alcohol-water mixture on the molecular level reveals the dynamic nature of the equilibrium state. The same type of argument is applicable to vapor pressure. 

The simple form of the equilibrium expression (4) follows directly from the dynamic nature of the solubility equilibrium. There must be a dynamic balance between the rate that iodine molecules leave the ciystal and the rate that iodine molecules return to the crystal. To understand this dynamic balance, we must consider the factors that determine these two rates. 

Expression (2) applies to a solubility equilibrium, provided we write the chemical reaction to show the important molecular species present. In Section 10-1 we considered the solubility of iodine in alcohol. Since iodine dissolves to give a solution containing molecules of iodine, the concentration of iodine itself fixed the solubility. The situation is quite different for substances that dissolve to form ions. When silver chloride dissolves in water, no molecules of silver chloride, AgCl, seem to be present. Instead, silver ions, Ag+, and chloride ions, Cl-, are found in the solution. The concentrations of these species, Ag+ and Cl-, are the ones which fix the equilibrium solubility. The counterpart of equation (7) will be... 

This may be illustrated by the following example. Suppose that 50 mL of water containing 0.1 g of iodine are shaken with 25 mL of carbon tetrachloride. The distribution coefficient of iodine between water and carbon tetrachloride at the ordinary laboratory temperature is 1 /85, i.e. at equilibrium the iodine concentration in the aqueous layer is 1 /85th of that in the carbon tetrachloride layer. The weight of iodine remaining in the aqueous layer after one extraction with 25 mL, and also after three extractions with 8.33 mL of the solvent, can be calculated by application of the above formula. In the first case, if x, g of iodine remains in the 50 mL of water, its concentration is x,/50 gmL 1 the concentration in the carbon tetrachloride layer will be (0.1 —x1)/25gmL 1. 

The resulting solution has a much lower vapour pressure than a solution of iodine in pure water, and consequently the loss by volatilisation is considerably diminished. Nevertheless, the vapour pressure is still appreciable so that precautions should always be taken to keep vessels containing iodine closed except during the actual titrations. When an iodide solution of iodine is titrated with a reductant, the free iodine reacts with the reducing agent, this displaces the equilibrium to the left, and eventually all the tri-iodide is decomposed the solution therefore behaves as though it were a solution of free iodine. 

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