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The solubilities of iodine in $\ce{CCl4,n-Hex,Et2O,PhCH3}$ were given respectively as 19 g/kg, 13 g/kg, 337 g/kg, and 182 g/kg. These results puzzle me for the following reasons:

1) Iodine, being a non-polar substance, should dissolve the most in nonpolar solvents. Yet diethyl ether the best solvent for iodine.

2) Toluene and n-Hexane, both being hydrocarbons, should have similar solubilities of iodine. Yet the solubility of Iodine in Toluene almost 20 times that of n-Hexane.

How do we qualitatively predict these results?

Note: This question's source is from HBCSE INCHO 2012, a chemistry test for high school students, without fancy tools such as the Abrahams' equation.

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  • $\begingroup$ Note that the values order or differences will look different once the density of the solvent or iodine/solvent is accounted. Your data are expressed in mass to mass. At the denominator I am not sure what to be intended, mass of the solution or mass of the solvent. Probably the last so one can proceed with knowing individual densities. I think this might impact your question though there is not an order of ten in density values difference. $\endgroup$ – Alchimista Apr 22 at 6:47
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Iodine as a halogen has the strong electron affinity, what is the best met by the free electron pair of diethylether, then by $\pi$ electron density of toluene.

Note that iodine is also well soluble in ethanol, commercially used in medical treatment of skin injuries.

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Iodine forms charge transfer complexes, most notably with starch. In these complexes, a fraction of the electronic charge is transferred to the other molecule. The formation of these complexes increase the solubility of iodine. This has also been called solvation, see for instance this article.

One effect of the formation of these complexes is the change in colour. When there is no interaction with the solvent, the colour of the solutions is violet. This colour changes to red-violet and brown when there is increasing charge transfer.

The order of solubility in the different solvents can be rationalized to some extent looking at the relative permittivity (also known as dielectric constant):

compound               solubility  permittivity   colour
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carbon tetrachloride       19          2.24       violet
hexane                     16          0.44       violet
diethyl ether             337          9.83       brown
toluene                   182          2.4        red-violet

Of course, carbon tetrachloride seems an outlier. What happens here is that the dielectric constant is high due to the high $\delta+$ on the carbon. But, since the $\delta-$ is shared by four chlorine atoms and the molecule is symmetrical, there is no effect on the iodine.

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