Doubt on solubility and how to predict it

In summary, Benzoic acid is a polar molecule and is soluble in a non-polar solvent, chloroform. This is because the intermolecular interactions between the molecules of benzoic acid are equal to those formed between the molecules of chloroform. However, there is some confusion in the technique to be used to predict solubility.
  • #1
pisluca99
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Two substances are known to be miscible when the intermolecular interactions formed between the molecules of one substance are equal to those formed between the molecules of the other substance.
That said, why is benzoic acid, which is predominantly apolar, so it mainly forms London interactions, soluble in chloroform, which mainly forms dipole-dipole interactions? For this reason, benzoic acid should not be well solvated by chloroform molecules, which will prefer to interact with each other to form stronger interactions than they would form with benzoic acid.

Also, having said that, I sense some confusion in the technique to be used to predict solubility. Does one exploit, as in the example given, the balance of intermolecular forces (which does not always seem to work, though)? Or is it better to use the dielectric constant to estimate the polarity of a solvent and then see which solutes dissolve in it (by taking a balance between polar and apolar groups in the solute itself)?

Also, chloroform is theoretically polar, having a dipole moment, but is evaluated as a non-polar solvent. I find so many contradictions.
 
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  • #2
These are just basic rules of thumb, so don't expect them to be very consistent.

Compare interactions not by type, but by the energy, it is thermodynamics that counts.
 
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  • #3
Borek said:
These are just basic rules of thumb, so don't expect them to be very consistent.

Compare interactions not by type, but by the energy, it is thermodynamics that counts.
Ok thanks!

Still referring to the concept of solubility, is there a relationship between the dielectric constant and the magnitude of hydrogen bonds?

For example: water has a high dielectric constant, so it is a strongly polar solvent, so it will dissolve equally strongly polar solutes (i.e., with high dielectric constant). If a molecule is soluble in water, then it will have to form several hydrogen bonds. So it can be concluded that the dielectric constant is proportional to the ability of a molecule to form hydrogen bonds. Can this be considered as an accurate reasoning?
Otherwise it would not be explained why polar substances (with a high dielectric constant) are soluble in water.
 
  • #4
No hydrogen bonds in bromine trifluoride, yet is has dielectric constant even higher than water.
 
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  • #5
Borek said:
No hydrogen bonds in bromine trifluoride, yet is has dielectric constant even higher than water.
Oh perfect.
But then how do you explain that polar/strongly polar molecules, therefore with a high dielectric constant, dissolve in water? They must necessarily form many hydrogen bonds. So there must be a sort of relationship between dielectric constant and hydrogen bonds..
 
  • #6
As far as I remember there is some correlation between the dielectric constant and the dipole moment of molecules.

And there is no need for hydrogen bonds, solvation of partial charges present in polar molecules will definitely help in dissolving.
 
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  • #7
Ok, thanks for your time!
 
  • #8
Borek said:
As far as I remember there is some correlation between the dielectric constant and the dipole moment of molecules.
Some correlation, but widely scattered. Like this:
https://depts.washington.edu/eooptic/linkfiles/dielectric_chart[1].pdf
The biggest dipole moment is hexamethylphosphamide - 5,54 D but only 30. No option for hydrogen bonds (no free hydrogens). Highest dielectric constant here is formamide - 111 but just 3,37 D.
 
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