Qualitative understanding of partition preferences

Introduction
Cavity model
Rules for partitioning
The cavity model in quantitative terms

Selftest
     1) What does the cavity model say?
           Answer
     2) Main interactions ... ?
           Answer
     3) Size of a solute  molecule ... ?
           Answer
     4) Size of the solvent  molecule ... ?
           Answer
     5) Interpretation of data
           Answer
     6) "Like dissolves like"
           Answer
     7) Concept maps
     8) Functional groups
           Answer
     9) Illustration by given data?
           Answer
     10) Evaluation of the software PcKocWIN
     11) H-bonds between given substances?
     12) Tendency to distribute
           Answer
     13) Gas chromatography
           Answer
     14) Henry's Law constant
           Answer
     15) Quiz
Problems
Intermolecular interactions in every day life
FAQ

6) "Like dissolves like"

 

Question:

You can often find that the rule "like dissolves like" is interpreted as "non-polar chemicals dissolve best in non-polar solvents" and "polar chemicals dissolve best in polar solvents". See for example the Wikipedia page on "Solvent" accessed in January 2008 (see Solvent-Wikipedia.pdf section marked in yellow). In this context polar solvents are typically defined as those that contain oxygen and/or nitrogen while non-polar solvents are defined as those that do not contain oxygen and/or nitrogen. Can you name some general cases that will proof these statements wrong? Use the classification that was introduced here.

b) In the same paragraph of the Wikipedia entry you can read that "The polarity can be measured as the dielectric constant or the dipole moment of a compound." Does that agree with what you have learned in this chapter?

 

Answer:

a) CHCl3 and CHCl2 are typically considered to be non-polar solvents but they will dissolve monopolar H-bond acceptor solutes much better than polar solvents such as esters, ethers or ketones will do because the latter are H-bond acceptors and therefore cannot form H-bonds with an H-bond acceptor solute.

b) There is no useful connection between the dipole moment or the dielectric constant of the solute or the solvent and partitioning of non-ionic organic compounds. This is illustrated by the data in the Table in FAQ 2 that show that the partitioning of the isomers of Dichlorobenzenes in various polar partition systems does not differ substantially despite the fact that all 3 isomers have distinctly different dipol moments while there vdW interactions are the same, and H-bonds do not exist.* The dielectric constant and the dipole moment is relevant for a solvent´s ability to dissolve charged compounds such as salts. This is beyond the scope of our text.


 

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